Reactive glia not only associates with plaques but also parallels tangles in Alzheimer's disease

Markers of microglia in post-mortem brain samples from patients with Alzheimer's disease: a systematic review

Neuroinflammation is proposed as one of the mechanisms by which Alzheimer's disease pathology, including amyloid-β plaques, leads to neuronal death and dysfunction. Increases in the expression of markers of microglia, the main neuroinmmune cell, are widely reported in brains from patients with Alzheimer's disease, but the literature has not yet been systematically reviewed to determine whether this is a consistent pathological feature. A systematic search was conducted in Medline, Embase and PsychINFO for articles published up to 23 February 2017. Papers were included if they quantitatively compared microglia markers in post-mortem brain samples from patients with Alzheimer's disease and aged controls without neurological disease. A total of 113 relevant articles were identified. Consistent increases in markers related to activation, such as major histocompatibility complex II (36/43 studies) and cluster of differentiation 68 (17/21 studies), were identified relative to nonneurological aged controls, whereas other common markers that stain both resting and activated microglia, such as ionized calcium-binding adaptor molecule 1 (10/20 studies) and cluster of differentiation 11b (2/5 studies), were not consistently elevated. Studies of ionized calcium-binding adaptor molecule 1 that used cell counts almost uniformly identified no difference relative to control, indicating that increases in activation occurred without an expansion of the total number of microglia. White matter and cerebellum appeared to be more resistant to these increases than other brain regions. Nine studies were identified that included high pathology controls, patients who remained free of dementia despite Alzheimer's disease pathology. The majority (5/9) of these studies reported higher levels of microglial markers in Alzheimer's disease relative to controls, suggesting that these increases are not solely a consequence of Alzheimer's disease pathology. These results show that increased markers of microglia are a consistent feature of Alzheimer's disease, though this seems to be driven primarily by increases in activation-associated markers, as opposed to markers of all microglia.

A Model of Systemic Inflammation to Study Neuroinflammation

Increasing evidence suggests that neurodegeneration occurs in part because the environment is affected during disease in a cascade of processes collectively termed neuroinflammation. This is a reactive response of the central nervous system against noxious elements that interfere with tissue homeostasis. Neuroinflammation is mediated by inflammatory molecules released by microglial cells. Understanding and controlling interactions between the immune system and microglial activation might represent the key to prevent or delay the onset of central nervous system diseases. This chapter details techniques to generate and characterize an in vivo model of neuroinflammation based on a single intraperitoneal injection of lipopolysaccharide, which can be used to understand the wide variety of cellular and molecular mechanisms of neuroinflammation, as well as to identify new therapies by testing the anti-inflammatory properties of synthetic and natural molecules.

Early changes in CSF sTREM2 in dominantly inherited Alzheimer's disease occur after amyloid deposition and neuronal injury

Emerging evidence supports a role for innate immunity and microglia in Alzheimer's disease (AD) pathophysiology. However, no marker related to microglia has been included in the temporal evolution models of AD. TREM2 is a transmembrane protein involved in innate immunity and is selectively expressed by microglia and genetically linked to AD and other neurodegenerative disorders. Its ectodomain is released by proteolysis as a soluble variant (sTREM2) and can be detected in the cerebrospinal fluid (CSF). In patients with autosomal dominant AD, we tested how many years before the expected symptom onset did CSF sTREM2 increase in mutation carriers (MCs) compared to noncarriers (NCs). We also determined the temporal sequence of changes in CSF sTREM2 and markers for amyloid deposition and neurodegeneration as well as cognitive performance. We included 218 participants consisting of 127 MC and 91 NC siblings from the Dominantly Inherited Alzheimer Network. We observed that CSF sTREM2 increased in MCs compared to NCs 5 years before the expected symptom onset and this difference remained significant until 5 years after the expected symptom onset. Changes in CSF sTREM2 occurred after alterations were observed in markers for brain amyloidosis and neuronal injury. We propose that microglial activation occurs several years before the expected symptom onset, but after amyloidosis and neuronal injury have already occurred.

Role of the peripheral innate immune system in the development of Alzheimer's disease

Alzheimer's disease is one of the most devastating neurodegenerative diseases. The exact cause of the disease is still not known although many scientists believe in the beta amyloid hypothesis which states that the accumulation of the amyloid peptide beta (Aβ) in brain is the initial cause which consequently leads to pathological neuroinflammation. However, it was recently shown that Aβ may have an important role in defending the brain against infections. Thus, the balance between positive and negative impact of Aβ may determine disease progression. Microglia in the brain are innate immune cells, and brain-initiated inflammatory responses reflected in the periphery suggests that Alzheimer's disease is to some extent also a systemic inflammatory disease. Greater permeability of the blood brain barrier facilitates the transport of peripheral immune cells to the brain and vice versa so that a vicious circle originating on the periphery may contribute to the development of overt clinical AD. Persistent inflammatory challenges by pathogens in the periphery, increasing with age, may also contribute to the central propagation of the pathological changes seen clinically. Therefore, the activation status of peripheral innate immune cells may represent an early biomarker of the upcoming impact on the brain. The modulation of these cells may thus become a useful mechanism for modifying disease progression.

Early and progressive deficit of neuronal activity patterns in a model of local amyloid pathology in mouse prefrontal cortex

Alzheimer's Disease (AD) is the most common form of dementia. The condition predominantly affects the cerebral cortex and hippocampus and is characterized by the spread of amyloid plaques and neurofibrillary tangles (NFTs). But soluble amyloid-β (Aβ) oligomers have also been identified to accumulate in the brains of AD patients and correlate with cognitive dysfunction more than the extent of plaque deposition. Here, we developed an adeno-associated viral vector expressing the human mutated amyloid precursor protein (AAV-hAPP). Intracranial injection of the AAV into the prefrontal cortex (PFC) allowed the induction of AD-like deficits in adult mice, thereby modelling human pathology. AAV-hAPP expression caused accumulation of Aβ oligomers, microglial activation, astrocytosis and the gradual formation of amyloid plaques and NFTs. In vivo two-photon imaging revealed an increase in neuronal activity, a dysfunction characteristic of the pathology, already during the accumulation of soluble oligomers. Importantly, we found that Aβ disrupts the synchronous spontaneous activity of neurons in PFC that, as in humans, is characterized by ultraslow fluctuation patterns. Our work allowed us to track brain activity changes during disease progression and provides new insight into the early deficits of synchronous ongoing brain activity, the “default network”, in the presence of Aβ peptide.

Imaging Translocator Protein as a Biomarker of Neuroinflammation in Dementia

Neuroinflammation has long been considered a potential contributor to neurodegenerative disorders that result in dementia. Accumulation of abnormal protein aggregates in Alzheimer's disease, frontotemporal dementia, and dementia with Lewy bodies is associated with the activation of microglia and astrocytes into proinflammatory states, and chronic low-level activation of glial cells likely contributes to the pathological changes observed in these and other neurodegenerative diseases. The 18kDa translocator protein (TSPO) is a key biomarker for measuring inflammation in the brain via positron emission tomography (PET). Increased TSPO density has been observed in brain tissue from patients with neurodegenerative diseases and colocalizes to activated microglia and reactive astrocytes. Several radioligands have been developed to measure TSPO density in vivo with PET, and these have been used in clinical studies of different dementia syndromes. However, TSPO radioligands have limitations, including low specific-to-nonspecific signal and differential affinity to a polymorphism on the TSPO gene, which must be taken into consideration in designing and interpreting human PET studies. Nonetheless, most PET studies have shown that increased TSPO binding is associated with various dementias, suggesting that TSPO has potential as a biomarker to further explore the role of neuroinflammation in dementia pathogenesis and may prove useful in monitoring disease progression.

