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

Amyloid-aβ Peptide in olfactory mucosa and mesenchymal stromal cells of mild cognitive impairment and Alzheimer's disease patients

Patients with mild cognitive impairment (MCI) or Alzheimer's disease (AD) might develop olfactory dysfunction that correlates with progression of disease. Alteration of olfactory neuroepithelium associated with MCI may be useful as predictor of cognitive decline. Biomarkers with higher sensitivity and specificity would allow to understand the biological progression of the pathology in association with the clinical course of the disease. In this study, magnetic resonance images, apolipoprotein E (ApoE) load, Olfactory Connecticut test and Montreal Cognitive Assessment (MoCA) indices were obtained from noncognitive impaired (NCI), MCI and AD patients. We established a culture of patient-derived olfactory stromal cells from biopsies of olfactory mucosa (OM) to test whether biological properties of mesenchymal stromal cells (MSC) are concurrent with MCI and AD psychophysical pathology. We determined the expression of amyloid Aβ peptides in the neuroepithelium of tissue sections from MCI and AD, as well as in cultured cells of OM. Reduced migration and proliferation of stromal (CD90(+) ) cells in MCI and AD with respect to NCI patients was determined. A higher proportion of anosmic MCI and AD cases were concurrent with the ApoE ε4 allele. In summary, dysmetabolism of amyloid was concurrent with migration and proliferation impairment of patient-derived stem cells.

Insufficient resolution response in the hippocampus of a senescence-accelerated mouse model--SAMP8

Aging is the primary risk factor for Alzheimer’s disease (AD), and it is known that inflammation is associated with both aging and AD. To resolve inflammation, biosynthesis of the specialized pro-resolving mediators (SPMs) is enhanced in a programmed and active manner. We investigated the effect of age on resolution by analyzing hippocampal tissue from 2- and 9-month-old senescence-accelerated mouse prone 8 (SAMP8), as well as age-matched senescence-accelerated mouse resistant 1 (SAMR1). Pro-inflammatory markers increased upon age in SAMP8 mice and were also higher than those in age-matched SAMR1 mice. However, neither SPMs nor their receptors were enhanced upon age in SAMP8 mice compared to age-matched SAMR1 mice. Analysis of SPM biosynthetic enzymes revealed elevated levels of leukocyte type 12-lipoxygenase (L12-LOX) and decreased 5-LOX levels upon age in SAMR1 mice, whereas they remained unchanged in SAMP8 mice. Moreover, we found partial co-localization of L12-LOX and amyloid beta (Aβ) staining, as well as correlation between L12-LOX and phosphorylated tau levels in SAMP8, but not SAMR1 mice. Thus, we conclude that the resolution response in SAMP8 mice is insufficient to counteract the increased inflammation with age, and this may have a role in the development of AD-like pathologies.

The intersection of amyloid beta and tau at synapses in Alzheimer's disease

The collapse of neural networks important for memory and cognition, including death of neurons and degeneration of synapses, causes the debilitating dementia associated with Alzheimer's disease (AD). We suggest that synaptic changes are central to the disease process. Amyloid beta and tau form fibrillar lesions that are the classical hallmarks of AD. Recent data indicate that both molecules may have normal roles at the synapse, and that the accumulation of soluble toxic forms of the proteins at the synapse may be on the critical path to neurodegeneration. Further, the march of neurofibrillary tangles through brain circuits appears to take advantage of recently described mechanisms of transsynaptic spread of pathological forms of tau. These two key phenomena, synapse loss and the spread of pathology through the brain via synapses, make it critical to understand the physiological and pathological roles of amyloid beta and tau at the synapse.

