Microglia contributes to plaque growth by cell death due to uptake of amyloid β in the brain of Alzheimer's disease mouse model

Pathological hallmarks of Alzheimer's disease (AD) include extracellularly accumulated amyloid β (Aβ) plaques and intracellular neurofibrillary tangles in the brain. Activated microglia, brain‐resident macrophages, are also found surrounding Aβ plaques. The study of the brain of AD mouse models revealed that Aβ plaque formation is completed by the consolidation of newly generated plaque clusters in vicinity of existed plaques. However, the dynamics of Aβ plaque formation, growth and the mechanisms by which microglia contribute to Aβ plaque formation are unknown. In the present study, we confirmed how microglia are involved in Aβ plaque formation and their growth in the brain of 5XFAD mice, the Aβ‐overexpressing AD transgenic mouse model, and performed serial intravital two‐photon microscopy (TPM) imaging of the brains of 5XFAD mice crossed with macrophage/microglia‐specific GFP‐expressing CX3CR1^GFP/GFP mice. We found that activated microglia surrounding Aβ plaques take up Aβ, which are clusters developed inside activated microglia in vivo and this was followed by microglial cell death. These dying microglia release the accumulated Aβ into the extracellular space, which contributes to Aβ plaque growth. This process was confirmed by live TPM in vivo imaging and flow cytometry. These results suggest that activated microglia can contribute to formation and growth of Aβ plaques by causing microglial cell death in the brain. GLIA 2016;64:2274–2290     

Microglial CX3CR1 production increases in Alzheimer's disease and is regulated by noradrenaline

The loss of noradrenergic neurons and subsequent reduction of brain noradrenaline (NA) levels are associated with the progression of Alzheimer's disease (AD). This seems to be due mainly to the ability of NA to reduce the activation of microglial cells. We previously observed that NA induces the production of the chemokine Fractalkine/CX3CL1 in neurons. The activation of microglial CX3CR1, sole receptor for CX3CL1, reduces the activation of microglia, which is known to largely contribute to the neuronal damage characteristic of AD. Therefore, alterations of CX3CR1 production in microglia could translate into the enhancement or inhibition of CX3CL1 anti‐inflammatory effects. In order to determine if microglial CX3CR1 production is altered in AD and if NA can control it, CX3CR1 expression and synthesis were analyzed in 5xFAD mice and human AD brain samples. In addition, the effects of NA and its reuptake inhibitor reboxetine were analyzed in microglial cultures and mice respectively. Our results indicate that in AD CX3CR1 production is increased in the brain cortex and that reboxetine administration further increases it and enhances microglial reactivity toward amyloid beta plaques. However, direct administration of NA to primary rat microglia or human HMC3 cells inhibits CX3CR1 production, suggesting that microglia responses to NA may be altered in the absence of CX3CL1‐producing neurons or other nonmicroglial external factors.     

Adhesion of exogenous human microglia and THP-1 cells to amyloid plaques of postmortem Alzheimer's disease brain

Microglial phagocytosis of amyloid-beta (Abeta) deposits is involved in Abeta clearance in vivo. To explore the ability of microglia to phagocytose beta, we cultured human microglia or human monocytic THP-1 cells directly on unfixed frontal cortex sections of an Alzheimer disease (AD) case. We found that when these cells were activated by lipopolysaccharide (LPS) plus interferon (IFN)-gamma, they developed ameboid morphology and formed clusters around and attaching to amyloid plaques in the tissue. Some cells adhering to these plaques internalized Abeta and some appeared to be degraded. Nevertheless, no significant reduction of the overall Abeta burden was observed. If the cells were not stimulated, they adhered poorly to the sections. We quantified THP-1 cell adhesion to an AD brain section compared with a normal brain section and found it to be significantly increased. If a brain section was rinsed with phosphate buffered saline containing 0.1% Triton X-100, most LPS/IFN-gamma-activated THP-1 cells failed to adhere. However, in co-culture with human astrocytes, the number of adherent THP-1 cells was significantly increased. These results suggest that human microglial cells are capable of adhering to and phagocytosing post mortem AD plaque material but activation may be necessary. Astrocytes may further enhance the process.     

