Slow degradation of aggregates of the Alzheimer's disease amyloid beta-protein by microglial cells

Evaluation of oxidative stress in the brain of a transgenic mouse model of Alzheimer disease by in vivo electron paramagnetic resonance imaging

Alzheimer disease (AD) is a neurodegenerative disease clinically characterized by progressive cognitive dysfunction. Deposition of amyloid-β (Aβ) peptides is the most important pathophysiological hallmark of AD. Oxidative stress induced by reactive oxygen species is prominent in AD, and several reports suggest the relationship between a change in redox status and AD pathology containing progressive Aβ deposition, the activation of glial cells, and mitochondrial dysfunction. Therefore, we performed immunohistochemical analysis using a transgenic mouse model of AD (APdE9) and evaluated the activity of superoxide dismutase in brain tissue homogenates of APdE9 mice in vitro. Together with those analyses, in vivo changes in redox status with age in both wild-type (WT) and APdE9 mouse brains were measured noninvasively by three-dimensional electron paramagnetic resonance (EPR) imaging using nitroxide (3-methoxycarbonyl-2,2,5,5-tetramethylpyrrolidine-1-yloxy) as a redox-sensitive probe. Both methods found similar changes in redox status with age, and in particular a significant change in redox status in the hippocampus was observed noninvasively by EPR imaging between APdE9 mice and age-matched WT mice from 9 to 18 months of age. EPR imaging clearly visualized the accelerated change in redox status of APdE9 mouse brain compared with WT. The evaluation of the redox status in the brain of AD model rodents by EPR imaging should be useful for diagnostic study of AD.

Microglial cell dysregulation in brain aging and neurodegeneration

Aging is the main risk factor for neurodegenerative diseases. In aging, microglia undergoes phenotypic changes compatible with their activation. Glial activation can lead to neuroinflammation, which is increasingly accepted as part of the pathogenesis of neurodegenerative diseases, including Alzheimer’s disease (AD). We hypothesize that in aging, aberrant microglia activation leads to a deleterious environment and neurodegeneration. In aged mice, microglia exhibit an increased expression of cytokines and an exacerbated inflammatory response to pathological changes. Whereas LPS increases nitric oxide (NO) secretion in microglia from young mice, induction of reactive oxygen species (ROS) predominates in older mice. Furthermore, there is accumulation of DNA oxidative damage in mitochondria of microglia during aging, and also an increased intracellular ROS production. Increased ROS activates the redox-sensitive nuclear factor kappa B, which promotes more neuroinflammation, and can be translated in functional deficits, such as cognitive impairment. Mitochondria-derived ROS and cathepsin B, are also necessary for the microglial cell production of interleukin-1β, a key inflammatory cytokine. Interestingly, whereas the regulatory cytokine TGFβ1 is also increased in the aged brain, neuroinflammation persists. Assessing this apparent contradiction, we have reported that TGFβ1 induction and activation of Smad3 signaling after inflammatory stimulation are reduced in adult mice. Other protective functions, such as phagocytosis, although observed in aged animals, become not inducible by inflammatory stimuli and TGFβ1. Here, we discuss data suggesting that mitochondrial and endolysosomal dysfunction could at least partially mediate age-associated microglial cell changes, and, together with the impairment of the TGFβ1-Smad3 pathway, could result in the reduction of protective activation and the facilitation of cytotoxic activation of microglia, resulting in the promotion of neurodegenerative diseases.

Prophylactic liraglutide treatment prevents amyloid plaque deposition, chronic inflammation and memory impairment in APP/PS1 mice

Type 2 diabetes is a risk factor for Alzheimer's disease (AD). Previously, we have shown that the diabetes drug liraglutide is protective in middle aged and in old APP/PS1 mice. Here, we show that liraglutide has prophylactic properties. When injecting liraglutide once-daily ip. in two months old mice for 8 months, the main hallmarks of AD were much reduced. Memory formation in object recognition and Morris water maze were normalised and synapse loss and the loss of synaptic plasticity was prevented. In addition, amyloid plaque load, including dense core congophilic plaques, was much reduced. Chronic inflammation (activated microglia) was also reduced in the cortex, and neurogenesis was enhanced in the dentate gyrus. The results demonstrate that liraglutide may protect from progressive neurodegeneration that develops in AD. The drug is currently in clinical trials in patients with AD.

The Distinct Role of ADAM17 in APP Proteolysis and Microglial Activation Related to Alzheimer's Disease

Alzheimer's disease (AD) is a progressive neurodegenerative disease with the symptom of cognitive impairment. The deposition of amyloid β (Aβ) peptide is believed to be the primary cause to neuronal dystrophy and eventually dementia. Aβ is the proteolytic product from its precursor amyloid precursor protein (APP) by β- and γ- secretase. An optional cleavage by α-secretase happens inside the Aβ domain. ADAM17 is supposed to be the regulated α-secretase of APP. Enhanced activity of ADAM17 leads to the increasing secretion of neuroprotective soluble APP α fragment and reduction of Aβ generation, which may be benefit to the disease. ADAM17 is then considered the potential therapeutic target for AD. Microglia activation and neuroinflammation is another important event in AD pathogenesis. Interestingly, ADAM17 also participates in the cleavage of many other membrane-bound proteins, especially some inflammatory factors related to microglia activation. The facilitating role of ADAM17 in inflammation and further neuronal damage has also been illustrated. In results, the activation of ADAM17 as the solution to AD may be a tricky task. The comprehensive consideration and evaluation has to be carried out carefully before the final treatment. In the present review, the distinct role of ADAM17 in AD-related APP shedding and neuroinflammatory microglial activation will be carefully discussed.

In Vitro Studies on Accelerating the Degradation and Clearance of Amyloid-β Fibrils by an Antiamyloidogenic Peptide

The clearance of overloaded amyloid-β (Aβ) species, especially the toxic aggregates, was thought to be an attractive and promising strategy for Alzheimer's disease (AD) therapy in the past decade. In this work, an active Aβ inhibitor decapeptide RR was used to transform mature Aβ fibrils (fAβ) into nanorod-like Aβ assemblies (rAβ) as well as loosen the β-structure of rAβ. Compared with fAβ, rAβ could be engulfed by PC12 cells more efficiently and showed a 1.46-fold difference. More importantly, the rAβ was colocated with lysosomes after endocytosis, and in vitro study illustrated that rAβ were easily degraded by lysosome protease cathepsin B when compared with the fibrils. Thus, our study indicated the potential application of RR in Aβ fibrils clearance by a cell-participated and enzyme-mediated pathway.

Microglia display modest phagocytic capacity for extracellular tau oligomers

Background

Abnormal misfolded tau protein is a driving force of neurofibrillary degeneration in Alzheimer’s disease. It has been shown that tau oligomers play a crucial role in the formation of intracellular neurofibrillary tangles. They are intermediates between soluble tau monomers and insoluble tau filaments and are suspected contributors to disease pathogenesis. Oligomeric tau can be released into the extracellular space and spread throughout the brain. This finding opens the question of whether brain macrophages or blood monocytes have the potential to phagocytose extracellular oligomeric tau.

Methods

We have used stable rat primary microglial cells, rat peripheral monocytes-derived macrophages, BV2 microglial and TIB67 macrophage immortalized cell lines that were challenged by tau oligomers prepared by an in vitro aggregation reaction. The efficiency of cells to phagocytose oligomeric protein was evaluated with confocal microscopy. The ability to degrade tau protein was analyzed by immunoblotting.

Results

Confocal microscopy analyses showed that macrophages were significantly more efficient in phagocytosing oligomerized tau proteins than microglial cells. In contrast to macrophages, microglia are able to degrade the internalized oligomeric tau only after stimulation with lipopolysaccharide (LPS).