Multifunctional neuroprotective effect of Withanone, a compound from Withania somnifera roots in alleviating cognitive dysfunction

Alzheimer's disease (AD) is a chronic disorder that slowly worsens and impairs the person's memory, learning, reasoning, judgment, communication and familiar tasks with loss of orientation. AD is characterized clinically by cognitive deficit and pathologically by the deposition of β amyloid plaques, neurofibrillary tangles, associated with degeneration of the cholinergic forebrain. Withanone (WS-2), a compound isolated from root extract of Withania somnifera at doses administered orally/day to wistar rats for duration of 21 days showed significant improvement in the cognitive skill by inhibiting amyloid β-42 and attenuated the elevated levels of pro-inflammatory cytokines like TNF alpha, IL-1 beta, IL-6, MCP-1, Nitric oxide, lipid peroxidation and both β- and γ- secretase enzymatic activity. Administration of WS-2 also significantly reversed the decline in acetyl choline and Glutathione (GSH) activity. None of the treatments that are available today alter the underlying causes of this terminal disease. Few preliminary clinical treatments have demonstrated that some plant medicines do ameliorate and improve memory and learning in patients with mild-to-moderate AD. WS-2 showed promise in AD treatment because of cognitive benefits and more importantly, mechanisms of action with respect to the fundamental pathophysiology of the disease, not limited to the inhibition of AChE, but also include the modification of Aβ processing, protection against oxidative stress and anti-inflammatory effects.

TREM2 deficiency attenuates neuroinflammation and protects against neurodegeneration in a mouse model of tauopathy

Alzheimer’s disease (AD) is the most common cause of dementia and is a major public health problem for which there is currently no disease-modifying treatment. There is an urgent need for greater understanding of the molecular mechanisms underlying neurodegeneration in patients to create better therapeutic options. Recently, genetic studies uncovered novel AD risk variants in the microglial receptor, triggering receptor expressed on myeloid cells 2 (TREM2). Previous studies suggested that loss of TREM2 function worsens amyloid-β (Aβ) plaque-related toxicity. In contrast, we observe TREM2 deficiency mitigates neuroinflammation and protects against brain atrophy in the context of tau pathology. These findings indicate dual roles for TREM2 and microglia in the context of amyloid versus tau pathology, which are important to consider for potential treatments targeting TREM2.

Variants in the gene encoding the triggering receptor expressed on myeloid cells 2 (TREM2) were recently found to increase the risk for developing Alzheimer’s disease (AD). In the brain, TREM2 is predominately expressed on microglia, and its association with AD adds to increasing evidence implicating a role for the innate immune system in AD initiation and progression. Thus far, studies have found TREM2 is protective in the response to amyloid pathology while variants leading to a loss of TREM2 function impair microglial signaling and are deleterious. However, the potential role of TREM2 in the context of tau pathology has not yet been characterized. In this study, we crossed Trem2^+/+ (T2^+/+) and Trem2^−/− (T2^−/−) mice to the PS19 human tau transgenic line (PS) to investigate whether loss of TREM2 function affected tau pathology, the microglial response to tau pathology, or neurodegeneration. Strikingly, by 9 mo of age, T2^−/−PS mice exhibited significantly less brain atrophy as quantified by ventricular enlargement and preserved cortical volume in the entorhinal and piriform regions compared with T2^+/+PS mice. However, no TREM2-dependent differences were observed for phosphorylated tau staining or insoluble tau levels. Rather, T2^−/−PS mice exhibited significantly reduced microgliosis in the hippocampus and piriform cortex compared with T2^+/+PS mice. Gene expression analyses and immunostaining revealed microglial activation was significantly attenuated in T2^−/−PS mice, and there were lower levels of inflammatory cytokines and astrogliosis. These unexpected findings suggest that impairing microglial TREM2 signaling reduces neuroinflammation and is protective against neurodegeneration in the setting of pure tauopathy.

Neuropsychiatric Disturbances in Alzheimer's Disease: What Have We Learned from Neuropathological Studies?

Neuropsychiatric symptoms (NPS) are an integral part of the dementia syndrome and were therefore recently included in the core diagnostic criteria of dementia. The near universal prevalence of NPS in Alzheimer's disease (AD), combined with their disabling effects on patients and caregivers, is contrasted by the fact that few effective and safe treatments exist, which is in part to be attributed to our incomplete understanding of the neurobiology of NPS. In this review, we describe the pathological alterations typical for AD, including spreading and evolution of burden, effect on the molecular and cellular integrity, functional consequences and atrophy of NPS-relevant brain regions and circuits in correlation with specific NPS assessments. It is thereby clearly established that NPS are fundamental expressions of the underlying neurodegenerative brain disease and not simply reflect the patients' secondary response to their illness. Neuropathological studies, moreover, include a majority of end-stage patient samples, which may not correctly represent the pathophysiological environment responsible for particular NPS that may already be present in an early stage, or even prior to AD diagnosis. The burdensome nature and high prevalence of NPS, in combination with the absence of effective and safe pharmacotherapies, provide a strong incentive to continue neuropathological and neurochemical, as well as imaging and other relevant approaches to further improve our apprehension of the neurobiology of NPS.

Correlations of 18F-THK5351 PET with Postmortem Burden of Tau and Astrogliosis in Alzheimer Disease

Clinical PET studies using ¹⁸F-THK5351 have demonstrated significant tracer retention in sites susceptible to tau burden in Alzheimer disease (AD). However, the in vivo PET signal to reflect tau aggregates remains controversial. Methods: We examined the spatial pattern of tracer binding, amyloid-β, tau, and gliosis in an autopsy-confirmed AD patient who underwent ¹⁸F-THK5351 and ¹¹C-Pittsburgh compound B PET before death. Results: Regional in vivo ¹⁸F-THK5351 retention was significantly correlated with the density of tau aggregates in the neocortex and monoamine oxidase-B in the whole brain, but not correlated with that of insoluble amyloid-β. Furthermore, significant association was observed between the density of tau aggregates, monoamine oxidase-B, and glial fibrillary acidic protein, suggesting that neocortical tau would strongly influence the formation of reactive astrocytes. Conclusion:¹⁸F-THK5351 PET may have limited utility as a biomarker of tau pathology in AD; however, it could be used to monitor the neuroinflammatory processes in the living brain.

Diversity of astroglial responses across human neurodegenerative disorders and brain aging

Astrogliopathy refers to alterations of astrocytes occurring in diseases of the nervous system, and it implies the involvement of astrocytes as key elements in the pathogenesis and pathology of diseases and injuries of the central nervous system. Reactive astrocytosis refers to the response of astrocytes to different insults to the nervous system, whereas astrocytopathy indicates hypertrophy, atrophy/degeneration and loss of function and pathological remodeling occurring as a primary cause of a disease or as a factor contributing to the development and progression of a particular disease. Reactive astrocytosis secondary to neuron loss and astrocytopathy due to intrinsic alterations of astrocytes occur in neurodegenerative diseases, overlap each other, and, together with astrocyte senescence, contribute to disease-specific astrogliopathy in aging and neurodegenerative diseases with abnormal protein aggregates in old age. In addition to the well-known increase in glial fibrillary acidic protein and other proteins in reactive astrocytes, astrocytopathy is evidenced by deposition of abnormal proteins such as β-amyloid, hyper-phosphorylated tau, abnormal α-synuclein, mutated huntingtin, phosphorylated TDP-43 and mutated SOD1, and PrPres , in Alzheimer's disease, tauopathies, Lewy body diseases, Huntington's disease, amyotrophic lateral sclerosis and Creutzfeldt-Jakob disease, respectively. Astrocytopathy in these diseases can also be manifested by impaired glutamate transport; abnormal metabolism and release of neurotransmitters; altered potassium, calcium and water channels resulting in abnormal ion and water homeostasis; abnormal glucose metabolism; abnormal lipid and, particularly, cholesterol metabolism; increased oxidative damage and altered oxidative stress responses; increased production of cytokines and mediators of the inflammatory response; altered expression of connexins with deterioration of cell-to-cell networks and transfer of gliotransmitters; and worsening function of the blood brain barrier, among others. Increased knowledge of these aspects will permit a better understanding of brain aging and neurodegenerative diseases in old age as complex disorders in which neurons are not the only players.