Microglial activation in early stages of amyloid β protein deposition

The present study was undertaken to investigate the relationship of microglial activation to amyloid β protein (Aβ) deposition, particularly at the early stage. Using single and double immunostaining methods with a panel of microglia markers and antibodies against Aβ and amyloid β protein precursor (APP), we examined the cerebrum and cerebella of both Alzheimer’s disease (AD) and non-demented subjects obtained at autopsy. In non-demented, middle-aged subjects that had small amounts of cerebral Aβ deposits, approximately 70% of the diffuse plaques contained ramified microglia. However, no evidence of microglial activation was found in diffuse plaques in any of the non-demented subjects. Dual immunostaining of sections of cerebral cortex using antibodies against Aβ and major histocompatibility complex class II antigen showed that in AD subjects, approximately 20% of total diffuse plaques contained a few, activated microglia. Most of these plaques were defined as a transitional form between diffuse and primitive plaques. Both primitive and classic plaques in the cerebral cortex of AD subjects consistently contained clusters of activated microglia. Subpial Aβ deposits without neuritic changes lacked microglial activation. In the cerebellum, all of the diffuse plaques lacked microglial activation, and activated microglia in the compact plaques were not as hypertrophic as those in cerebral primitive/classic plaques. Our findings indicate that microglial reactions are absent in the early stages of Aβ deposition, and it occurs during the transition from diffuse to primitive plaques, when amounts of Aβ deposits and the degree of neuritic changes increase. Received: 17 January 1997 / Revised, accepted: 7 April 1997     

Microglial alterations in human Alzheimer's disease following Aβ42 immunization

E. Zotova, C. Holmes, D. Johnston, J. W. Neal, J. A. R. Nicoll and D. Boche (2011) Neuropathology and Applied Neurobiology 37, 513–524 Microglial alterations in human Alzheimer's disease following Aβ42 immunization Aims: In Alzheimer's disease (AD), microglial activation prompted by the presence of amyloid has been proposed as an important contributor to the neurodegenerative process. Conversely following Aβ immunization, phagocytic microglia have been implicated in plaque removal, potentially a beneficial effect. We have investigated the effects of Aβ42 immunization on microglial activation and the relationship with Aβ42 load in human AD. Methods: Immunostaining against Aβ42 and microglia (CD68 and HLA‐DR) was performed in nine immunized AD cases (iAD – AN1792, Elan Pharmaceuticals) and eight unimmunized AD (cAD) cases. Results: Although the Aβ42 load (% area stained of total area examined) was lower in the iAD than the cAD cases (P = 0.036), the CD68 load was higher (P = 0.046). In addition, in the iAD group, the CD68 level correlated with the Aβ42 load, consistent with the immunization upregulating microglial phagocytosis when plaques are present. However, in two long‐surviving iAD patients in whom plaques had been extensively cleared, the CD68 load was less than in controls. HLA‐DR quantification did not show significant difference implying that the microglial activation may have related specifically to their phagocytic function. CD68 and HLA‐DR loads in the pons were similar in both groups, suggesting that the differences in microglial activation in the cortex were due to the presence of AD pathology. Conclusion: Our findings suggest that Aβ42 immunization modifies the function of microglia by increasing their phagocytic activity and when plaques have been cleared, the level of phagocytosis is decreased below that seen in unimmunized AD.     

Early and rapid de novo synthesis of Alzheimer βA4-Amyloid precursor protein (APP) in activated microglia