Tau-tubulin kinase

Tau-tubulin kinase (TTBK) belongs to casein kinase superfamily and phosphorylates microtubule-associated protein tau and tubulin. TTBK has two isoforms, TTBK1 and TTBK2, which contain highly homologous catalytic domains but their non-catalytic domains are distinctly different. TTBK1 is expressed specifically in the central nervous system and is involved in phosphorylation and aggregation of tau. TTBK2 is ubiquitously expressed in multiple tissues and genetically linked to spinocerebellar ataxia type 11. TTBK1 directly phosphorylates tau protein, especially at Ser422, and also activates cycline-dependent kinase 5 in a unique mechanism. TTBK1 protein expression is significantly elevated in Alzheimer’s disease (AD) brains, and genetic variations of the TTBK1 gene are associated with late-onset Alzheimer’s disease in two cohorts of Chinese and Spanish populations. TTBK1 transgenic mice harboring the entire 55-kilobase genomic sequence of human TTBK1 show progression of tau accumulation, neuroinflammation, and neurodegeneration when crossed with tau mutant mice. Our recent study shows that there is a striking switch in mononuclear phagocyte and activation phenotypes in the anterior horn of the spinal cord from alternatively activated (M2-skewed) microglia in P301L tau mutant mice to pro-inflammatory (M1-skewed) infiltrating peripheral monocytes by crossing the tau mice with TTBK1 transgenic mice. TTBK1 is responsible for mediating M1-activated microglia-induced neurotoxicity, and its overexpression induces axonal degeneration in vitro. These studies suggest that TTBK1 is an important molecule for the inflammatory axonal degeneration, which may be relevant to the pathobiology of tauopathy including AD.

The dual face of connexin-based astroglial Ca(2+) communication: a key player in brain physiology and a prime target in pathology

For decades, studies have been focusing on the neuronal abnormalities that accompany neurodegenerative disorders. Yet, glial cells are emerging as important players in numerous neurological diseases. Astrocytes, the main type of glia in the central nervous system , form extensive networks that physically and functionally connect neuronal synapses with cerebral blood vessels. Normal brain functioning strictly depends on highly specialized cellular cross-talk between these different partners to which Ca(2+), as a signaling ion, largely contributes. Altered intracellular Ca(2+) levels are associated with neurodegenerative disorders and play a crucial role in the glial responses to injury. Intracellular Ca(2+) increases in single astrocytes can be propagated toward neighboring cells as intercellular Ca(2+) waves, thereby recruiting a larger group of cells. Intercellular Ca(2+) wave propagation depends on two, parallel, connexin (Cx) channel-based mechanisms: i) the diffusion of inositol 1,4,5-trisphosphate through gap junction channels that directly connect the cytoplasm of neighboring cells, and ii) the release of paracrine messengers such as glutamate and ATP through hemichannels ('half of a gap junction channel'). This review gives an overview of the current knowledge on Cx-mediated Ca(2+) communication among astrocytes as well as between astrocytes and other brain cell types in physiology and pathology, with a focus on the processes of neurodegeneration and reactive gliosis. Research on Cx-mediated astroglial Ca(2+) communication may ultimately shed light on the development of targeted therapies for neurodegenerative disorders in which astrocytes participate. This article is part of a Special Issue entitled: Calcium signaling in health and disease. Guest Editors: Geert Bultynck, Jacques Haiech, Claus W. Heizmann, Joachim Krebs, and Marc Moreau.

Rare Variants and Transcriptomics in Alzheimer disease

Alzheimer disease (AD) is the most common dementia in the elderly, still without effective treatment. Early-onset AD (EOAD) is caused by mutations in the genes APP, PSEN1 and PSEN2. Genome-wide association studies have identified >20 late-onset AD (LOAD) susceptibility genes with common variants of small risk, with the exception of APOE. We review rare susceptibility variants in LOAD with larger effects that have been recently identified in the EOAD gene APP and the newly discovered AD genes TREM2 and PLD3. Human genetic studies now consistently support the amyloid hypothesis of AD for both EOAD and LOAD. Moreover, they identified biological processes that overlap with human transcriptomics studies in AD across different tissues, such as inflammation, cytoskeletal organization, synaptic functions, etc. Transcriptomic profiles of pre-symptomatic AD-associated variant carriers already reflect specific molecular mechanisms reminiscent to those of AD patients. This might provide an avenue for personalized medicine.