Impact of aging immune system on neurodegeneration and potential immunotherapies

The interaction between the nervous and immune systems during aging is an area of avid interest, but many aspects remain unclear. This is due, not only to the complexity of the aging process, but also to a mutual dependency and reciprocal causation of alterations and diseases between both the nervous and immune systems. Aging of the brain drives whole body systemic aging, including aging-related changes of the immune system. In turn, the immune system aging, particularly immunosenescence and T cell aging initiated by thymic involution that are sources of chronic inflammation in the elderly (termed inflammaging), potentially induces brain aging and memory loss in a reciprocal manner. Therefore, immunotherapeutics including modulation of inflammation, vaccination, cellular immune therapies and "protective autoimmunity" provide promising approaches to rejuvenate neuroinflammatory disorders and repair brain injury. In this review, we summarize recent discoveries linking the aging immune system with the development of neurodegeneration. Additionally, we discuss potential rejuvenation strategies, focusing aimed at targeting the aging immune system in an effort to prevent acute brain injury and chronic neurodegeneration during aging.

Aberrant Wnt signaling: a special focus in CNS diseases

Wnt signals regulate cell proliferation, migration and differentiation during development, as well as synaptic transmission and plasticity in the adult brain. Abnormal Wnt signaling is central to a number of brain pathologies. We review here, the significance of this pathway focused in the contribution of the most frequent alterations in receptors, secretable modulators and downstream targets in Alzheimer's disease (AD) and Glioblastoma (GBM). β-catenin and GSK3 levels are pivotal in the neurodegeneration associated to AD contributing to memory deficits, tau phosphorylation, increased β-amyloid production and modulation of Apolipoprotein E in the brain. In consequence, β-catenin and GSK3 are targets for potential treatments in AD. Also, Wnt pathway components and secreted molecules interfering with this signaling contribute to the progression of tumoral cells. Wnt pathway activation is a bad prognosis in brain cancer; however, mutations in WNT or Frizzled (FZD) genes do not account for the cases of GBM. Instead, recent studies indicate that epigenetic modifications contribute to the development of GBMs opening novel strategies to study GBM progression.

Higher visceral fat is associated with lower cerebral N-acetyl-aspartate ratios in middle-aged adults

Excessive adipose tissue, particularly with a central distribution, consists of visceral fat, which is metabolically active and could impinge upon central nervous system functioning. The aim of the current study was to examine levels of visceral adiposity in relation to key cerebral metabolite ratios localized in the occipitoparietal grey matter. Seventy-three adults, aged between 40 and 60 years, underwent structural magnetic resonance imaging and single voxel ¹H Magnetic Resonance Spectroscopy (¹H MRS). Visceral fat was assessed using Dual Energy X Ray Absorptiometry (DXA). Individuals with higher visceral fat mass and volume had significantly lower ratios of N-acetyl-aspartate to total creatine (phosphocreatine + creatine, PCr + Cr) (NAA/PCr + Cr) (β = -0.29, p = 0.03, β = -0.28, p = 0.04). They also had significantly higher ratios of myo-inositol to total creatine (mI/PCr + Cr ) (β = 0.36, p = 0.01, β = 0.36, p = 0.01). Visceral fat mass and volume were not significantly related to ratios of glutamate to total creatine (Glu/PCr + Cr). While future studies are necessary, these results indicate central adiposity is associated with metabolic changes that could impinge upon the central nervous system in middle age.

Different Molecular Mechanisms Mediate Direct or Glia-Dependent Prion Protein Fragment 90-231 Neurotoxic Effects in Cerebellar Granule Neurons

Glia over-stimulation associates with amyloid deposition contributing to the progression of central nervous system neurodegenerative disorders. Here we analyze the molecular mechanisms mediating microglia-dependent neurotoxicity induced by prion protein (PrP)90-231, an amyloidogenic polypeptide corresponding to the protease-resistant portion of the pathological prion protein scrapie (PrP^Sc). PrP90-231 neurotoxicity is enhanced by the presence of microglia within neuronal culture, and associated to a rapid neuronal [Ca⁺⁺] _i increase. Indeed, while in "pure" cerebellar granule neuron cultures, PrP90-231 causes a delayed intracellular Ca⁺⁺ entry mediated by the activation of NMDA receptors; when neuron and glia are co-cultured, a transient increase of [Ca⁺⁺] _i occurs within seconds after treatment in both granule neurons and glial cells, then followed by a delayed and sustained [Ca⁺⁺] _i raise, associated with the induction of the expression of inducible nitric oxide synthase and phagocytic NADPH oxidase. [Ca⁺⁺] _i fast increase in neurons is dependent on the activation of multiple pathways since it is not only inhibited by the blockade of voltage-gated channel activity and NMDA receptors but also prevented by the inhibition of nitric oxide and PGE₂ release from glial cells. Thus, Ca⁺⁺ homeostasis alteration, directly induced by PrP90-231 in cerebellar granule cells, requires the activation of NMDA receptors, but is greatly enhanced by soluble molecules released by activated glia. In glia-enriched cerebellar granule cultures, the activation of inducible nitric oxide (iNOS) and NADPH oxidase represents the main mechanism of toxicity since their pharmacological inhibition prevented PrP90-231 neurotoxicity, whereas NMDA blockade by D(-)-2-amino-5-phosphonopentanoic acid is ineffective; conversely, in pure cerebellar granule cultures, NMDA blockade but not iNOS inhibition strongly reduced PrP90-231 neurotoxicity. These data indicate that amyloidogenic peptides induce neurotoxic signals via both direct neuron interaction and glia activation through different mechanisms responsible of calcium homeostasis disruption in neurons and potentiating each other: the activation of excitotoxic pathways via NMDA receptors and the release of radical species that establish an oxidative milieu.

Innate immunity in Alzheimer's disease: the relevance of animal models?

The mouse is one of the organisms most widely used as an animal model in biomedical research, due to the particular ease with which it can be handled and reproduced in laboratory. As a member of the mammalian class, mice share with humans many features regarding metabolic pathways, cell morphology and anatomy. However, important biological differences between mice and humans exist and must be taken into consideration when interpreting research results, to properly translate evidence from experimental studies into information that can be useful for human disease prevention and/or treatment. With respect to Alzheimer’s disease (AD), much of the experimental information currently known about this disease has been gathered from studies using mainly mice as models. Therefore, it is notably important to fully characterise the differences between mice and humans regarding important aspects of the disease. It is now widely known that inflammation plays an important role in the development of AD, a role that is not only a response to the surrounding pathological environment, but rather seems to be strongly implicated in the aetiology of the disease as indicated by the genetic studies. This review highlights relevant differences in inflammation and in microglia, the innate immune cell of the brain, between mice and humans regarding genetics and morphology in normal ageing, and the relationship of microglia with AD-like pathology, the inflammatory profile, and cognition. We conclude that some noteworthy differences exist between mice and humans regarding microglial characteristics, in distribution, gene expression, and states of activation. This may have repercussions in the way that transgenic mice respond to, and influence, the AD-like pathology. However, despite these differences, human and mouse microglia also show similarities in morphology and behaviour, such that the mouse is a suitable model for studying the role of microglia, as long as these differences are taken into consideration when delineating new strategies to approach the study of neurodegenerative diseases.

Astrocyte dysfunction in Alzheimer disease

Astrocytes are glial cells that are distributed throughout the central nervous system in an arrangement optimal for chemical and physical interaction with neuronal synapses and brain blood supply vessels. Neurotransmission modulates astrocytic excitability by activating an array of cell surface receptors and transporter proteins, resulting in dynamic changes in intracellular Ca²⁺ or Na⁺ . Ionic and electrogenic astrocytic changes, in turn, drive vital cell nonautonomous effects supporting brain function, including regulation of synaptic activity, neuronal metabolism, and regional blood supply. Alzheimer disease (AD) is associated with aberrant oligomeric amyloid β generation, which leads to extensive proliferation of astrocytes with a reactive phenotype and abnormal regulation of these processes. Astrocytic morphology, Ca²⁺ responses, extracellular K⁺ removal, glutamate transport, amyloid clearance, and energy metabolism are all affected in AD, resulting in a deleterious set of effects that includes glutamate excitotoxicity, impaired synaptic plasticity, reduced carbon delivery to neurons for oxidative phosphorylation, and dysregulated linkages between neuronal energy demand and regional blood supply. This review summarizes how astrocytes are affected in AD and describes how these changes are likely to influence brain function. © 2017 Wiley Periodicals, Inc.