Upon acute activation, microglia, the immuneffector cells of the brain parenchyma, express the amyloid precursor protein (APP) that is otherwise prominent in pathological structures related to Alzheimer's disease. In this disease complex amyloidbearing neuritic plaques contain βA4-amyloid protein, the APP, and numerous inflammatory proteins. The accompanying activation of microglia has mostly been viewed as a secondary reaction to amyloid deposits. Activation of microglia was performed in a graded fashion. Transection of peripheral nerves such as the facial or sciatic nerve causes a microglial reaction within hours in the nucleus of origin or in projection areas of the CNS. A predominantly glial up-regulation of APP mRNA and protein could be detected as early as 6 h post lesion not only at the site of affected neuronal cell bodies but also in corresponding projection areas. Its time course suggests rapid transneuronal signalling to glial cells in the projection area. Light and electron microscopy demonstrate that microglia, which are cells of mononuclear phagocyte lineage and comprise up to 20% of all glial cells, are the dominant source for non-neuronal APP expression. Ultrastructurally, brain perivascular cells within the basal lamina constitutively express APP and thus are a possible source of vascular amyloid. Additionally, microglia express leukocyte-derived (L)-APP mRNA and protein that have recently been described in mononuclear cells of the immune system. Increased L-APP expression may serve as a potential marker for glial/microglial activation. Such immune-mediated amyloidogenesis initiated by microglia might have implications for the treatment of neurodegenerative diseases. © 1993 Wiley-Liss, Inc.     

Microglia and cytokines in neurological disease,with special reference to AIDS and Alzheimer's disease

Microglia are associated with central nervous system (CNS) pathology of both Alzheimer's disease (AD) and the acquired immunodeficiency syndrome (AIDS). In AD, microglia, especially those associated with amyloid deposits, have a phenotype that is consistent with a state of activation, including immunoreactivity with antibodies to class II major histocompatibility antigens and to inflammatory cytokines (interleukin-1-β and tumor necrosis factor-α). Evidence from other studies in rodents indicate that microglia can be activated by neuronal degeneration. These results suggest that microglial activation in AD may be secondary to neurodegeneration and that, once activated, microglia may participate in a local inflammatory cascade that promotes tissue damage and contributes to amyloid formation. In AIDS, microglia are the primary target of retroviral infection. Both ramified and ameboid microglia, in addition to multinucleated giant cells, are infected by the human immunodeficiency virus (HIV-1). The mechanism of microglial infection is not known since microglia lack CD4, the HIV-1 receptor. Microglia display high affinity receptors for immunoglobulins, which makes antibody-mediated viral uptake a possible mechanism of infection. In AIDS, the extent of active viral infection and cytokine production may be critically dependent upon other factors, such as the presence of coinfecting agents. In the latter circumstance, very severe CNS pathology may emerge, including necrotizing lesions. In other circumstances, HIV infection microglia probably leads to CNS pathology by indirect mechanisms, including release of viral proteins (gp120) and toxic cytokines. Such a mechanism is the best hypothesis for the pathogenesis of vacuolar myelopathy in adults and the diffuse gliosis that characterizes pediatric AIDS, in which very little viral antigen can be detected.     

Plaque‐dependent morphological and electrophysiological heterogeneity of microglia in an Alzheimer's disease mouse model

Microglia, the central nervous system resident innate immune cells, cluster around Aβ plaques in Alzheimer's disease (AD). The activation phenotype of these plaque‐associated microglial cells, and their differences to microglia distant to Aβ plaques, are incompletely understood. We used novel three‐dimensional cell analysis software to comprehensively analyze the morphological properties of microglia in the TgCRND8 mouse model of AD in spatial relation to Aβ plaques. We found strong morphological changes exclusively in plaque‐associated microglia, whereas plaque‐distant microglia showed only minor changes. In addition, patch‐clamp recordings of microglia in acute cerebral slices of TgCRND8 mice revealed increased K⁺ currents in plaque‐associated but not plaque‐distant microglia. Within the subgroup of plaque‐associated microglia, two different current profiles were detected. One subset of cells displayed only increased inward currents, while a second subset showed both increased inward and outward currents, implicating that the plaque microenvironment differentially impacts microglial ion channel expression. Using pharmacological channel blockers, multiplex single‐cell PCR analysis and RNA fluorescence in situ hybridization, we identified Kir and Kv channel types contributing to the in‐ and outward K⁺ conductance in plaque‐associated microglia. In summary, we have identified a previously unrecognized level of morphological and electrophysiological heterogeneity of microglia in relation to amyloid plaques, suggesting that microglia may display multiple activation states in AD.     