Conclusions

Our data suggests that microglia may not be the principal phagocytic cells able to target extracellular oligomeric tau. We found that peripheral macrophages display a high potency for elimination of oligomeric tau and therefore could play an important role in the modulation of neurofibrillary pathology in Alzheimer’s disease.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-014-0161-z) contains supplementary material, which is available to authorized users.

Silver and gold nanoparticles exposure to in vitro cultured retina--studies on nanoparticle internalization, apoptosis, oxidative stress, glial- and microglial activity

The complex network of neuronal cells in the retina makes it a potential target of neuronal toxicity – a risk factor for visual loss. With growing use of nanoparticles (NPs) in commercial and medical applications, including ophthalmology, there is a need for reliable models for early prediction of NP toxicity in the eye and retina. Metal NPs, such as gold and silver, gain much of attention in the ophthalmology community due to their potential to cross the barriers of the eye. Here, NP uptake and signs of toxicity were investigated after exposure to 20 and 80 nm Ag- and AuNPs, using an in vitro tissue culture model of the mouse retina. The model offers long-term preservation of retinal cell types, numbers and morphology and is a controlled system for delivery of NPs, using serum-free defined culture medium. AgNO₃-treatment was used as control for toxicity caused by silver ions. These end-points were studied; gross morphological organization, glial activity, microglial activity, level of apoptosis and oxidative stress, which are all well described as signs of insult to neural tissue. TEM analysis demonstrated cellular- and nuclear uptake of all NP types in all neuronal layers of the retina. Htx-eosin staining showed morphological disruption of the normal complex layered retinal structure, vacuole formation and pyknotic cells after exposure to all Ag- and AuNPs. Significantly higher numbers of apoptotic cells as well as an increased number of oxidative stressed cells demonstrated NP-related neuronal toxicity. NPs also caused increased glial staining and microglial cell activation, typical hallmarks of neural tissue insult. This study demonstrates that low concentrations of 20 and 80 nm sized Ag- and AuNPs have adverse effects on the retina, using an organotypic retina culture model. Our results motivate careful assessment of candidate NP, metallic or-non-metallic, to be used in neural systems for therapeutic approaches.

Membrane pathology and microglial activation of mice expressing membrane anchored or membrane released forms of Aβ and mutated human Alzheimer's precursor protein (APP)

AIMS

Alzheimer's disease and the transmissible spongiform encephalopathies or prion diseases accumulate misfolded and aggregated forms of neuronal cell membrane proteins. Distinctive membrane lesions caused by the accumulation of disease-associated prion protein (PrP(d)) are found in prion disease but morphological changes of membranes are not associated with Aβ in Alzheimer's disease. Membrane changes occur in all prion diseases where PrP(d) is attached to cell membranes by a glycosyl-phosphoinositol (GPI) anchor but are absent from transgenic mice expressing anchorless PrP(d). Here we investigate whether GPI membrane attached Aβ may also cause prion-like membrane lesions.

METHODS

We used immunogold electron microscopy to determine the localization and pathology of Aβ accumulation in groups of transgenic mice expressing anchored or unanchored forms of Aβ or mutated human Alzheimer's precursor protein.

RESULTS

GPI attached Aβ did not replicate the membrane lesions of PrP(d). However, as with PrP(d) in prion disease, Aβ peptides derived from each transgenic mouse line initially accumulated on morphologically normal neurite membranes, elicited rapid glial recognition and neurite Aβ was transferred to attenuated microglial and astrocytic processes.

CONCLUSIONS

GPI attachment of misfolded membrane proteins is insufficient to cause prion-like membrane lesions. Prion disease and murine Aβ amyloidosis both accumulate misfolded monomeric or oligomeric membrane proteins that are recognized by glial processes and acquire such misfolded proteins prior to their accumulation in the extracellular space. In contrast to prion disease where glial cells efficiently endocytose PrP(d) to endolysosomes, activated microglial cells in murine Aβ amyloidosis are not as efficient phagocytes.

Lixisenatide, a drug developed to treat type 2 diabetes, shows neuroprotective effects in a mouse model of Alzheimer's disease

Type 2 diabetes is a risk factor for developing Alzheimer's disease (AD). In the brains of AD patients, insulin signalling is desensitised. The incretin hormone Glucagon-like peptide-1 (GLP-1) facilitates insulin signalling, and analogues such as liraglutide are on the market as treatments for type 2 diabetes. We have previously shown that liraglutide showed neuroprotective effects in the APPswe/PS1ΔE9 mouse model of AD. Here, we test the GLP-1 receptor agonist lixisenatide in the same mouse model and compare the effects to liraglutide. After ten weeks of daily i.p. injections with liraglutide (2.5 or 25 nmol/kg) or lixisenatide (1 or 10 nmol/kg) or saline of APP/PS1 mice at an age when amyloid plaques had already formed, performance in an object recognition task was improved in APP/PS1 mice by both drugs at all doses tested. When analysing synaptic plasticity in the hippocampus, LTP was strongly increased in APP/PS1 mice by either drug. Lixisenatide (1 nmol/kg) was most effective. The reduction of synapse numbers seen in APP/PS1 mice was prevented by the drugs. The amyloid plaque load and dense-core Congo red positive plaque load in the cortex was reduced by both drugs at all doses. The chronic inflammation response (microglial activation) was also reduced by all treatments. The results demonstrate that the GLP-1 receptor agonists liraglutide and lixisenatide which are on the market as treatments for type 2 diabetes show promise as potential drug treatments of AD. Lixisenatide was equally effective at a lower dose compared to liraglutide in some of the parameters measured.

Role of microglia adenosine A(2A) receptors in retinal and brain neurodegenerative diseases

Neuroinflammation mediated by microglial cells in the brain has been commonly associated with neurodegenerative diseases. Whether this microglia-mediated neuroinflammation is cause or consequence of neurodegeneration is still a matter of controversy. However, it is unequivocal that chronic neuroinflammation plays a role in disease progression and halting that process represents a potential therapeutic strategy. The neuromodulator adenosine emerges as a promising targeting candidate based on its ability to regulate microglial proliferation, chemotaxis, and reactivity through the activation of its G protein coupled A2A receptor (A2AR). This is in striking agreement with the ability of A2AR blockade to control several brain diseases. Retinal degenerative diseases have been also associated with microglia-mediated neuroinflammation, but the role of A2AR has been scarcely explored. This review aims to compare inflammatory features of Parkinson's and Alzheimer's diseases with glaucoma and diabetic retinopathy, discussing the therapeutic potential of A2AR in these degenerative conditions.

Phagocytosis of microglia in the central nervous system diseases

Microglia, the resident macrophages of the central nervous system, rapidly activate in nearly all kinds of neurological diseases. These activated microglia become highly motile, secreting inflammatory cytokines, migrating to the lesion area, and phagocytosing cell debris or damaged neurons. During the past decades, the secretory property and chemotaxis of microglia have been well-studied, while relatively less attention has been paid to microglial phagocytosis. So far there is no obvious concordance with whether it is beneficial or detrimental in tissue repair. This review focuses on phagocytic phenotype of microglia in neurological diseases such as Alzheimer's disease, multiple sclerosis, Parkinson's disease, traumatic brain injury, ischemic and other brain diseases. Microglial morphological characteristics, involved receptors and signaling pathways, distribution variation along with time and space changes, and environmental factors that affecting phagocytic function in each disease are reviewed. Moreover, a comparison of contributions between macrophages from peripheral circulation and the resident microglia to these pathogenic processes will also be discussed.