CB2 receptor deficiency increases amyloid pathology and alters tau processing in a transgenic mouse model of Alzheimer's disease

The endocannabinoid CB2 receptor system has been implicated in the neuropathology of Alzheimer's disease (AD). In order to investigate the impact of the CB2 receptor system on AD pathology, a colony of mice with a deleted CB2 receptor gene, CNR2, was established on a transgenic human mutant APP background for pathological comparison with CB2 receptor-sufficient transgenic mice. J20 APP (PDGFB-APPSwInd) mice were bred over two generations with CNR2(-/-) (Cnr2(tm1Dgen)/J) mice to produce a colony of J20 CNR2(+/+) and J20 CNR2(-/-) mice. Seventeen J20 CNR2(+/+) mice (12 females, 5 males) and 16 J20 CNR2(-/-) mice (11 females, 5 males) were killed at 12 months, and their brains were interrogated for AD-related pathology with both biochemistry and immunocytochemistry (ICC). In addition to amyloid-dependent endpoints such as soluble Aβ production and plaque deposition quantified with 6E10 staining, the effect of CB2 receptor deletion on total soluble mouse tau production was assayed by using a recently developed high-sensitivity assay. Results revealed that soluble Aβ42 and plaque deposition were significantly increased in J20 CNR2(-/-) mice relative to CNR2(+/+) mice. Microgliosis, quantified with ionized calcium-binding adapter molecule 1 (Iba-1) staining, did not differ between groups, whereas plaque associated microglia was more abundant in J20 CNR2(-/-) mice. Total tau was significantly suppressed in J20 CNR2(-/-) mice relative to J20 CNR2(+/+) mice. The results confirm the constitutive role of the CB2 receptor system both in reducing amyloid plaque pathology in AD and also support tehpotential of cannabinoid therapies targeting CB2 to reduce Aβ; however, the results suggest that interventions may have a divergent effect on tau pathology.

Examination of the clinicopathologic continuum of Alzheimer disease in the autopsy cohort of the National Alzheimer Coordinating Center

To test the hypothesis that Alzheimer disease (AD) is a clinical and pathologic continuum between normal aging and end-stage dementia, we selected a convenience sample of subjects from the National Alzheimer Coordinating Center 2005 to 2012 autopsy cohort (n = 2,083) with the last clinical evaluation within 2 years before autopsy and no other primary neuropathologic diagnosis. Demographic and neuropathologic characteristics were correlated with the Clinical Dementia Rating-Sum of Boxes in the 835 subjects meeting these criteria. Both neuritic plaques and neurofibrillary tangles independently predicted Clinical Dementia Rating-Sum of Boxes. Severe small-vessel disease, severe amyloid angiopathy, and hippocampal sclerosis were also independently associated with the degree of cognitive impairment. By contrast, education was a strong independent protective factor against cognitive deficits. The cause of mild to moderate dementia remained uncertain in 14% of the patients. Inverse probability weighting suggests the generalizability of these results to nonautopsied cohorts. These data indicate that plaques and tangles independently contribute to cognitive impairment, that concurrent vascular disease strongly correlates with cognitive dysfunction even in a sample selected to represent the AD pathologic continuum, and that education further modifies clinical expression. Thus, multiple concomitant etiologies of brain damage and premorbid characteristics contribute to the uncertainty of AD clinicopathologic correlations based only on tangles and plaques.

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.

Iron deposits in the chronically inflamed central nervous system and contributes to neurodegeneration

Neurodegenerative disorders are characterized by the presence of inflammation in areas with neuronal cell death and a regional increase in iron that exceeds what occurs during normal aging. The inflammatory process accompanying the neuronal degeneration involves glial cells of the central nervous system (CNS) and monocytes of the circulation that migrate into the CNS while transforming into phagocytic macrophages. This review outlines the possible mechanisms responsible for deposition of iron in neurodegenerative disorders with a main emphasis on how iron-containing monocytes may migrate into the CNS, transform into macrophages, and die out subsequently to their phagocytosis of damaged and dying neuronal cells. The dying macrophages may in turn release their iron, which enters the pool of labile iron to catalytically promote formation of free-radical-mediated stress and oxidative damage to adjacent cells, including neurons. Healthy neurons may also chronically acquire iron from the extracellular space as another principle mechanism for oxidative stress-mediated damage. Pharmacological handling of monocyte migration into the CNS combined with chelators that neutralize the effects of extracellular iron occurring due to the release from dying macrophages as well as intraneuronal chelation may denote good possibilities for reducing the deleterious consequences of iron deposition in the CNS.