Microglia‐targeted stem cell therapies for Alzheimer disease: A preclinical data review

Alzheimer disease (AD) is a severe, life‐threatening illness characterized by gradual memory loss. The classic histological features of AD include extracellular formation of β‐amyloid plaques (Aβ), intracellular neurofibrillary tangles (NFT), and synaptic loss. Recently, accumulated evidence has confirmed the critical role of microglia in the development and exacerbation of AD. When Aβ forms deposits, microglia quickly respond to restore brain physiology by activating a series of repair mechanisms. However, prolonged microglial activation is considered detrimental and may aggravate AD progression. To date, there are no curative therapies for AD. The advent of stem cell transplantation offers novel strategies to treat AD in animal models. Furthermore, studies have reported that transplanted stem cells might ameliorate AD symptoms by regulating microglial functions, from detrimental to protective. This review focuses on the crucial functions of microglia in AD and examines the reactions of microglia to transplanted stem cells.     

Influence of microglia and astrocyte activation in the neuroinflammatory pathogenesis of Alzheimer’s disease: Rational insights for the therapeutic approaches

Alzheimer's disease (AD), the most common progressive neurodegenerative disorder is characterized by the formation of extracellular amyloid plaques and intracellular neurofibrillary tangles (NFTs). Increasing evidences suggest a link between neuroinflammation and neuronal dysfunction in AD, orchestrated by the progressive activation of microglial cells and astrocytes with the consequent overproduction of proinflammatory molecules. The concomitant release of anti-inflammatory mediators antagonizes the inflammatory processes and leading to the severity of the AD pathology. The simultaneous detection of these inflammatory molecules in the pre-symptomatic stage may help in the early diagnosis of the AD. We have discussed the impact of microglia and astrocytic cells, the principal agents in the neuroinflammation process, in relation to the progression of the AD. Modulation of the risk factors and targeting of these immune mechanisms could lead to better therapeutic or preventive strategies for the AD. Further studies need to determine, how the inhibition of inflammatory factors could be used for the AD alternative therapies.     

Apolipoproteins E and J interfere with amyloid‐beta uptake by primary human astrocytes and microglia in vitro

Defective clearance of the amyloid‐β peptide (Aβ) from the brain is considered a strong promoter in Alzheimer's disease (AD) pathogenesis. Astrocytes and microglia are important mediators of Aβ clearance and Aβ aggregation state and the presence of amyloid associated proteins (AAPs), such as Apolipoproteins E and J (ApoE and ApoJ), may influence Aβ clearance by these cells. Here we set out to investigate whether astrocytes and microglia differ in uptake efficiency of Aβ oligomers (Aβ_oligo) and Aβ fibrils (Aβ_fib), and whether the Aβ aggregation state and/or presence of AAPs affect Aβ uptake in these cells in vitro. Adult human primary microglia and astrocytes, isolated from short delay post‐mortem brain tissue, were exposed to either Aβ_oligo or Aβ_fib alone or combined with a panel of certain AAPs whereafter Aβ‐positive cells were quantified using flow cytometry. Upon exposure to Aβ combined with ApoE, ApoJ, α1‐antichymotrypsin (ACT) and a combination of serum amyloid P and complement C1q (SAP‐C1q), a clear reduction in astrocytic but not microglial Aβ_oligo uptake, was observed. In contrast, Aβ_fib uptake was strongly reduced in the presence of AAPs in microglia, but not in astrocytes. These data provide the first evidence of distinct roles of microglia and astrocytes in Aβ clearance. More importantly we show that Aβ clearance by glial cells is negatively affected by AAPs like ApoE and ApoJ. Thus, targeting the association of Aβ with AAPs, such as ApoE and ApoJ, could serve as a therapeutic strategy to increase Aβ clearance by glial cells. GLIA 2014;62:493–503     