Environmental factors in the development and progression of late-onset Alzheimer's disease

Late-onset Alzheimer's disease (LOAD) is an age-related neurodegenerative disorder characterized by gradual loss of synapses and neurons, but its pathogenesis remains to be clarified. Neurons live in an environment constituted by neurons themselves and glial cells. In this review, we propose that the neuronal degeneration in the AD brain is partially caused by diverse environmental factors. We first discuss various environmental stresses and the corresponding responses at different levels. Then we propose some mechanisms underlying the specific pathological changes, in particular, hypothalamic-pituitary adrenal axis dysfunction at the systemic level; cerebrovascular dysfunction, metal toxicity, glial activation, and Aβ toxicity at the intercellular level; and kinase-phosphatase imbalance and epigenetic modification at the intracellular level. Finally, we discuss the possibility of developing new strategies for the prevention and treatment of LOAD from the perspective of environmental stress. We conclude that environmental factors play a significant role in the development of LOAD through multiple pathological mechanisms.

Innate immunity in Alzheimer's disease: a complex affair

Alzheimer's disease (AD) is characterized by three major histopathological hallmarks: β-amyloid deposits, neurofibrillary tangles and gliosis. While neglected for decades, the neuroinflammatory processes coordinated by microglia are now accepted as etiologic events in AD evolution. Microglial cells are found in close vicinity to amyloid plaques and display various activation phenotypes determined by the expression of a wide range of cytokines, chemokines, and innate immune surface receptors. During the development of AD pathology, microglia fail to restrict amyloid plaques and may contribute to neurotoxicity and cognitive deficit. Nevertheless, under specific activation states, microglia can participate in cerebral amyloid clearance. This review focuses on the complex relationship between microglia and Aβ pathology, and highlights both deleterious and beneficial roles of microglial activation states in the context of AD. A deeper understanding of microglial biology will hopefully pave the way for next-generation AD therapeutic approaches aimed at harnessing these enigmatic innate immune cells of the central nervous system.

Microglia convert aggregated amyloid-β into neurotoxic forms through the shedding of microvesicles

Alzheimer's disease (AD) is characterized by extracellular amyloid-β (Aβ) deposition, which activates microglia, induces neuroinflammation and drives neurodegeneration. Recent evidence indicates that soluble pre-fibrillar Aβ species, rather than insoluble fibrils, are the most toxic forms of Aβ. Preventing soluble Aβ formation represents, therefore, a major goal in AD. We investigated whether microvesicles (MVs) released extracellularly by reactive microglia may contribute to AD degeneration. We found that production of myeloid MVs, likely of microglial origin, is strikingly high in AD patients and in subjects with mild cognitive impairment and that AD MVs are toxic for cultured neurons. The mechanism responsible for MV neurotoxicity was defined in vitro using MVs produced by primary microglia. We demonstrated that neurotoxicity of MVs results from (i) the capability of MV lipids to promote formation of soluble Aβ species from extracellular insoluble aggregates and (ii) from the presence of neurotoxic Aβ forms trafficked to MVs after Aβ internalization into microglia. MV neurotoxicity was neutralized by the Aβ-interacting protein PrP and anti-Aβ antibodies, which prevented binding to neurons of neurotoxic soluble Aβ species. This study identifies microglia-derived MVs as a novel mechanism by which microglia participate in AD degeneration, and suggest new therapeutic strategies for the treatment of the disease.

Liraglutide can reverse memory impairment, synaptic loss and reduce plaque load in aged APP/PS1 mice, a model of Alzheimer's disease

Type 2 diabetes is a risk factor in the development of Alzheimer's disease (AD). It has been shown that insulin signalling is desensitised in the brains of AD patients. The incretin hormone Glucagon-like peptide-1 (GLP-1) facilitates insulin signalling, and long-lasting analogues such as liraglutide (Victoza(®)) are on the market as type 2 diabetes treatments. We have previously shown that liraglutide improved cognitive function, reduced amyloid plaque deposition, inflammation, overall APP and oligomer levels and enhanced LTP when injected peripherally for two months in 7 month old APPswe/PS1ΔE9 (APP/PS1) mice. This showed that liraglutide has preventive effects at the early stage of AD development. The current study investigated whether Liraglutide would have restorative effects in late-stage Alzheimer's disease in mice. Accordingly, 14-month-old APP/PS1 and littermate control mice were injected with Liraglutide (25 nmol/kg bw) ip. for 2 months. Spatial memory was improved by Liraglutide-treatment in APP/PS1 mice compared with APP/PS1 saline-treated mice. Overall plaque load was reduced by 33%, and inflammation reduced by 30%, while neuronal progenitor cell count in the dentate gyrus was increased by 50%. LTP was significantly enhanced in APP/PS1 liraglutide-treated mice compared with APP/PS1 saline mice, corroborated with increased synapse numbers in hippocampus and cortex. Total brain APP and beta-amyloid oligomer levels were reduced in Liraglutide-treated APP/PS1 mice while IDE levels were increased. These results demonstrate that Liraglutide not only has preventive properties, but also can reverse some of the key pathological hallmarks of AD. Liraglutide is now being tested in clinical trials in AD patients. This article is part of the Special Issue entitled 'The Synaptic Basis of Neurodegenerative Disorders'.

Inflammatory components in human Alzheimer's disease and after active amyloid-β42 immunization

Inflammatory processes are important in the pathogenesis of Alzheimer's disease and in response to amyloid-β immunotherapy. We investigated the expression of multiple inflammatory markers in the brains of 28 non-immunized patients with Alzheimer's disease and 11 patients with Alzheimer's disease immunized against amyloid-β42 (AN1792): microglial ionized calcium-binding adaptor Iba-1, lysosome marker CD68, macrophage scavenger receptor A, Fcγ receptors I (CD64) and II (CD32); and also immunoglobulin IgG, complement C1q and the T lymphocyte marker CD3 using immunohistochemistry. The data were analysed with regard to amyloid-β and phospho-tau pathology, severity of cerebral amyloid angiopathy and cortical microhaemorrhages. In non-immunized Alzheimer's disease cases, amyloid-β42 correlated inversely with CD32 and Iba-1, whereas phospho-tau correlated directly with all microglial markers, IgG, C1q and the number of T cells. In immunized Alzheimer's disease cases, amyloid-β42 load correlated directly with macrophage scavenger receptor A-positive clusters and inversely with C1q. The severity of cerebral amyloid angiopathy and microhaemorrhages did not relate to any of the analysed markers. Overall, the levels of CD68, macrophage scavenger receptor A, CD64, CD32 and the number of macrophage scavenger receptor A-positive plaque-related clusters were significantly lower in immunized than non-immunized cases, although there was no significant difference in Iba-1 load, number of Iba-1-positive cells, IgG load, C1q load or number of T cells. Our findings indicate that different microglial populations co-exist in the Alzheimer's disease brain, and that the local inflammatory status within the grey matter is importantly linked with tau pathology. After amyloid-β immunization, the microglial functional state is altered in association with reduced amyloid-β and tau pathology. The results suggest that, in the long term, amyloid-β immunotherapy results in downregulation of microglial activation and potentially reduces the inflammation-mediated component of the neurodegeneration of Alzheimer's disease.