Astrocytic LRP1 Mediates Brain Aβ Clearance and Impacts Amyloid Deposition

Accumulation and deposition of amyloid-β (Aβ) in the brain represent an early and perhaps necessary step in the pathogenesis of Alzheimer's disease (AD). Aβ accumulation leads to the formation of Aβ aggregates, which may directly and indirectly lead to eventual neurodegeneration. While Aβ production is accelerated in many familial forms of early-onset AD, increasing evidence indicates that impaired clearance of Aβ is more evident in late-onset AD. To uncover the mechanisms underlying impaired Aβ clearance in AD, we examined the role of low-density lipoprotein receptor-related protein 1 (LRP1) in astrocytes. Although LRP1 has been shown to play critical roles in brain Aβ metabolism in neurons and vascular mural cells, its role in astrocytes, the most abundant cell type in the brain responsible for maintaining neuronal homeostasis, remains unclear. Here, we show that astrocytic LRP1 plays a critical role in brain Aβ clearance. LRP1 knockdown in primary astrocytes resulted in decreased cellular Aβ uptake and degradation. In addition, silencing of LRP1 in astrocytes led to downregulation of several major Aβ-degrading enzymes, including matrix metalloproteases MMP2, MMP9, and insulin-degrading enzyme. More important, conditional knock-out of the Lrp1 gene in astrocytes in the background of APP/PS1 mice impaired brain Aβ clearance, exacerbated Aβ accumulation, and accelerated amyloid plaque deposition without affecting its production. Together, our results demonstrate that astrocytic LRP1 plays an important role in Aβ metabolism and that restoring LRP1 expression and function in the brain could be an effective strategy to facilitate Aβ clearance and counter amyloid pathology in AD.SIGNIFICANCE STATEMENT Astrocytes represent a major cell type regulating brain homeostasis; however, their roles in brain clearance of amyloid-β (Aβ) and underlying mechanism are not clear. In this study, we used both cellular models and conditional knock-out mouse models to address the role of a critical Aβ receptor, the low-density lipoprotein receptor-related protein 1 (LRP1) in astrocytes. We found that LRP1 in astrocytes plays a critical role in brain Aβ clearance by modulating several Aβ-degrading enzymes and cellular degradation pathways. Our results establish a critical role of astrocytic LRP1 in brain Aβ clearance and shed light on specific Aβ clearance pathways that may help to establish new targets for AD prevention and therapy.

Excessive tau accumulation in the parieto-occipital cortex characterizes early-onset Alzheimer's disease

Early-onset Alzheimer's disease (EOAD) is characterized by greater nonmemory dysfunctions, more rapid progression, and greater hypometabolism and atrophy than late-onset AD (LOAD). We sought to investigate the differences in tau accumulation patterns between early- and late-onset patients with AD and mild cognitive impairment (MCI). In 90 patients who completed ¹⁸F-AV-1451 and ¹⁸F-florbetaben positron emission tomography scans, only 59 amyloid-positive patients (11 EOAD, 10 EOMCI, 21 LOAD, and 17 LOMCI) were included in this study. We compared cortical ¹⁸F-AV-1451 binding between each patient group and corresponding amyloid-negative age-matched controls. In contrast to no difference in cortical binding between the EOMCI and LOMCI groups, EOAD showed greater binding in the parieto-occipital cortex than LOAD. The parieto-occipital binding correlated with visuospatial dysfunction in the EOAD spectrum, whereas binding in the temporal cortex correlated with verbal memory dysfunction in the LOAD spectrum. Our findings suggest that distinct topographic distribution of tau may influence the nature of cognitive impairment in EOAD patients.

A state of delirium: Deciphering the effect of inflammation on tau pathology in Alzheimer's disease

Alzheimer's disease (AD), the predominant form of dementia, is highly correlated with the abnormal hyperphosphorylation and aggregation of tau. Immune responses are key drivers of AD and how they contribute to tau pathology in human disease remains largely unknown. This review summarises current knowledge on the association between inflammatory processes and tau pathology. While, preclinical evidence suggests that inflammation can indeed induce tau hyperphosphorylation at both pre- and post-tangles epitopes, a better understanding of whether this develops into advanced pathological features such as neurofibrillary tangles is needed. Microglial cells, the immune phagocytes in the central nervous system, appear to play a key role in regulating tau pathology, but the underlying mechanisms are not fully understood. Their activation can be detrimental via the secretion of pro-inflammatory mediators, particularly interleukin-1β, but also potentially beneficial through phagocytosis of extracellular toxic tau oligomers. Nevertheless, anti-inflammatory treatments in animal models were found protective, but whether or not they affect microglial phagocytosis of tau species is unknown. However, one major challenge to our understanding of the role of inflammation in the progression of tau pathology is the preclinical models used to address this question. They mostly rely on the use of septic doses of lipopolysaccharide that do not reflect the inflammatory conditions experienced AD patients, questioning whether the impact of inflammation on tau pathology in these models is dose-dependent and relevant to the human disease. The use of more translational models of inflammation corroborated with verification in clinical investigations are necessary to progress our understanding of the interplay between inflammation and tau pathology.

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Inflammation modulates tau function in Alzheimer's disease.

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LPS induces tau phosphorylation in vivo.

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Modulation of late stage tau pathology is less clear.

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Microglial shows potential to slow spread of extracellular tau.

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A holistic approach will determine the role of inflammation in Alzheimer's disease.

Soluble phospho-tau from Alzheimer's disease hippocampus drives microglial degeneration

The role of microglial cells in the development and progression of Alzheimer's disease (AD) has not been elucidated. Here, we demonstrated the existence of a weak microglial response in human AD hippocampus which is in contrast to the massive microglial activation observed in APP-based models. Most importantly, microglial cells displayed a prominent degenerative profile (dentate gyrus > CA3 > CA1 > parahippocampal gyrus), including fragmented and dystrophic processes with spheroids, a reduced numerical density, and a significant decrease in the area of surveillance ("microglial domain"). Consequently, there was a substantial decline in the area covered by microglia which may compromise immune protection and, therefore, neuronal survival. In vitro experiments demonstrated that soluble fractions (extracellular/cytosolic) from AD hippocampi were toxic for microglial cells. This toxicity was abolished by AT8 and/or AT100 immunodepletion, validating that soluble phospho-tau was the toxic agent. These results were reproduced using soluble fractions from phospho-tau-positive Thy-tau22 hippocampi. Cultured microglial cells were not viable following phagocytosis of SH-SY5Y cells expressing soluble intracellular phospho-tau. Because the phagocytic capacity of microglial cells is highly induced by apoptotic signals in the affected neurons, we postulate that accumulation of intraneuronal soluble phospho-tau might trigger microglial degeneration in the AD hippocampus. This microglial vulnerability in AD pathology provides new insights into the immunological mechanisms underlying the disease progression and highlights the need to improve or develop new animal models, as the current models do not mimic the microglial pathology observed in the hippocampus of AD patients.

Amyloid-β 1-24 C-terminal truncated fragment promotes amyloid-β 1-42 aggregate formation in the healthy brain

Substantial data indicate that amyloid-β (Aβ), the major component of senile plaques, plays a central role in Alzheimer’s Disease and indeed the assembly of naturally occurring amyloid peptides into cytotoxic aggregates is linked to the disease pathogenesis. Although Aβ42 is a highly aggregating form of Aβ, the co-occurrence of shorter Aβ peptides might affect the aggregation potential of the Aβ pool. In this study we aimed to assess whether the structural behavior of human Aβ42 peptide inside the brain is influenced by the concomitant presence of N-terminal fragments produced by the proteolytic activity of glial cells. We show that the occurrence of the human C-terminal truncated 1–24 Aβ fragment impairs Aβ42 clearance through blood brain barrier and promotes the formation of Aβ42 aggregates even in the healthy brain. By showing that Aβ1-24 has seeding properties for aggregate formation in intracranially injected wild type mice, our study provide the proof-of-concept that peptides produced upon Aβ42 cleavage by activated glial cells may cause phenotypic defects even in the absence of genetic mutations associated with Alzheimer’s Disease, possibly contributing to the development of the sporadic form of the pathology.