Dystrophic microglia in late-onset Alzheimer's disease

Here, we summarize current understanding of functional involvement of microglial cells in the most common neurodegenerative disease to affect humans, which is sporadic or late-onset Alzheimer's disease (LOAD). Our review narrowly focuses on insights obtained from post-mortem neuropathological examinations of human brains paying particular attention to microglia as these cells have long been implicated as pivotal players in the cellular processes that lead to AD-type neurodegeneration. Although complete understanding of the roles played by microglia in AD neurodegeneration remains elusive, our studies thus far have illuminated microglial involvement in LOAD, showing that microglial dystrophy, the morphological manifestation of senescence, can be integrated with other hallmark pathological features of AD, such as intraneuronal neurofibrillary degeneration (NFD) and extracellular deposits of amyloid-beta (Aβ) protein. We have demonstrated an in situ correlation between microglial dystrophy and presence of NFD suggesting that neurodegeneration is secondary to aging-related microglial deterioration, a concept founded on the notion that proper neuronal function is dependent on presence of healthy microglia. Diseased or weakened glia are detrimental for neuronal well-being because their ability to provide neuronal support may be impaired. Our most recent work also links microglial dystrophy with Aβ deposits by showing that there is a chronic, yet futile microglial reaction to insoluble amyloid deposits. This inability of microglia to remove aggregated amyloid (a foreign body) causes microglial exhaustion and thereby exacerbates already ongoing aging-dependent microglial deterioration. An eventual total loss of functional microglia in advanced LOAD promotes widespread NFD, dementia, and brain failure.     

Expression of glucose transporter 5 by microglia in human gliomas

Our previous studies indicate that glucose transporter 5 (GLUT5) is a microglial marker in routine paraffin sections, and is rarely present in monocytes/macrophages of the peripheral organs. We examined the expression of GLUT5 in 91 cases of human gliomas to characterize the microglial phenotype in glioma tissues. Immunohistochemistry was performed on formalin-fixed, paraffin-embedded sections using such antibodies as a GLUT5 antibody, two markers for activated microglia: major histocompatibility complex (MHC) class II Ag and macrophage scavenger receptor class A (MSR-A), and MIB-1 antibody. The immunoreactivity of GLUT5 was present in three microglial phenotypes: ramified (resting), activated, and ameboid (macrophagic) microglia in most of the cases. A double-labelling study of astrocytic tumours using GLUT5 and MIB-1 antibodies demonstrated a proportion of proliferating microglia. However, no morphological difference between MIB-1-positive, microglial cells and MIB-1-negative, microglial cells was found. The number of GLUT5-positive microglia was significantly (P < 0.001) higher in astrocytic tumours than in oligodendroglial tumours. Many GLUT5-positive microglia (up to 52% in total cells) were often observed in pilocytic astrocytomas, where microglial cells were predominantly ramified, and the number of MHC class II- or MSR-A-positive microglia was less than GLUT5-positive microglia. Thus, the present study indicated that intrinsic microglia can be a source of microglia/macrophages cell populations in astrocytic tumours, and that pilocytic astrocytomas often have a high proportion of microglial cells with mild activation.     

Time-dependent reduction in Abeta levels after intracranial LPS administration in APP transgenic mice

Inflammation has been argued to play a primary role in the pathogenesis of Alzheimer's disease (AD). Lipopolysaccharide (LPS) activates the innate immune system, triggering gliosis and inflammation when injected in the central nervous system. In studies described here, APP transgenic mice were injected intrahippocampally with 4 or 10 microg of LPS and evaluated 1, 3, 7, 14, or 28 days later. Abeta load was significantly reduced at 3, 7, and 14 days but surprisingly returned near baseline 28 days after the injection. No effects of LPS on congophilic amyloid deposits could be detected. LPS also activated both microglia and astrocytes in a time-dependent manner. The GFAP astrocyte reaction and the Fcgamma receptor microglial reaction peaked at 7 days after LPS injection, returning to baseline by 2 weeks postinjection. When stained for CD45, microglial activation was detected at all time points, although the morphology of these cells transitioned from an ameboid to a ramified and bushy appearance between 7 and 14 days postinjection. These results indicate that activation of brain glia can rapidly and transiently clear diffuse Abeta deposits but has no effect on compacted fibrillar amyloid.     