Decoding Alzheimer's disease from perturbed cerebral glucose metabolism: implications for diagnostic and therapeutic strategies

Alzheimer's disease (AD) is an age-related devastating neurodegenerative disorder, which severely impacts on the global economic development and healthcare system. Though AD has been studied for more than 100 years since 1906, the exact cause(s) and pathogenic mechanism(s) remain to be clarified. Also, the efficient disease-modifying treatment and ideal diagnostic method for AD are unavailable. Perturbed cerebral glucose metabolism, an invariant pathophysiological feature of AD, may be a critical contributor to the pathogenesis of this disease. In this review, we firstly discussed the features of cerebral glucose metabolism in physiological and pathological conditions. Then, we further reviewed the contribution of glucose transportation abnormality and intracellular glucose catabolism dysfunction in AD pathophysiology, and proposed a hypothesis that multiple pathogenic cascades induced by impaired cerebral glucose metabolism could result in neuronal degeneration and consequently cognitive deficits in AD patients. Among these pathogenic processes, altered functional status of thiamine metabolism and brain insulin resistance are highly emphasized and characterized as major pathogenic mechanisms. Finally, considering the fact that AD patients exhibit cerebral glucose hypometabolism possibly due to impairments of insulin signaling and altered thiamine metabolism, we also discuss some potential possibilities to uncover diagnostic biomarkers for AD from abnormal glucose metabolism and to develop drugs targeting at repairing insulin signaling impairment and correcting thiamine metabolism abnormality. We conclude that glucose metabolism abnormality plays a critical role in AD pathophysiological alterations through the induction of multiple pathogenic factors such as oxidative stress, mitochondrial dysfunction, and so forth. To clarify the causes, pathogeneses and consequences of cerebral hypometabolism in AD will help break the bottleneck of current AD study in finding ideal diagnostic biomarker and disease-modifying therapy.

Inflammation in Alzheimer disease-a brief review of the basic science and clinical literature

Biochemical and neuropathological studies of brains from individuals with Alzheimer disease (AD) provide clear evidence for an activation of inflammatory pathways, and long-term use of anti-inflammatory drugs is linked with reduced risk to develop the disease. As cause and effect relationships between inflammation and AD are being worked out, there is a realization that some components of this complex molecular and cellular machinery are most likely promoting pathological processes leading to AD, whereas other components serve to do the opposite. The challenge will be to find ways of fine tuning inflammation to delay, prevent, or treat AD.

Amyloid β precursor protein as a molecular target for amyloid β--induced neuronal degeneration in Alzheimer's disease

A role of amyloid β (Aβ) peptide aggregation and deposition in Alzheimer's disease (AD) pathogenesis is widely accepted. Significantly, abnormalities induced by aggregated Aβ have been linked to synaptic and neuritic degeneration, consistent with the "dying-back" pattern of degeneration that characterizes neurons affected in AD. However, molecular mechanisms underlying the toxic effect of aggregated Aβ remain elusive. In the last 2 decades, a variety of aggregated Aβ species have been identified and their toxic properties demonstrated in diverse experimental systems. Concurrently, specific Aβ assemblies have been shown to interact and misregulate a growing number of molecular effectors with diverse physiological functions. Such pleiotropic effects of aggregated Aβ posit a mayor challenge for the identification of the most cardinal Aβ effectors relevant to AD pathology. In this review, we discuss recent experimental evidence implicating amyloid β precursor protein (APP) as a molecular target for toxic Aβ assemblies. Based on a significant body of pathologic observations and experimental evidence, we propose a novel pathologic feed-forward mechanism linking Aβ aggregation to abnormalities in APP processing and function, which in turn would trigger the progressive loss of neuronal connectivity observed early in AD.

Long-term immunomodulatory effect of amniotic stem cells in an Alzheimer's disease model

Amyloid beta (Aβ) plays a major role in Alzheimer's disease (AD), and neuroinflammatory processes mediated by Aβ plaque-induced microglial cells and astrocytes contribute to AD pathogenesis. The present study examined human placenta amniotic membrane-derived mesenchymal stem cells (AMSCs), which have potent immunomodulatory and paracrine effects in a Tg2576 (APPswe) transgenic mouse model of AD. AMSCs secreted high levels of transforming growth factor-β under in vitro inflammatory environment conditions. Six weeks after the intravenous injection of AMSCs, APPswe mice showed evidence of improved spatial learning, which significantly correlated with the observation of fewer Aβ plaques in brain. The number of ED1-positive phagocytic microglial cells associated with Aβ plaques was higher in AMSC-injected mice than in phosphate-buffered saline-injected mice, and the level of Aβ-degrading enzymes (matrix metallopeptidase-9 and insulin-degrading enzyme) was also significantly higher. Furthermore, the level of proinflammatory cytokines, interleukin-1 and tumor necrosis factor-α, was lower and that of anti-inflammatory cytokines, interleukin-10 and transforming growth factor-β, was higher in AMSC-injected mice than phosphate-buffered saline-injected mice. These effects lasted until 12 weeks after AMSC injection. Taken together, these results collectively suggest that injection of AMSCs might show significant long-lasting improvement in AD pathology and memory function via immunomodulatory and paracrine mechanisms.

Neuroprotective effects of D-Ala(2)GIP on Alzheimer's disease biomarkers in an APP/PS1 mouse model

Introduction

Type 2 diabetes mellitus has been identified as a risk factor for Alzheimer's disease (AD). An impairment of insulin signaling as well as a desensitization of its receptor has been found in AD brains. Glucose-dependent insulinotropic polypeptide (GIP) normalises insulin signaling by facilitating insulin release. GIP directly modulates neurotransmitter release, LTP formation, and protects synapses from the detrimental effects of beta-amyloid fragments on LTP formation, and cell proliferation of progenitor cells in the dentate gyrus. Here we investigate the potential therapeutic property of the new long lasting incretin hormone analogue D-Ala²GIP on key symptoms found in a mouse model of Alzheimer' disease (APPswe/PS1detaE9).

Methods

D-Ala²GIP was injected for 21 days at 25 nmol/kg ip once daily in APP/PS1 male mice and wild type (WT) littermates aged 6 or 12 months of age. Amyloid plaque load, inflammation biomarkers, synaptic plasticity in the brain (LTP), and memory were measured.

Results

D-Ala²GIP improved memory in WT mice and rescued the cognitive decline of 12 months old APP/PS1 mice in two different memory tasks. Furthermore, deterioration of synaptic function in the dentate gyrus and cortex was prevented in 12 months old APP/PS1 mice. D-Ala²GIP facilitated synaptic plasticity in APP/PS1 and WT mice and reduced the number of amyloid plaques in the cortex of D-Ala²GIP injected APP/PS1 mice. The inflammatory response in microglia was also reduced.

Conclusion

The results demonstrate that D-Ala²GIP has neuroprotective properties on key hallmarks found in AD. This finding shows that novel GIP analogues have the potential as a novel therapeutic for AD.

Pathology associated with AAV mediated expression of beta amyloid or C100 in adult mouse hippocampus and cerebellum

Accumulation of beta amyloid (Aβ) in the brain is a primary feature of Alzheimer’s disease (AD) but the exact molecular mechanisms by which Aβ exerts its toxic actions are not yet entirely clear. We documented pathological changes 3 and 6 months after localised injection of recombinant, bi-cistronic adeno-associated viral vectors (rAAV2) expressing human Aβ40-GFP, Aβ42-GFP, C100-GFP or C100^V717F-GFP into the hippocampus and cerebellum of 8 week old male mice. Injection of all rAAV2 vectors resulted in wide-spread transduction within the hippocampus and cerebellum, as shown by expression of transgene mRNA and GFP protein. Despite the lack of accumulation of Aβ protein after injection with AAV vectors, injection of rAAV2-Aβ42-GFP and rAAV2- C100^V717F-GFP into the hippocampus resulted in significantly increased microgliosis and altered permeability of the blood brain barrier, the latter revealed by high levels of immunoglobulin G (IgG) around the injection site and the presence of IgG positive cells. In comparison, injection of rAAV2-Aβ40-GFP and rAAV2-C100-GFP into the hippocampus resulted in substantially less neuropathology. Injection of rAAV2 vectors into the cerebellum resulted in similar types of pathological changes, but to a lesser degree. The use of viral vectors to express different types of Aβ and C100 is a powerful technique with which to examine the direct in vivo consequences of Aβ expression in different regions of the mature nervous system and will allow experimentation and analysis of pathological AD-like changes in a broader range of species other than mouse.