Regional and Gender Study of Neuronal Density in Brain during Aging and in Alzheimer's Disease

Background: Learning processes or language development are only some of the cognitive functions that differ qualitatively between men and women. Gender differences in the brain structure seem to be behind these variations. Indeed, this sexual dimorphism at neuroanatomical level is accompanied unequivocally by differences in the way that aging and neurodegenerative diseases affect men and women brains.

Objective: The aim of this study is the analysis of neuronal density in four areas of the hippocampus, and entorhinal and frontal cortices to analyze the possible gender influence during normal aging and in Alzheimer's disease (AD).

Methods: Human brain tissues of different age and from both sexes, without neurological pathology and with different Braak's stages of AD, were studied. Neuronal density was quantified using the optical dissector.

Results: Our results showed the absence of a significant neuronal loss during aging in non-pathological brains in both sexes. However, we have demonstrated specific punctual significant variations in neuronal density related with the age and gender in some regions of these brains. In fact, we observed a higher neuronal density in CA3 and CA4 hippocampal areas of non-pathological brains of young men compared to women. During AD, we observed a negative correlation between Braak's stages and neuronal density in hippocampus, specifically in CA1 for women and CA3 for men, and in frontal cortex for both, men and women.

Conclusion: Our data demonstrated a sexual dimorphism in the neuronal vulnerability to degeneration suggesting the need to consider the gender of the individuals in future studies, regarding neuronal loss in aging and AD, in order to avoid problems in interpreting data.

Alzheimer's Disease Cerebrospinal Fluid and Neuroimaging Biomarkers: Diagnostic Accuracy and Relationship to Drug Efficacy

Widely researched Alzheimer's disease (AD) biomarkers include in vivo brain imaging with PET and MRI, imaging of amyloid plaques, and biochemical assays of Aβ 1 - 42, total tau, and phosphorylated tau (p-tau-181) in cerebrospinal fluid (CSF). In this review, we critically evaluate these biomarkers and discuss their clinical utility for the differential diagnosis of AD. Current AD biomarker tests are either highly invasive (requiring CSF collection) or expensive and labor-intensive (neuroimaging), making them unsuitable for use in the primary care, clinical office-based setting, or to assess drug efficacy in clinical trials. In addition, CSF and neuroimaging biomarkers continue to face challenges in achieving required sensitivity and specificity and minimizing center-to-center variability (for CSF-Aβ 1 - 42 biomarkers CV = 26.5% ; http://www.alzforum.org/news/conference-coverage/paris-standardization-hurdle-spinal-fluid-imaging-markers). Although potentially useful for selecting patient populations for inclusion in AD clinical trials, the utility of CSF biomarkers and neuroimaging techniques as surrogate endpoints of drug efficacy needs to be validated. Recent trials of β- and γ-secretase inhibitors and Aβ immunization-based therapies in AD showed no significant cognitive improvements, despite changes in CSF and neuroimaging biomarkers. As we learn more about the dysfunctional cellular and molecular signaling processes that occur in AD, and how these processes are manifested in tissues outside of the brain, new peripheral biomarkers may also be validated as non-invasive tests to diagnose preclinical and clinical AD.

Physical activity and exercise attenuate neuroinflammation in neurological diseases

Major depressive disorder (MDD), schizophrenia (SCH), Alzheimer's disease (AD), and Parkinson's disease (PD) are devastating neurological disorders, which increasingly contribute to global morbidity and mortality. Although the pathogenic mechanisms of these conditions are quite diverse, chronic neuroinflammation is one underlying feature shared by all these diseases. Even though the specific root causes of these diseases remain to be identified, evidence indicates that the observed neuroinflammation is initiated by unique pathological features associated with each specific disease. If the initial acute inflammation is not resolved, a chronic neuroinflammatory state develops and ultimately contributes to disease progression. Chronic neuroinflammation is characterized by adverse and non-specific activation of glial cells, which can lead to collateral damage of nearby neurons and other glia. This misdirected neuroinflammatory response is hypothesized to contribute to neuropathology in MDD, SCH, AD, and PD. Physical activity (PA), which is critical for maintenance of whole body and brain health, may also beneficially modify neuroimmune responses. Since PA has neuroimmune-modifying properties, and the common underlying feature of MDD, SCH, AD, and PD is chronic neuroinflammation, we hypothesize that PA could minimize brain diseases by modifying glia-mediated neuroinflammation. This review highlights current evidence supporting the disease-altering potential of PA and exercise through modifications of neuroimmune responses, specifically in MDD, SCH, AD and PD.

Differential Roles of M1 and M2 Microglia in Neurodegenerative Diseases

One of the most striking hallmarks shared by various neurodegenerative diseases, including Parkinson's disease, Alzheimer's disease (AD), and amyotrophic lateral sclerosis, is microglia-mediated neuroinflammation. Increasing evidence indicates that microglial activation in the central nervous system is heterogeneous, which can be categorized into two opposite types: M1 phenotype and M2 phenotype. Depending on the phenotypes activated, microglia can produce either cytotoxic or neuroprotective effects. In this review, we focus on the potential role of M1 and M2 microglia and the dynamic changes of M1/M2 phenotypes that are critically associated with the neurodegenerative diseases. Generally, M1 microglia predominate at the injury site at the end stage of disease, when the immunoresolution and repair process of M2 microglia are dampened. This phenotype transformation is very complicated in AD due to the phagocytosis of regionally distributed β-amyloid (Aβ) plaque and tangles that are released into the extracellular space. The endogenous stimuli including aggregated α-synuclein, mutated superoxide dismutase, Aβ, and tau oligomers exist in the milieu that may persistently activate M1 pro-inflammatory responses and finally lead to irreversible neuron loss. The changes of microglial phenotypes depend on the disease stages and severity; mastering the stage-specific switching of M1/M2 phenotypes within appropriate time windows may provide better therapeutic benefit.

Actin dynamics and cofilin-actin rods in alzheimer disease

Cytoskeletal abnormalities and synaptic loss, typical of both familial and sporadic Alzheimer disease (AD), are induced by diverse stresses such as neuroinflammation, oxidative stress, and energetic stress, each of which may be initiated or enhanced by proinflammatory cytokines or amyloid-β (Aβ) peptides. Extracellular Aβ-containing plaques and intracellular phospho-tau-containing neurofibrillary tangles are postmortem pathologies required to confirm AD and have been the focus of most studies. However, AD brain, but not normal brain, also have increased levels of cytoplasmic rod-shaped bundles of filaments composed of ADF/cofilin-actin in a 1:1 complex (rods). Cofilin, the major ADF/cofilin isoform in mammalian neurons, severs actin filaments at low cofilin/actin ratios and stabilizes filaments at high cofilin/actin ratios. It binds cooperatively to ADP-actin subunits in F-actin. Cofilin is activated by dephosphorylation and may be oxidized in stressed neurons to form disulfide-linked dimers, required for bundling cofilin-actin filaments into stable rods. Rods form within neurites causing synaptic dysfunction by sequestering cofilin, disrupting normal actin dynamics, blocking transport, and exacerbating mitochondrial membrane potential loss. Aβ and proinflammatory cytokines induce rods through a cellular prion protein-dependent activation of NADPH oxidase and production of reactive oxygen species. Here we review recent advances in our understanding of cofilin biochemistry, rod formation, and the development of cognitive deficits. We will then discuss rod formation as a molecular pathway for synapse loss that may be common between all three prominent current AD hypotheses, thus making rods an attractive therapeutic target. © 2016 Wiley Periodicals, Inc.