Minocycline affects microglia activation, Abeta deposition, and behavior in APP-tg mice

Activated microglia and reactive astrocytes invade and surround cerebral beta amyloid (Abeta) plaques in Alzheimer's disease (AD), but the role of microglia in plaque development is still unclear. In this study, minocycline was administered for 3 months, prior to and early in Abeta plaque formation in amyloid precursor protein transgenic mice (APP-tg). When minocycline was given to younger mice, there was a small but significant increase in Abeta deposition in the hippocampus, concurrent with improved cognitive performance relative to vehicle treated mice. If APP-tg mice received minocycline after Abeta deposition had begun, microglial activation was suppressed but this did not affect Abeta deposition or improve cognitive performance. In vitro studies demonstrated that minocycline suppressed microglial production of IL-1beta, IL-6, TNF, and NGF. Thus, minocycline has different effects on Abeta plaque deposition and microglia activation depending on the age of administration. Our data suggest that this may be due to the effects of minocycline on microglial function. Therefore, anti-inflammatory therapies to suppress microglial activation or function may reduce cytokine production but enhance Abeta plaque formation early in AD.     

Viral-induced inflammation is accompanied by beta-amyloid plaque reduction in brains of amyloid precursor protein transgenic Tg2576 mice

Amyloid plaques, one of the neuropathological hallmarks of Alzheimer's disease, and their main constituent, the amyloid beta-peptide (Abeta), are triggers of the activation of innate inflammatory mechanisms involving the activation of microglia. To dissect the effects of a non-Abeta-specific microglial activation on the Abeta metabolism, we employed a viral infection-based model. Transgenic mice expressing a mutated form of the human amyloid precursor protein (Tg2576) were used. In preceding experiments, 2-week-old transgenic mice and non-transgenic littermates were infected intracerebrally with the neurotropic Borna disease virus and investigated at 2, 4 and 14 weeks post-infection. The Borna disease virus-inoculated mice showed a persisting, subclinical infection of cortical and limbic brain areas characterized by slight T-cell infiltrates, expression of cytokines and a massive microglial activation in the hippocampus and neocortex. Viral-induced effects reached their peak at 4 weeks post-infection. In 14-month-old Tg2576 mice, characterized by the deposition of diffuse and dense-core amyloid plaques in cortical brain regions, Borna disease virus-induced microglial activation in the vicinity of Abeta deposits was used to investigate the influence of a local inflammatory response on these deposits. At 4 weeks post-infection, histometric analyses employing Abeta immunohistochemistry revealed a decrease of the cortical and hippocampal Abeta-immunopositive area. This overall decrease was accompanied by a decrease of parenchymal thioflavin-S-positive amyloid deposits and an increase of such deposits in the walls of cerebral vessels, which indicates that the elicitation of a non-Abeta-specific microglial activation may contribute to a reduction of Abeta in the brain parenchyma.     

Cytokine production by a human microglial cell line: Effects of ßamyloid and α-melanocyte-stimulating hormone

Senile plaques in the Alzheimer's disease (AD) are formed by aggregation of beta-amyloid (Abeta) peptide. Abeta peptide has been shown to activate microglia and stimulate their production of inflammatory factors, such as cytokines. In the AD brain, the continued presence of amyloid plaques may keep microglia persistently activated, leading to chronic inflammation in the CNS. It is well established that alpha-melanocyte-stimulating hormone (alpha-MSH) gives rise to anti-inflammatory and anti-pyretic effects. The biological activities of alpha-MSH are mediated by one or more of the melanocortin receptor (MCR) subtypes, i.e. MCR1 - MCR5. The aim of the present study was to determine the effect of alpha-MSH alone and on Abeta-activated microglial cells with regard to the secretion of inflammatory cytokines, such as interleukin-6 (IL-6), and to determine which receptor subtype mediates the effects of alpha-MSH. The human microglial cell line, CHME3, was incubated for 24 h with freshly dissolved Abeta(1-40), interferon-gamma (IFN-gamma) and/or alpha-MSH. Freshly dissolved Abeta(1-40) (5-60 microM) resulted in a dose-dependent decrease in cell viability, along with a dose-dependent increase in IL-6 release. Neither IFN-gamma nor alpha-MSH affected the Abeta-induced secretion of IL-6, but resulted in a dose-dependent increase in basal IL-6 release. Agouti, the endogenous antagonist of MCR1 and 4, further increased the alpha-MSH-induced secretion of IL-6. RT-PCR showed the expression of MCR1, MCR3, MCR4 and MCR5 mRNA. The combined data suggest that the effect of alpha-MSH in increasing IL-6 release from the human microglial cell line is mediated by MCR3 or MCR5.     