A novel retro-inverso peptide inhibitor reduces amyloid deposition, oxidation and inflammation and stimulates neurogenesis in the APPswe/PS1ΔE9 mouse model of Alzheimer's disease

Previously, we have developed a retro-inverso peptide inhibitor (RI-OR2, rGffvlkGr) that blocks the in vitro formation and toxicity of the Aβ oligomers which are thought to be a cause of neurodegeneration and memory loss in Alzheimer’s disease. We have now attached a retro-inverted version of the HIV protein transduction domain ‘TAT’ to RI-OR2 to target this new inhibitor (RI-OR2-TAT, Ac-rGffvlkGrrrrqrrkkrGy-NH₂) into the brain. Following its peripheral injection, a fluorescein-labelled version of RI-OR2-TAT was found to cross the blood brain barrier and bind to the amyloid plaques and activated microglial cells present in the cerebral cortex of 17-months-old APPswe/PS1ΔE9 transgenic mice. Daily intraperitoneal injection of RI-OR2-TAT (at 100 nmol/kg) for 21 days into 10-months-old APPswe/PS1ΔE9 mice resulted in a 25% reduction (p<0.01) in the cerebral cortex of Aβ oligomer levels, a 32% reduction (p<0.0001) of β-amyloid plaque count, a 44% reduction (p<0.0001) in the numbers of activated microglial cells, and a 25% reduction (p<0.0001) in oxidative damage, while the number of young neurons in the dentate gyrus was increased by 210% (p<0.0001), all compared to control APPswe/PS1ΔE9 mice injected with vehicle (saline) alone. Our data suggest that oxidative damage, inflammation, and inhibition of neurogenesis are all a downstream consequence of Aβ aggregation, and identify a novel brain-penetrant retro-inverso peptide inhibitor of Aβ oligomer formation for further testing in humans as a potential disease-modifying treatment for Alzheimer’s disease.

The type 2 diabetes drug liraglutide reduces chronic inflammation induced by irradiation in the mouse brain

Chronic inflammation in the brain is found in a range of neurodegenerative diseases such as Parkinson's or Alzheimer's disease. We have recently shown that analogues of the glucagon-like polypeptide 1 (GLP-1) such as liraglutide have potent neuroprotective properties in a mouse model of Alzheimer's disease. We also found a reduction of activated microglia in the brain. This finding suggests that GLP-1 analogues such as liraglutide have anti-inflammatory properties. To further characterise this property, we tested the effects of liraglutide on the chronic inflammation response induced by exposure of the brain to 6 Gy (X-ray). Animals were injected i.p. with 25 nmol/kg once daily for 30 days. Brains were analysed for cytokine levels, activated microglia and astrocyte levels, and nitrite levels as a measure for nitric oxide production and protein expression of iNOS. Exposure of the brain to 6 Gy induced a pronounced chronic inflammation response in the brain. The activated microglia load in the cortex and dentate gyrus region of hippocampus (P<0.001), and the activated astrocyte load in the cortex (P<0.01) was reduced by liraglutide. Furthermore, the pro-inflammatory cytokine levels of IL-6 (P<0.01), IL-12p70 (P<0.01), IL-1β (P<0.05), and total nitrite concentration were reduced in the brains of animals treated with liraglutide. The results demonstrate that liraglutide is effective in reducing a number of parameters linked to the chronic inflammation response. Liraglutide or similar GLP-1 analogues may be a suitable treatment for reducing the chronic inflammatory response in the brain found in several neurodegenerative conditions.

Multiple low-dose infusions of human umbilical cord blood cells improve cognitive impairments and reduce amyloid-β-associated neuropathology in Alzheimer mice

Alzheimer's disease (AD) is the most common progressive age-related dementia in the elderly and the fourth major cause of disability and mortality in that population. The disease is pathologically characterized by deposition of β-amyloid plaques neurofibrillary tangles in the brain. Current strategies for the treatment of AD are symptomatic only. As such, they are less than efficacious in terms of significantly slowing or halting the underlying pathophysiological progression of the disease. Modulation by cell therapy may be new promising disease-modifying therapy. Recently, we showed reduction in amyloid-β (Aβ) levels/β-amyloid plaques and associated astrocytosis following low-dose infusions of mononuclear human umbilical cord blood cells (HUCBCs). Our current study extended our previous findings by examining cognition via (1) the rotarod test, (2) a 2-day version of the radial-arm water maze test, and (3) a subsequent observation in an open pool platform test to characterize the effects of monthly peripheral HUCBC infusion (1×10(6) cells/μL) into the transgenic PSAPP mouse model of cerebral amyloidosis (bearing mutant human APP and presenilin-1 transgenes) from 6 to 12 months of age. We show that HUCBC therapy correlates with decreased (1) cognitive impairment, (2) Aβ levels/β-amyloid plaques, (3) amyloidogenic APP processing, and (4) reactive microgliosis after a treatment of 6 or 10 months. As such, this report lays the groundwork for an HUCBC therapy as potentially novel alternative to oppose AD at the disease-modifying level.

Selenium-enhanced electron microscopic imaging of different aggregate forms of a segment of the amyloid β peptide in cells

The aggregation of misfolded proteins is a common feature underlying a wide range of age-related degenerative disorders, including Alzheimer's and Parkinson's diseases. A key aspect of understanding the molecular origins of these conditions is to define the manner in which specific types of protein aggregates influence disease pathogenesis through their interactions with cells. We demonstrate how selenium-enhanced electron microscopy (SE-EM), combined with tomographic reconstruction methods, can be used to image, here at a resolution of 5-10 nm, the interaction with human macrophage cells of amyloid aggregates formed from Aβ(25-36), a fragment of the Aβ peptide whose self-assembly is associated with Alzheimer's disease. We find that prefibrillar aggregates and mature fibrils are distributed into distinct subcellular compartments and undergo varying degrees of morphological change over time, observations that shed new light on the origins of their differential toxicity and the mechanisms of their clearance. In addition, the results show that SE-EM provides a powerful and potentially widely applicable means to define the nature and location of protein assemblies in situ and to provide detailed and specific information about their partitioning and processing.

Microglial Amyloid-β1-40 Phagocytosis Dysfunction Is Caused by High-Mobility Group Box Protein-1: Implications for the Pathological Progression of Alzheimer's Disease

In Alzheimer disease (AD) patient brains, the accumulation of amyloid-β (Aβ) peptides is associated with activated microglia. Aβ is derived from the amyloid precursor protein; two major forms of Aβ, that is, Aβ1-40 (Aβ40) and Aβ1-42 (Aβ42), exist. We previously reported that rat microglia phagocytose Aβ42, and high mobility group box protein 1 (HMGB1), a chromosomal protein, inhibits phagocytosis. In the present study, we investigated the effects of exogenous HMGB1 on rat microglial Aβ40 phagocytosis. In the presence of exogenous HMGB1, Aβ40 markedly increased in microglial cytoplasm, and the reduction of extracellular Aβ40 was inhibited. During this period, HMGB1 was colocalized with Aβ40 in the cytoplasm. Furthermore, exogenous HMGB1 inhibited the degradation of Aβ40 induced by the rat microglial cytosolic fraction. Thus, extracellular HMGB1 may internalize with Aβ40 in the microglial cytoplasm and inhibit Aβ40 degradation by microglia. This may subsequently delay Aβ40 clearance. We further confirmed that in AD brains, the parts of senile plaques surrounded by activated microglia are composed of Aβ40, and extracellular HMGB1 is deposited on these plaques. Taken together, microglial Aβ phagocytosis dysfunction may be caused by HMGB1 that accumulates extracellularly on Aβ plaques, and it may be critically involved in the pathological progression of AD.