Abnormal tau induces cognitive impairment through two different mechanisms: synaptic dysfunction and neuronal loss

The hyperphosphorylated microtubule-associated protein tau is present in several neurodegenerative diseases, although the causal relationship remains elusive. Few mouse models used to study Alzheimer-like dementia target tau phosphorylation. We created an inducible pseudophosphorylated tau (Pathological Human Tau, PH-Tau) mouse model to study the effect of conformationally modified tau in vivo. Leaky expression resulted in two levels of PH-Tau: low basal level and higher upon induction (4% and 14% of the endogenous tau, respectively). Unexpectedly, low PH-Tau resulted in significant cognitive deficits, decrease in the number of synapses (seen by EM in the CA1 region), reduction of synaptic proteins, and localization to the nucleus. Induction of PH-Tau triggered neuronal death (60% in CA3), astrocytosis, and loss of the processes in CA1. These findings suggest, that phosphorylated tau is sufficient to induce neurodegeneration and that two different mechanisms can induce cognitive impairment depending on the levels of PH-Tau expression.

Noninvasive and targeted delivery of therapeutics to the brain using focused ultrasound

The range of therapeutic treatment options for central nervous system (CNS) diseases is greatly limited by the blood-brain barrier (BBB). While a variety of strategies to circumvent the blood-brain barrier for drug delivery have been investigated, little clinical success has been achieved. Focused ultrasound (FUS) is a unique approach whereby the transcranial application of acoustic energy to targeted brain areas causes a noninvasive, safe, transient, and targeted opening of the BBB, providing an avenue for the delivery of therapeutic agents from the systemic circulation into the brain. There is a great need for viable treatment strategies for CNS diseases, and we believe that the preclinical success of this technique should encourage a rapid movement towards clinical testing. In this review, we address the versatile applications of FUS-mediated BBB opening, the safety profile of the technique, and the physical and biological mechanisms that drive this process. This article is part of the Special Issue entitled "Beyond small molecules for neurological disorders".

Astrogliosis: An integral player in the pathogenesis of Alzheimer's disease

Alzheimer's disease is the main cause of dementia in the elderly and begins with a subtle decline in episodic memory followed by a more general decline in overall cognitive abilities. Though the exact trigger for this cascade of events remains unknown the presence of the misfolded amyloid-beta protein triggers reactive gliosis, a prominent neuropathological feature in the brains of Alzheimer's patients. The cytoskeletal and morphological changes of astrogliosis are its evident features, while changes in oxidative stress defense, cholesterol metabolism, and gene transcription programs are less manifest. However, these latter molecular changes may underlie a disruption in homeostatic regulation that keeps the brain environment balanced. Astrocytes in Alzheimer's disease show changes in glutamate and GABA signaling and recycling, potassium buffering, and in cholinergic, purinergic, and calcium signaling. Ultimately the dysregulation of homeostasis maintained by astrocytes can have grave consequences for the stability of microcircuits within key brain regions. Specifically, altered inhibition influenced by astrocytes can lead to local circuit imbalance with farther reaching consequences for the functioning of larger neuronal networks. Healthy astrocytes have a role in maintaining and modulating normal neuronal communication, synaptic physiology and energy metabolism, astrogliosis interferes with these functions. This review considers the molecular and functional changes occurring during astrogliosis in Alzheimer's disease, and proposes that astrocytes are key players in the development of dementia.

Genetic and degenerative disorders primarily causing dementia

Neuroimaging comprises a powerful set of instruments to diagnose the different causes of dementia, clarify their neurobiology, and monitor their treatment. Magnetic resonance imaging (MRI) depicts volume changes with neurodegeneration and inflammation, as well as abnormalities in functional and structural connectivity. MRI arterial spin labeling allows for the quantification of regional cerebral blood flow, characteristically altered in Alzheimer's disease, diffuse Lewy-body disease, and the frontotemporal dementias. Positron emission tomography allows for the determination of regional metabolism, with similar abnormalities as flow, and for the measurement of β-amyloid and abnormal tau deposition in the brain, as well as regional inflammation. These instruments allow for the quantification in vivo of most of the pathologic features observed in disorders causing dementia. Importantly, they allow for the longitudinal study of these abnormalities, having revealed, for instance, that the deposition of β-amyloid in the brain can antecede by decades the onset of dementia. Thus, a therapeutic window has been opened and the efficacy of immunotherapies directed at removing β-amyloid from the brain of asymptomatic individuals is currently being tested. Tau and inflammation imaging, still in their infancy, combined with genomics, should provide powerful insights into these disorders and facilitate their treatment.

LPS-induced Murine Neuroinflammation Model: Main Features and Suitability for Pre-clinical Assessment of Nutraceuticals

Neuroinflammation is an important feature in the pathogenesis and progression of neurodegenerative diseases such as Alzheimer´s disease (AD), Parkinson´s disease (PD), frontotemporal dementia and amyotrophic lateral sclerosis. Based on current knowledge in the field, suggesting that targeting peripheral inflammation could be a promising additional treatment/prevention approach for neurodegenerative diseases, drugs and natural products with anti-inflammatory properties have been evaluated in animal models of neuroinflammation and neurodegeneration. In this review, we provide an extensive analysis of one of the most important and widely-used animal models of peripherally induced neuroinflammation and neurodegeneration - lipopolysaccharide (LPS)-treated mice, and address the data reproducibility in published research. We also summarize briefly basic features of various natural products, nutraceuticals, with known anti-inflammatory effects and present an overview of data on their therapeutic potential for reducing neuroinflammation in LPS-treated mice.

Plaque-Associated Local Toxicity Increases over the Clinical Course of Alzheimer Disease

Amyloid (senile) plaques, one of the two pathologic hallmarks of Alzheimer disease (AD), are associated with dystrophic neurites and glial responses, both astrocytic and microglial. Although plaque burden remains relatively stable through the clinical course of AD, whether these features of local plaque toxicity continue to worsen over the course of the disease is unclear. We performed an unbiased plaque-centered quantification of SMI312(+) dystrophic neurites, GFAP(+) reactive astrocytes, and IBA1(+) and CD68(+) activated microglia in randomly selected dense-core (Thioflavin-S(+)) plaques from the temporal neocortex of 40 AD subjects with a symptom duration ranging from 4 to 20 years, and nine nondemented control subjects with dense-core plaques. Dystrophic neurites (Kendall τ = 0.34, P = 0.001), reactive astrocytes (Kendall τ = 0.30, P = 0.003), and CD68(+) (Kendall τ = 0.48, P < 0.0001), but not IBA1 microglia (Kendall τ = 0.045, P = 0.655), exhibited a significant positive correlation with symptom duration. When excluding control subjects, only the positive association between CD68(+) microglia and symptom duration remained significant (Kendall τ = 0.39, P = 0.0003). The presence of the APOEε4 allele did not affect these results. We conclude that plaques exert an increasing toxicity in the surrounding neuropil over the clinical course of AD, thereby potentially contributing to cognitive decline.

Tau Oligomers: The Toxic Player at Synapses in Alzheimer's Disease

Alzheimer’s disease (AD) is a progressive disorder in which the most noticeable symptoms are cognitive impairment and memory loss. However, the precise mechanism by which those symptoms develop remains unknown. Of note, neuronal loss occurs at sites where synaptic dysfunction is observed earlier, suggesting that altered synaptic connections precede neuronal loss. The abnormal accumulation of amyloid-β (Aβ) and tau protein is the main histopathological feature of the disease. Several lines of evidence suggest that the small oligomeric forms of Aβ and tau may act synergistically to promote synaptic dysfunction in AD. Remarkably, tau pathology correlates better with the progression of the disease than Aβ. Recently, a growing number of studies have begun to suggest that missorting of tau protein from the axon to the dendrites is required to mediate the detrimental effects of Aβ. In this review we discuss the novel findings regarding the potential mechanisms by which tau oligomers contribute to synaptic dysfunction in AD.

Statins induce insulin-degrading enzyme secretion from astrocytes via an autophagy-based unconventional secretory pathway

Background

Insulin degrading enzyme (IDE) is a major protease of amyloid beta peptide (Aβ), a prominent toxic protein in Alzheimer’s disease (AD) pathogenesis. Previous studies suggested that statins promote IDE secretion; however, the underlying mechanism is unknown, as IDE has no signal sequence.