Transgenic AD model mice, effects of potential anti-AD treatments on inflammation, and pathology

The deposition of amyloid-β (Aβ) peptides in plaques and intracellular neurofibrillary tangles are the two main characteristic pathological features of Alzheimer's disease (AD). Significantly, plaques are surrounded by activated astrocytes, microglia, and possibly, macrophages, and it has been suggested that this activity contributes to the pathology. Whether this will lead to a decrease or an increase in the amount of Aβ deposition is not clear. To investigate the relation between amyloid neuropathology and inflammation, we examined the changes in amyloid pathology in the hippocampus and neocortex following three anti-inflammatory treatments aimed at reducing the amyloid burden. In these studies we treated mice with different non-steroidal anti-inflammatory drugs for several months (i.e., from 8 through 14 months of age), and studied the Aβ pathology and inflammation in the brain. Sham treatment and flurbiprofen treatment did not affect Aβ pathology, and a low dose HCT 1026 (10 mg/kg; a nitric oxide-donating flurbiprofen analog that has additional useful properties, including a remarkable gastrointestinal safety) did not affect pathology either, however a higher dose of HCT 1026 (30 mg/kg) did reduce the Aβ load. Furthermore, this treatment reduced the amount of microglial activation surrounding plaques. In contrast, the low dose of HCT 1026 increased GFAP activation, but did not change microglial activation. Together the data indicate that changing the activity of glial cells can lead to both a decrease of the amyloid burden, and to detrimental changes, likely caused by the interplay between the activation levels of astrocytes and microglial cells.     

Ion channels on microglia: therapeutic targets for neuroprotection

Under pathological conditions microglia (resident CNS immune cells) become activated, and produce reactive oxygen and nitrogen species and pro-inflammatory cytokines: molecules that can contribute to axon demyelination and neuron death. Because some microglia functions can exacerbate CNS disorders, including stroke, traumatic brain injury, progressive neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, and multiple sclerosis, and several retinal diseases, controlling their activation might ameliorate immune-mediated CNS disorders. A growing body of evidence now points to ion channels on microglia as contributing to the above neuropathologies. For example, the ATP-gated P2X7 purinergic receptor cation channel is up-regulated around amyloid β-peptide plaques in transgenic mouse models of Alzheimer's disease and co-localizes to microglia and astrocytes. Upregulation of the P2X7 receptor subtype on microglia occurs also following spinal cord injury and after ischemia in the cerebral cortex of rats, while P2X7 receptor-like immunoreactivity is increased in activated microglial cells of multiple sclerosis and amyotrophic lateral sclerosis spinal cord. Utilizing neuron/microglia co-cultures as an in vitro model for neuroinflammation, P2X7 receptor activation on microglia appears necessary for microglial cell-mediated injury of neurons. A second example can be found in the chloride intracellular channel 1 (CLIC1), whose expression is related to macrophage activation, undergoes translocation from the cytosol to the plasma membrane (activation) of microglia exposed to amyloid β-peptide, and participates in amyloid β-peptide-induced neurotoxicity through the generation of reactive oxygen species. A final example is the small-conductance Ca2+/calmodulin-activated K+ channel KCNN4/KCa3.1/SK4/IK1, which is highly expressed in rat microglia. Lipopolysaccharide-activated microglia are capable of killing adjacent neurons in co-culture, and show markedly reduced toxicity when treated with an inhibitor of KCa3.1 channels. Moreover, blocking KCa3.1 channels mitigated the neurotoxicity of amyloid β-peptide-stimulated microglia. Excessive microglial cell activation and production of potentially neurotoxic molecules, mediated by ion channels, may thus constitute viable targets for the discovery and development of neurodegenerative disease therapeutics. This chapter will review recent data that reflect the prevailing approaches targeting neuroinflammation as a pathophysiological process contributing to the onset or progression of neurodegenerative diseases, with a focus on microglial ion channels and their neuroprotective potential.