Alzheimer's disease: redox dysregulation as a common denominator for diverse pathogenic mechanisms

Alzheimer's disease (AD) is the most common cause of dementia and a progressive neurodegeneration that appears to result from multiple pathogenic mechanisms (including protein misfolding/aggregation, involved in both amyloid β-dependent senile plaques and tau-dependent neurofibrillary tangles), metabolic and mitochondrial dysfunction, excitoxicity, calcium handling impairment, glial cell dysfunction, neuroinflammation, and oxidative stress. Oxidative stress, which could be secondary to several of the other pathophysiological mechanisms, appears to be a major determinant of the pathogenesis and progression of AD. The identification of oxidized proteins common for mild cognitive impairment and AD suggests that key oxidation pathways are triggered early and are involved in the initial progression of the neurodegenerative process. Abundant data support that oxidative stress, also considered as a main factor for aging, the major risk factor for AD, can be a common key element capable of articulating the divergent nature of the proposed pathogenic factors. Pathogenic mechanisms influence each other at different levels. Evidence suggests that it will be difficult to define a single-target therapy resulting in the arrest of progression or the improvement of AD deterioration. Since oxidative stress is present from early stages of disease, it appears as one of the main targets to be included in a clinical trial. Exploring the articulation of AD pathogenic mechanisms by oxidative stress will provide clues for better understanding the pathogenesis and progression of this dementing disorder and for the development of effective therapies to treat this disease.

The role of inflammatory processes in Alzheimer's disease

It has become increasingly clear that inflammatory processes play a significant role in the pathophysiology of Alzheimer's disease (AD). Neuroinflammation is characterized by the activation of astrocytes and microglia and the release of proinflammatory cytokines and chemokines. Vascular inflammation, mediated largely by the products of endothelial activation, is accompanied by the production and the release of a host of inflammatory factors which contribute to vascular, immune, and neuronal dysfunction. The complex interaction of these processes is still only imperfectly understood, yet as the mechanisms continue to be elucidated, targets for intervention are revealed. Although many of the studies to date on therapeutic or preventative strategies for AD have been narrowly focused on single target therapies, there is accumulating evidence to suggest that the most successful treatment strategy will likely incorporate a sequential, multifactorial approach, addressing direct neuronal support, general cardiovascular health, and interruption of deleterious inflammatory pathways.

The immunology of neurodegeneration

While immune responses in neurodegeneration were regarded as little more than a curiosity a decade ago, they are now increasingly moving toward center stage. Factors driving this movement include the recognition that most of the relevant immune molecules are produced within the brain, that microglia are proficient immune cells shaping neuronal circuitry and fate, and that systemic immune responses affect brain function. We will review this complex field from the perspective of neurons, extra-neuronal brain cells, and the systemic environment and highlight the possibility that cell intrinsic innate immune molecules in neurons may function in neurodegenerative processes.

Effects of non-steroidal anti-inflammatory drug treatments on cognitive decline vary by phase of pre-clinical Alzheimer disease: findings from the randomized controlled Alzheimer's Disease Anti-inflammatory Prevention Trial

OBJECTIVE

We examined the effects of non-steroidal anti-inflammatory drugs on cognitive decline as a function of phase of pre-clinical Alzheimer disease.

METHODS

Given recent findings that cognitive decline accelerates as clinical diagnosis is approached, we used rate of decline as a proxy for phase of pre-clinical Alzheimer disease. We fit growth mixture models of Modified Mini-Mental State (3MS) Examination trajectories with data from 2388 participants in the Alzheimer's Disease Anti-inflammatory Prevention Trial and included class-specific effects of naproxen and celecoxib.

RESULTS

We identified three classes: "no decline", "slow decline", and "fast decline", and examined the effects of celecoxib and naproxen on linear slope and rate of change by class. Inclusion of quadratic terms improved fit of the model (-2 log likelihood difference: 369.23; p < 0.001) but resulted in reversal of effects over time. Over 4 years, participants in the slow-decline class on placebo typically lost 6.6 3MS points, whereas those on naproxen lost 3.1 points (p-value for difference: 0.19). Participants in the fast-decline class on placebo typically lost 11.2 points, but those on celecoxib first declined and then gained points (p-value for difference from placebo: 0.04), whereas those on naproxen showed a typical decline of 24.9 points (p-value for difference from placebo: <0.0001).

CONCLUSIONS

Our results appeared statistically robust but provided some unexpected contrasts in effects of different treatments at different times. Naproxen may attenuate cognitive decline in slow decliners while accelerating decline in fast decliners. Celecoxib appeared to have similar effects at first but then attenuated change in fast decliners.

Clearance of amyloid-β peptides by microglia and macrophages: the issue of what, when and where

Accumulation of senile plaques consisting of amyloid-β peptide (Aβ) aggregates is a prominent pathological feature in Alzheimer's disease. Effective clearance of Aβ from the brain parenchyma is thought to regulate the development and progression of the disease. Macrophages in the brain play an important role in Aβ clearance by a variety of phagocytic and digestive mechanisms. Subpopulations of macrophages are heterogeneous such that resident microglia in the parenchyma, blood macrophages infiltrating from the periphery, and perivascular macrophages residing along cerebral vessels make functionally distinct contributions to Aβ clearance. Despite phenotypic similarities between the different macrophage subsets, a series of in vivo models have been derived to differentiate their relative impacts on Aβ dynamics as well as the molecular mechanisms underlying their activities. This review discusses the key findings from these models and recent research efforts to selectively enhance macrophage clearance of Aβ.

Animal Models of Alzheimer's Disease: Utilization of Transgenic Alzheimer's Disease Models in Studies of Amyloid Beta Clearance

Glial cells in Alzheimer’s disease (AD) have been shown to be capable of clearing or at least restricting the accumulation of toxic amyloid beta (Aβ) deposits. Recently, bone marrow (BM)–derived monocytic cells have been recognized in experimental studies to be superior in their phagocytic properties when compared to their brain endogenous counterparts. In human AD, BM-derived monocytic cells may have deficiencies in their capacity to restrict plaque growth. Therefore, enhancement of phagocytic properties of cells of monocyte origin, both brain endogenous microglia and BM-derived monocytic cells, offers an attractive therapeutic approach to fight off AD. Transgenic mouse models with aberrant Aβ deposition offer a valuable tool for discovery of novel pathways to facilitate cell-mediated Aβ uptake. This article reviews the most recent findings on the phagocytic capacity of cells with monocytic origin in various transgenic AD models and describes the methods to study phagocytic activity of these cells.

Aβ-degrading enzymes: potential for treatment of Alzheimer disease

There is increasing evidence that deficient clearance of β-amyloid (Aβ) contributes to its accumulation in late-onset Alzheimer disease (AD). Several Aβ-degrading enzymes, including neprilysin (NEP), insulin-degrading enzyme, and endothelin-converting enzyme reduce Aβ levels and protect against cognitive impairment in mouse models of AD. The activity of several Aβ-degrading enzymes rises with age and increases still further in AD, perhaps as a physiological response to minimize the buildup of Aβ. The age- and disease-related changes in expression of more recently recognized Aβ-degrading enzymes (e.g. NEP-2 and cathepsin B) remain to be investigated, and there is strong evidence that reduced NEP activity contributes to the development of cerebral amyloid angiopathy. Regardless of the role of Aβ-degrading enzymes in the development of AD, experimental data indicate that increasing the activity of these enzymes (NEP in particular) has therapeutic potential in AD, although targeting their delivery to the brain remains a major challenge. The most promising current approaches include the peripheral administration of agents that enhance the activity of Aβ-degrading enzymes and the direct intracerebral delivery of NEP by convection-enhanced delivery. In the longer term, genetic approaches to increasing the intracerebral expression of NEP or other Aβ-degrading enzymes may offer advantages.