Results

In this study, we found that simvastatin (0.2 μM for 12 h) induced the degradation of extracellular Aβ₄₀, which depended on IDE secretion from primary astrocytes. In addition, simvastatin increased IDE secretion from astrocytes in a time- and dose-dependent manner. Moreover, simvastatin-mediated IDE secretion was mediated by an autophagy-based unconventional secretory pathway, and autophagic flux regulated simvastatin-mediated IDE secretion. Finally, simvastatin activated autophagy via the LKB1-AMPK-mTOR signaling pathway in astrocytes.

Conclusions

These results demonstrate a novel pathway for statin-mediated IDE secretion from astrocytes. Modulation of this pathway could provide a potential therapeutic target for treatment of Aβ pathology by enhancing extracellular clearance of Aβ.

Electronic supplementary material

The online version of this article (doi:10.1186/s13024-015-0054-3) contains supplementary material, which is available to authorized users.

Depletion of microglia and inhibition of exosome synthesis halt tau propagation

Accumulation of pathological tau protein is a major hallmark of Alzheimer's disease. Tau protein spreads from the entorhinal cortex to the hippocampal region early in the disease. Microglia, the primary phagocytes in the brain, are positively correlated with tau pathology, but their involvement in tau propagation is unknown. We developed an adeno-associated virus-based model exhibiting rapid tau propagation from the entorhinal cortex to the dentate gyrus in 4 weeks. We found that depleting microglia dramatically suppressed the propagation of tau and reduced excitability in the dentate gyrus in this mouse model. Moreover, we demonstrate that microglia spread tau via exosome secretion, and inhibiting exosome synthesis significantly reduced tau propagation in vitro and in vivo. These data suggest that microglia and exosomes contribute to the progression of tauopathy and that the exosome secretion pathway may be a therapeutic target.

Cytokine-producing microglia have an altered beta-amyloid load in aged APP/PS1 Tg mice

Beta-amyloid (Aβ) plaques and chronic neuroinflammation are significant neuropathological features of Alzheimer's disease. Microglial cells in aged brains have potential to produce cytokines such as TNF and IL-1 family members (IL-1α, IL-1β, and IL-1Ra) and to phagocytose Aβ in Alzheimer's disease, however the inter-relationship between these processes is poorly understood. Here we show that % Aβ plaque load followed a sigmoidal trajectory with age in the neocortex of APPswe/PS1ΔE9 Tg mice, and correlated positively with soluble Aβ40 and Aβ42. Aβ measures were moderately correlated with mRNA levels of CD11b, TNF, and IL-1Ra. Cytokine production and Aβ load were assessed in neocortical CD11b(+)(CD45(+)) microglia by flow cytometry. Whereas most microglia in aged mice produced IL-1Ra, relatively low proportions of microglia produced TNF, IL-1α, and IL-1β. However, microglial production of these latter cytokines was generally increased in APP/PS1 Tg mice. Microglia that phagocytosed endogenously-produced Aβ were only observed in APP/PS1 Tg mice. Differences in phagocytic index and total Aβ load were observed in microglia with specific cytokine profiles. Both phagocytic index and total Aβ load were higher in IL-1α(+) and IL-1Ra(+) microglia, than microglia that did not produce these cytokines. In contrast, total Aβ load was lower in IL-1β(+) and TNF(+) microglia, compared to IL-1β(-) and TNF(-) microglia, and TNF(+) microglia also had a lower phagocytic index. Using GFP bone marrow chimeric mice, we confirmed that the majority of neocortical CD11b(+)(CD45(+)) microglia were resident cells (GFP(-)) in APP/PS1 Tg mice, even after selectively analysing CD11b(+)CD45(high) cells, which are typically considered to be infiltrating cells. Together, our data demonstrate that cytokine expression is selectively correlated with age and Aβ pathology, and is associated with an altered Aβ load in phagocytic microglia from APP/PS1 Tg mice. These findings have implications for understanding the regulation of microglial cytokine production and phagocytosis of Aβ in Alzheimer's disease.

Does neuroinflammation turn on the flame in Alzheimer's disease? Focus on astrocytes

Data from animal models and Alzheimer's disease (AD) subjects provide clear evidence for an activation of inflammatory pathways during the pathogenetic course of such illness. Biochemical and neuropathological studies highlighted an important cause/effect relationship between inflammation and AD progression, revealing a wide range of genetic, cellular, and molecular changes associated with the pathology. In this context, glial cells have been proved to exert a crucial role. These cells, in fact, undergo important morphological and functional changes and are now considered to be involved in the onset and progression of AD. In particular, astrocytes respond quickly to pathology with changes that have been increasingly recognized as a continuum, with potentially beneficial and/or negative consequences. Although it is now clear that activated astrocytes trigger the neuroinflammatory process, however, the precise mechanisms have not been completely elucidated. Neuroinflammation is certainly a multi-faceted and complex phenomenon and, especially in the early stages, exerts a reparative intent. However, for reasons not yet all well known, this process goes beyond the physiologic control and contributes to the exacerbation of the damage. Here we scrutinize some evidence supporting the role of astrocytes in the neuroinflammatory process and the possibility that these cells could be considered a promising target for future AD therapies.

Central Adiposity and Cortical Thickness in Midlife

OBJECTIVES

Excessive visceral fat is associated with greater metabolic fluctuation and increased risk for dementia in older adults. The aim of the current study is to directly determine the impact of central adiposity on brain structure at midlife by examining the thickness of the cerebral cortex.

METHODS

High-resolution magnetization-prepared rapid acquisition gradient-echo images were obtained from 103 participants aged 40 to 60 years (mean [standard deviation] = 49.63 [6.47] years) on a 3-T Siemens Skyra scanner. Visceral fat was measured using dual-energy x-ray absorptiometry.

RESULTS

Individuals with higher visceral fat mass and volume had significantly thicker cortex in the right posterior cingulate gyrus (β = 0.29 [p = .019] and β = 0.31 [p = .011], respectively), controlling for age, systolic blood pressure, total cholesterol level, and blood glucose level.

CONCLUSIONS

Visceral fat was significantly associated with thicker cortex in the posterior cingulate gyrus. Although future studies are necessary, these results indicate that central adiposity is associated with significant metabolic changes that impinge upon the central nervous system in middle age.

Does Anticholinergic Activity Affect Neuropathology? Implication of Neuroinflammation in Alzheimer's Disease

One characteristic neuropathological feature of Alzheimer's disease (AD) is profound neuronal loss in the nucleus basalis of Meynert, the major source of cholinergic innervation of the cerebral cortex. Clinically, anticholinergic activity causes a decline in cognitive function and increases the risk of dementia, thus possibly enhancing AD pathologies and neurodegeneration. Until now there has been insufficient human neuropathological data to support this conclusion. Experimental studies using a tauopathy mouse model demonstrated anticholinergics enhanced tau pathology and neurodegeneration corresponding to central anticholinergic activity. Additionally, donepezil, a cholinesterase inhibitor, ameliorated tau pathology and neurodegeneration in the same mouse model. These results indicate the balance between cholinergic and anticholinergic activities might affect neurodegeneration. Importantly, neurodegeneration observed in the mouse model seemed to correspond to the distribution of microglial activation, and it was reported that neuroinflammation plays an important role in the pathomechanism of AD, while anticholinergic activity augments inflammatory responses. Moreover, some studies indicated β-amyloid itself depletes cholinergic function similarly to anticholinergic activity. Thus, anticholinergic activity might initiate and/or accelerate AD pathology. Limited human data support the conclusion that anticholinergic activity enhances AD-related neuropathology and neurodegeneration. However, experimental data from a tauopathy mouse model indicated anticholinergic activity might enhance neurodegeneration with enhanced neuroinflammation including microglial activation.