CB₂ receptor deficiency increases amyloid pathology and alters tau processing in a transgenic mouse model of Alzheimer's disease

The endocannabinoid CB₂ receptor system has been implicated in the neuropathology of Alzheimer's disease (AD). In order to investigate the impact of the CB₂ receptor system on AD pathology, a colony of mice with a deleted CB₂ receptor gene, CNR2, was established on a transgenic human mutant APP background for pathological comparison with CB₂ receptor-sufficient transgenic mice. J20 APP (PDGFB-APPSwInd) mice were bred over two generations with CNR2⁻/⁻ (Cnr2(tm1Dgen)/J) mice to produce a colony of J20 CNR2⁺/⁺ and J20 CNR2⁻/⁻ mice. Seventeen J20 CNR2⁺/⁺ mice (12 females, 5 males) and 16 J20 CNR2⁻/⁻ mice (11 females, 5 males) were killed at 12 months, and their brains were interrogated for AD-related pathology with both biochemistry and immunocytochemistry (ICC). In addition to amyloid-dependent endpoints such as soluble Aβ production and plaque deposition quantified with 6E10 staining, the effect of CB2 receptor deletion on total soluble mouse tau production was assayed by using a recently developed high-sensitivity assay. Results revealed that soluble Aβ42 and plaque deposition were significantly increased in J20 CNR2⁻/⁻ mice relative to CNR2⁺/⁺ mice. Microgliosis, quantified with ionized calcium-binding adapter molecule 1 (Iba-1) staining, did not differ between groups, whereas plaque associated microglia was more abundant in J20 CNR2⁻/⁻ mice. Total tau was significantly suppressed in J20 CNR2⁻/⁻ mice relative to J20 CNR2⁺/⁺ mice. The results confirm the constitutive role of the CB₂ receptor system both in reducing amyloid plaque pathology in AD and also support tehpotential of cannabinoid therapies targeting CB₂ to reduce Aβ; however, the results suggest that interventions may have a divergent effect on tau pathology.

Amyloid β peptide-mediated neurotoxicity is attenuated by the proliferating microglia more potently than by the quiescent phenotype

Background

The specific role of microglia on Aβ-mediated neurotoxicity is difficult to assign in vivo due to their complicated environment in the brain. Therefore, most of the current microglia-related studies employed the isolated microglia. However, the previous in vitro studies have suggested either beneficial or destructive function in microglia. Therefore, to investigate the phenotypes of the isolated microglia which exert activity of neuroprotective or destructive is required.

Results

The present study investigates the phenotypes of isolated microglia on protecting neuron against Aβ-mediated neurotoxicity. Primary microglia were isolated from the mixed glia culture, and were further cultured to distinct phenotypes, designated as proliferating amoeboid microglia (PAM) and differentiated process-bearing microglia (DPM). Their inflammatory phenotypes, response to amyloid β (Aβ), and the beneficial or destructive effects on neurons were investigated. DPM may induce both direct neurotoxicity without exogenous stimulation and indirect neurotoxicity after Aβ activation. On the other hand, PAM attenuates Aβ-mediated neurotoxicity through Aβ phagocytosis and/or Aβ degradation.

Conclusions

Our results suggest that the proliferating microglia, but not the differentiated microglia, protect neurons against Aβ-mediated neurotoxicity. This discovery may be helpful on the therapeutic investigation of Alzheimer’s disease.

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.

³H-deprenyl and ³H-PIB autoradiography show different laminar distributions of astroglia and fibrillar β-amyloid in Alzheimer brain

BACKGROUND

The pathological features in Alzheimer's disease (AD) brain include the accumulation and deposition of β-amyloid (Aβ), activation of astrocytes and microglia and disruption of cholinergic neurotransmission. Since the topographical characteristics of these different pathological processes in AD brain and how these relate to each other is not clear, this motivated further exploration using binding studies in postmortem brain with molecular imaging tracers. This information could aid the development of specific biomarkers to accurately chart disease progression.