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.

Nuclear receptors as therapeutic targets for Alzheimer's disease

INTRODUCTION

Alzheimer's disease (AD) is characterized by the accumulation and extensive deposition of amyloid β (Aβ) in the parenchyma of the brain. This accumulation of amyloid is associated with perturbations in synaptic function, impairments in energy metabolism and induction of a chronic inflammatory response which acts to promote neuronal loss and cognitive impairment.

AREAS COVERED

Currently, there are no drugs that target the underlying mechanisms of AD. Here, we propose a class of nuclear receptors as novel and promising new therapeutic targets for AD. This review summarizes the literature on nuclear receptors and their effects on AD-related pathophysiology.

EXPERT OPINION

Nuclear receptors are attractive targets for the treatment of AD due to their ability to facilitate degradation of Aβ, affect microglial activation and suppress the inflammatory milieu of the brain. Liver X receptor agonists have proven difficult to move into clinical trials as long-term treatment results in hepatic steatosis. It is our view that PPAR-γ activation remains a promising avenue for the treatment for AD; however, the poor BBB permeability of the currently available agonists and the negative outcome of the Phase III clinical trials are likely to diminish interest in pursuing this target.

The diabetes drug liraglutide prevents degenerative processes in a mouse model of Alzheimer's disease

Type 2 diabetes is a risk factor for Alzheimer's disease, most likely linked to an impairment of insulin signaling in the brain. The incretin hormone glucagon-like peptide-1 (GLP-1) facilitates insulin signaling, and novel long-lasting GLP-1 analogs, such as liraglutide, are on the market as diabetes therapeutics. GLP-1 has been shown to have neuroprotective properties in vitro and in vivo. Here we tested the effects of peripherally injected liraglutide in an Alzheimer mouse model, APP(swe)/PS1(ΔE9) (APP/PS1). Liraglutide was shown to cross the blood-brain barrier in an acute study. Liraglutide was injected for 8 weeks at 25 nmol/kg body weight i.p. once daily in 7-month-old APP/PS1 and wild-type littermate controls. In APP/PS1 mice, liraglutide prevented memory impairments in object recognition and water maze tasks, and prevented synapse loss and deterioration of synaptic plasticity in the hippocampus, commonly observed in this model. Overall β-amyloid plaque count in the cortex and dense-core plaque numbers were reduced by 40-50%, while levels of soluble amyloid oligomers were reduced by 25%. The inflammation response as measured by activated microglia numbers was halved in liraglutide-treated APP/PS1 mice. Numbers of young neurons in the dentate gyrus were increased in APP/PS1 mice with treatment. Liraglutide treatment had little effect on littermate control mice, whose behavior was comparable to wild-type saline controls; however, synaptic plasticity was enhanced in the drug group. Our results show that liraglutide prevents key neurodegenerative developments found in Alzheimer's disease, suggesting that GLP-1 analogs represent a novel treatment strategy for Alzheimer's disease.

Degradation of Alzheimer's amyloid fibrils by microglia requires delivery of ClC-7 to lysosomes

Incomplete lysosomal acidification in microglia inhibits the degradation of fibrillar forms of Alzheimer's amyloid β peptide (fAβ). Here we show that in primary microglia a chloride transporter, ClC-7, is not delivered efficiently to lysosomes, causing incomplete lysosomal acidification. ClC-7 protein is synthesized by microglia but it is mistargeted and appears to be degraded by an endoplasmic reticulum-associated degradation pathway. Activation of microglia with macrophage colony-stimulating factor induces trafficking of ClC-7 to lysosomes, leading to lysosomal acidification and increased fAβ degradation. ClC-7 associates with another protein, Ostm1, which plays an important role in its correct lysosomal targeting. Expression of both ClC-7 and Ostm1 is increased in activated microglia, which can account for the increased delivery of ClC-7 to lysosomes. Our findings suggest a novel mechanism of lysosomal pH regulation in activated microglia that is required for fAβ degradation.

Tetrandrine suppresses amyloid-β-induced inflammatory cytokines by inhibiting NF-κB pathway in murine BV2 microglial cells

Microglial cells play an important role in mediating neuroinflammation in Alzheimer's disease (AD) by production of a series of proinflammatory mediators and clearance of Aβ peptides and senile plaques. Tetrandrine, a bisbenzylisoquinoline alkaloid isolated from the Chinese herb Radix Stephania tetrandra, has been demonstrated to decrease the expression of proinflammatory mediators by inhibition of NF-κB activation. Here we investigated whether tetrandrine may affect the phagocytosis of microglia and the expression of cytokines and NF-κB in murine BV2 microglial cells. We found that fibrillar Amyloid-β (fAβ) induced phagocytosis of microglia and dramatically increased the levels of interleukin 1 beta (IL-1β) and tumor necrosis factor alpha (TNF-α) as well as the expression of phospho NF-κB p65 in microglia cultures. The treatment with tetrandrine resulted in downregulation of phospho NF-κB p65 expression and strikingly reduced the production of IL-1β and TNF-α. However, tetrandrine did not affect fAβ induced phagocytosis of microglia. In conclusion, tetrandrine can decrease microglial detriment of neurotoxicity while maintaining microglial benefit of neuroprotection. Tetrandrine may be an efficacious and promising remedy in the treatment of AD.

Cell surface binding and internalization of aβ modulated by degree of aggregation

The amyloid peptides, Aβ40 and Aβ42, are generated through endoproteolytic cleavage of the amyloid precursor protein. Here we have developed a model to investigate the interaction of living cells with various forms of aggregated Aβ40/42. After incubation at endosomal pH 6, we observed a variety of Aβ conformations after 3 (Aβ³), 24 (Aβ²⁴), and 90 hours (Aβ⁹⁰). Both Aβ42²⁴ and Aβ40²⁴ were observed to rapidly bind and internalize into differentiated PC12 cells, leading to accumulation in the lysosome. In contrast, Aβ40/42⁹⁰ were both found to only weakly associate with cells, but were observed as the most aggregated using dynamic light scattering and thioflavin-T. Internalization of Aβ40/42²⁴ was inhibited with treatment of monodansylcadaverine, an endocytosis inhibitor. These studies indicate that the ability of Aβ40/42 to bind and internalize into living cells increases with degree of aggregation until it reaches a maximum beyond which its ability to interact with cells diminishes drastically.