Microglial internalization and degradation of pathological tau is enhanced by an anti-tau monoclonal antibody

Microglia have been shown to contribute to the clearance of brain amyloid β peptides (Aβ), the major component of amyloid plaques, in Alzheimer's disease (AD). However, it is not known whether microglia play a similar role in the clearance of tau, the major component of neurofibrillary tangles (NFTs). We now report that murine microglia rapidly internalize and degrade hyperphosphorylated pathological tau isolated from AD brain tissue in a time-dependent manner in vitro. We further demonstrate that microglia readily degrade human tau species released from AD brain sections and eliminate NFTs from brain sections of P301S tauopathy mice. The anti-tau monoclonal antibody MC1 enhances microglia-mediated tau degradation in an Fc-dependent manner. Our data identify a potential role for microglia in the degradation and clearance of pathological tau species in brain and provide a mechanism explaining the potential therapeutic actions of passively administered anti-tau monoclonal antibodies.

Astrocyte physiopathology: At the crossroads of intercellular networking, inflammation and cell death

Recent breakthroughs in neuroscience have led to the awareness that we should revise our traditional mode of thinking and studying the CNS, i.e. by isolating the privileged network of "intelligent" synaptic contacts. We may instead need to contemplate all the variegate communications occurring between the different neural cell types, and centrally involving the astrocytes. Basically, it appears that a single astrocyte should be considered as a core that receives and integrates information from thousands of synapses, other glial cells and the blood vessels. In turn, it generates complex outputs that control the neural circuitry and coordinate it with the local microcirculation. Astrocytes thus emerge as the possible fulcrum of the functional homeostasis of the healthy CNS. Yet, evidence indicates that the bridging properties of the astrocytes can change in parallel with, or as a result of, the morphological, biochemical and functional alterations these cells undergo upon injury or disease. As a consequence, they have the potential to transform from supportive friends and interactive partners for neurons into noxious foes. In this review, we summarize the currently available knowledge on the contribution of astrocytes to the functioning of the CNS and what goes wrong in various pathological conditions, with a particular focus on Amyotrophic Lateral Sclerosis, Alzheimer's Disease and ischemia. The observations described convincingly demonstrate that the development and progression of several neurological disorders involve the de-regulation of a finely tuned interplay between multiple cell populations. Thus, it seems that a better understanding of the mechanisms governing the integrated communication and detrimental responses of the astrocytes as well as their impact towards the homeostasis and performance of the CNS is fundamental to open novel therapeutic perspectives.

In vivo tau imaging: obstacles and progress

The military conflicts of the last decade have highlighted the growing problem of traumatic brain injury in combatants returning from the battlefield. The considerable evidence pointing at the accumulation of tau aggregates and its recognition as a risk factor in neurodegenerative conditions such as Alzheimer's disease have led to a major effort to develop selective tau ligands that would allow research into the physiopathologic underpinnings of traumatic brain injury and chronic traumatic encephalopathy in military personnel and the civilian population. These tracers will allow new insights into tau pathology in the human brain, facilitating research into causes, diagnosis, and treatment of traumatic encephalopathy and major neurodegenerative dementias, such as Alzheimer's disease and some variants of frontotemporal lobar degeneration, in which tau plays a role. The field of selective tau imaging has to overcome several obstacles, some of them associated with the idiosyncrasies of tau aggregation and others related to radiotracer design. A worldwide effort has focused on the development of imaging agents that will allow selective tau imaging in vivo. Recent progress in the development of these tracers is enabling the noninvasive assessment of the extent of tau pathology in the brain, eventually allowing the quantification of changes in tau pathology over time and its relation to cognitive performance, brain volumetrics, and other biomarkers, as well as assessment of efficacy and patient recruitment for antitau therapeutic trials.

A reduced astrocyte response to β-amyloid plaques in the ageing brain associates with cognitive impairment

Aims

β-amyloid (Aβ) plaques are a key feature of Alzheimer’s disease pathology but correlate poorly with dementia. They are associated with astrocytes which may modulate the effect of Aβ-deposition on the neuropil. This study characterised the astrocyte response to Aβ plaque subtypes, and investigated their association with cognitive impairment.

Methods

Aβ plaque subtypes were identified in the cingulate gyrus using dual labelling immunohistochemistry to Aβ and GFAP⁺ astrocytes, and quantitated in two cortical areas: the area of densest plaque burden and the deep cortex near the white matter border (layer VI). Three subtypes were defined for both diffuse and compact plaques (also known as classical or core-plaques): Aβ plaque with (1) no associated astrocytes, (2) focal astrogliosis or (3) circumferential astrogliosis.

Results

In the area of densest burden, diffuse plaques with no astrogliosis (β = -0.05, p = 0.001) and with focal astrogliosis (β = -0.27, p = 0.009) significantly associated with lower MMSE scores when controlling for sex and age at death. In the deep cortex (layer VI), both diffuse and compact plaques without astrogliosis associated with lower MMSE scores (β = -0.15, p = 0.017 and β = -0.81, p = 0.03, respectively). Diffuse plaques with no astrogliosis in layer VI related to dementia status (OR = 1.05, p = 0.025). In the area of densest burden, diffuse plaques with no astrogliosis or with focal astrogliosis associated with increasing Braak stage (β = 0.01, p<0.001 and β = 0.07, p<0.001, respectively), and ApoEε4 genotype (OR = 1.02, p = 0.001 and OR = 1.10, p = 0.016, respectively). In layer VI all plaque subtypes associated with Braak stage, and compact amyloid plaques with little and no associated astrogliosis associated with ApoEε4 genotype (OR = 1.50, p = 0.014 and OR = 0.10, p = 0.003, respectively).

Conclusions

Reactive astrocytes in close proximity to either diffuse or compact plaques may have a neuroprotective role in the ageing brain, and possession of at least one copy of the ApoEε4 allele impacts the astroglial response to Aβ plaques.

Kynurenine 3-Monooxygenase: An Influential Mediator of Neuropathology

Mounting evidence demonstrates that kynurenine metabolism may play an important pathogenic role in the development of multiple neurological and neuropsychiatric disorders. The kynurenine pathway consists of two functionally distinct branches that generate both neuroactive and oxidatively reactive metabolites. In the brain, the rate-limiting enzyme for one of these branches, kynurenine 3-monooxygenase (KMO), is predominantly expressed in microglia and has emerged as a pivotal point of metabolic regulation. KMO substrate and expression levels are upregulated by pro-inflammatory cytokines and altered by functional genetic mutations. Increased KMO metabolism results in the formation of metabolites that activate glutamate receptors and elevate oxidative stress, while recent evidence has revealed neurodevelopmental consequences of reduced KMO activity. Together, the evidence suggests that KMO is positioned at a critical metabolic junction to influence the development or trajectory of a myriad of neurological diseases. Understanding the mechanism(s) by which alterations in KMO activity are able to impair neuronal function, and viability will enhance our knowledge of related disease pathology and provide insight into novel therapeutic opportunities. This review will discuss the influence of KMO on brain kynurenine metabolism and the current understanding of molecular mechanisms by which altered KMO activity may contribute to neurodevelopment, neurodegenerative, and neuropsychiatric diseases.

Neuroinflammation in Alzheimer's disease

Amyloid-β plaques and neurofibrillary tangles are the main neuropathological hallmarks in Alzheimer's disease (AD), the most common cause of dementia in the elderly. However, it has become increasingly apparent that neuroinflammation plays a significant role in the pathophysiology of AD. This review summarizes the current status of neuroinflammation research related to AD, focusing on the connections between neuroinflammation and some inflammation factors in AD. Among these connections, we discuss the dysfunctional blood-brain barrier and alterations in the functional responses of microglia and astrocytes in this process. In addition, we summarize and discuss the role of intracellular signaling pathways involved in inflammatory responses in astrocytes and microglia, including the mitogen-activated protein kinase pathways, nuclear factor-kappa B cascade, and peroxisome proliferator-activated receptor-gamma transcription factors. Finally, the dysregulation of the control and release of pro- and anti-inflammatory cytokines and classic AD pathology (amyloid plaques and neurofibrillary tangles) in AD is also reviewed.