Morphologically distinct types of amyloid plaques point the way to a better understanding of Alzheimer's disease pathogenesis

The details of the sequence of pathological events leading to neuron death in Alzheimer's disease (AD) are not known. Even the formation of amyloid plaques, one of the major histopathological hallmarks of AD, is not clearly understood; both the origin of the amyloid and the means of its deposition remain unclear. It is still widely considered, however, that amyloid plaques undergo gradual growth in the interstitial space of the brain via continual extracellular deposition of amyloid beta peptides at "seeding sites," and that these growing plaques encroach progressively on neurons and their axons and dendritic processes, eventually leading to neuronal death. Actually, histopathological evidence to support this mechanism is sparse and of uncertain validity. The fact that the amyloid deposits in AD brains that are collectively referred to as plaques are of multiple types and that each seems to have a different origin often is overlooked. We have shown experimentally that many of the so-called "diffuse amyloid plaques," which lack associated inflammatory cells, are either the result of leaks of amyloid from blood vessels at focal sites of blood-brain barrier breaches or are artifacts resulting from grazing sections through the margins of dense core plaques. In addition, we have provided experimental evidence that neuronal death via necrosis leaves a residue that takes the form of a spheroid "cloud" of amyloid, released by cell lysis, surrounding a dense core that often contains neuronal nuclear material. Support for a neuronal origin for these "dense core amyloid plaques" includes their ability to attract inflammatory cells (microglia and immigrant macrophages) and that they contain nuclear and cytoplasmic components that are somewhat resistant to proteolysis by lysosomes released during neuronal cell lysis. We discuss here the clinical and therapeutic importance of recognizing that amyloid deposition occurs both within neurons (intracellular) and in the interstitial (extracellular) space of the brain. For dense core plaques, we propose that the latter location largely follows from the former. This scenario suggests that blocking intraneuronal amyloid deposition should be a primary therapeutic target. This strategy also would be effective for blocking the gradual compromise of neuronal function resulting from this intraneuronal deposition, and the eventual death and lysis of these amyloid-burdened neurons that leads to amyloid release and the appearance of dense core amyloid plaques in the interstitium of AD brains.

Microglia and neuroprotection: from in vitro studies to therapeutic applications

Microglia are the main immune cells in the brain, playing a role in both physiological and pathological conditions. Microglial involvement in neurodegenerative diseases is well-established, being microglial activation and neuroinflammation common features of these neuropathologies. Microglial activation has been considered harmful for neurons, but inflammatory state is not only associated with neurotoxic consequences, but also with neuroprotective effects, such as phagocytosis of dead neurons and clearance of debris. This brought to the idea of protective autoimmunity in the brain and to devise immunomodulatory therapies, aimed to specifically increase neuroprotective aspects of microglia. During the last years, several data supported the intrinsic neuroprotective function of microglia through the release of neuroprotective molecules. These data led to change the traditional view of microglia in neurodegenerative diseases: from the idea that these cells play an detrimental role for neurons due to a gain of their inflammatory function, to the proposal of a loss of microglial neuroprotective function as a causing factor in neuropathologies. This "microglial dysfunction hypothesis" points at the importance of understanding the mechanisms of microglial-mediated neuroprotection to develop new therapies for neurodegenerative diseases. In vitro models are very important to clarify the basic mechanisms of microglial-mediated neuroprotection, mainly for the identification of potentially effective neuroprotective molecules, and to design new approaches in a gene therapy set-up. Microglia could act as both a target and a vehicle for CNS gene delivery of neuroprotective factors, endogenously produced by microglia in physiological conditions, thus strengthening the microglial neuroprotective phenotype, even in a pathological situation.

The role of microglia in amyloid clearance from the AD brain

Alzheimer's disease (AD), the most prominent cause of senile dementia, is clinically characterized by the extracellular deposition of beta-amyloid (Abeta) and the intracellular neurofibrillary tangles. It has been well accepted that AD pathogenesis arises from perturbation in the homeostasis of Abeta in the brain. Abeta is normally produced at high levels in the brain and cleared in an equivalent rate. Thus, even a moderate decrease in the clearance leads to the accumulation of Abeta and subsequent amyloid deposition. Microglia are the tissue macrophages in the central nervous system (CNS) and have been shown to play major roles in internalization and degradation of Abeta. Abeta exists in the brain both in soluble and in fibrillar forms. Microglia interact with these two forms of Abeta in different ways. They take up soluble forms of Abeta through macropinocytosis and LDL receptor-related proteins (LRPs) mediated pathway. Fibrillar forms of Abeta interact with the cell surface innate immune receptor complex, initiating intracellular signaling cascades that stimulate phagocytosis. Inflammatory responses influence the activation status of microglia and subsequently regulate their ability to take up and degrade Abeta. ApoE and its receptors have been shown to play critical roles in these processes. In this review, we will explore the mechanisms that microglia utilize to clear Abeta and the effectors that modulate the processes.

Val(8)GLP-1 rescues synaptic plasticity and reduces dense core plaques in APP/PS1 mice

Diabetes is a risk factor for Alzheimer's disease. We tested the effects of Val(8)GLP-1, an enzyme-resistant analogue of the incretin hormone glucagon-like peptide 1 originally developed to treat diabetes in a mouse model of Alzheimer's disease that expresses mutated amyloid precursor protein (APP) and presenilin-1. We tested long term potentiation (LTP) of synaptic plasticity, inflammation response, and plaque formation. Val(8)GLP-1 crosses the blood-brain barrier when administered via intraperitoneal injection. Val(8)GLP-1 protected LTP in 9- and 18-month-old Alzheimer's disease mice when given for 3 weeks at 25 nmol/kg intraperitoneally. LTP was also enhanced in 18-month-old wild type mice, indicating that Val(8)GLP-1 also ameliorates age-related synaptic degenerative processes. Paired-pulse facilitation was also enhanced. The number of beta-amyloid plaques and microglia activation in the cortex increased with age but was not reduced by Val(8)GLP-1. In 18-month-old mice, however, the number of Congo red positive dense-core amyloid plaques was reduced. Treatment with Val(8)GLP-1 might prevent or delay neurodegenerative processes.

Immune activation in brain aging and neurodegeneration: too much or too little?

Until recently, the brain was studied almost exclusively by neuroscientists and the immune system by immunologists, fuelling the notion that these systems represented two isolated entities. However, as more data suggest an important role of the immune system in regulating the progression of brain aging and neurodegenerative disease, it has become clear that the crosstalk between these systems can no longer be ignored and a new interdisciplinary approach is necessary. A central question that emerges is whether immune and inflammatory pathways become hyperactivated with age and promote degeneration or whether insufficient immune responses, which fail to cope with age-related stress, may contribute to disease. We try to explore here the consequences of gain versus loss of function with an emphasis on microglia as sensors and effectors of immune function in the brain, and we discuss the potential role of the peripheral environment in neurodegenerative diseases.

A fibril-specific, conformation-dependent antibody recognizes a subset of Abeta plaques in Alzheimer disease, Down syndrome and Tg2576 transgenic mouse brain

Beta-amyloid (Abeta) is thought to be a key contributor to the pathogenesis of Alzheimer disease (AD) in the general population and in adults with Down syndrome (DS). Different assembly states of Abeta have been identified that may be neurotoxic. Abeta oligomers can assemble into soluble prefibrillar oligomers, soluble fibrillar oligomers and insoluble fibrils. Using a novel antibody, OC, recognizing fibrils and soluble fibrillar oligomers, we characterized fibrillar Abeta deposits in AD and DS cases. We further compared human specimens to those obtained from the Tg2576 mouse model of AD. Our results show that accumulation of fibrillar immunoreactivity is significantly increased in AD relative to nondemented aged subjects and those with select cognitive impairments (p < 0.0001). Further, there was a significant correlation between the extent of frontal cortex fibrillar deposit accumulation and dementia severity (MMSE r = -0.72). In DS, we observe an early age of onset and age-dependent accumulation of fibrillar OC immunoreactivity with little pathology in similarly aged non-DS individuals. Tg2576 mice show fibrillar accumulation that can be detected as young as 6 months. Interestingly, fibril-specific immunoreactivity was observed in diffuse, thioflavine S-negative Abeta deposits in addition to more mature neuritic plaques. These results suggest that fibrillar deposits are associated with disease in both AD and in adults with DS and their distribution within early Abeta pathology associated with diffuse plaques and correlation with MMSE suggest that these deposits may not be as benign as previously thought.