RESULTS

In vitro binding assays demonstrated increased [³H]-PIB (fibrillar Aβ) and [³H]-PK11195 (activated microglia) binding in the frontal cortex (FC) and hippocampus (HIP), as well as increased binding of [³H]-L-deprenyl (activated astrocytes) in the HIP, but a decreased [³H]-nicotine (α4β2 nicotinic acetylcholine receptor (nAChR)) binding in the FC of AD cases compared to age-matched controls. Quantitative autoradiography binding studies were also performed to investigate the regional laminar distributions of [³H]-L-deprenyl, [³H]-PIB as well as [¹²⁵I]-α-bungarotoxin (α7 nAChRs) and [³H]-nicotine in hemisphere brain of a typical AD case. A clear lamination pattern was observed with high [³H]-PIB binding in all layers and [³H]-deprenyl in superficial layers of the FC. In contrast, [³H]-PIB showed low binding to fibrillar Aβ, but [³H]-deprenyl high binding to activated astrocytes throughout the HIP. The [³H]-PIB binding was also low and the [³H]-deprenyl binding high in all layers of the medial temporal gyrus and insular cortex in comparison to the frontal cortex. Low [³H]-nicotine binding was observed in all layers of the frontal cortex in comparison to layers in the medial temporal gyrus, insular cortex and hippocampus. Immunohistochemical detection in the AD case revealed abundant glial fibrillary acidic protein positive (GFAP+) reactive astrocytes and α7 nAChR expressing GFAP+ astrocytes both in the vicinity and surrounding Aβ neuritic plaques in the FC and HIP. Although fewer Aβ plaques were observed in the HIP, some hippocampal GFAP+ astrocytes contained Aβ-positive (6 F/3D) granules within their somata.

CONCLUSIONS

Astrocytosis shows a distinct regional pattern in AD brain compared to fibrillar Aβ, suggesting that different types of astrocytes may be associated with the pathophysiological processes in AD.

Dissecting phenotypic traits linked to human resilience to Alzheimer's pathology

Clinico-pathological correlation studies and positron emission tomography amyloid imaging studies have shown that some individuals can tolerate substantial amounts of Alzheimer's pathology in their brains without experiencing dementia. Few details are known about the neuropathological phenotype of these unique cases that might prove relevant to understanding human resilience to Alzheimer's pathology. We conducted detailed quantitative histopathological and biochemical assessments on brains from non-demented individuals before death whose brains were free of substantial Alzheimer's pathology, non-demented individuals before death but whose post-mortem examination demonstrated significant amounts of Alzheimer's changes ('mismatches'), and demented Alzheimer's cases. Quantification of amyloid-β plaque burden, stereologically-based counts of neurofibrillary tangles, neurons and reactive glia, and morphological analyses of axons were performed in the multimodal association cortex lining the superior temporal sulcus. Levels of synaptic integrity markers, and soluble monomeric and multimeric amyloid-β and tau species were measured. Our results indicate that some individuals can accumulate equivalent loads of amyloid-β plaques and tangles to those found in demented Alzheimer's cases without experiencing dementia. Analyses revealed four main phenotypic differences among these two groups: (i) mismatches had striking preservation of neuron numbers, synaptic markers and axonal geometry compared to demented cases; (ii) demented cases had significantly higher burdens of fibrillar thioflavin-S-positive plaques and of oligomeric amyloid-β deposits reactive to conformer-specific antibody NAB61 than mismatches; (iii) strong and selective accumulation of hyperphosphorylated soluble tau multimers into the synaptic compartment was noted in demented cases compared with controls but not in mismatches; and (iv) the robust glial activation accompanying amyloid-β and tau pathologies in demented cases was remarkably reduced in mismatches. Further biochemical measurements of soluble amyloid-β species-monomers, dimers and higher molecular weight oligomers-in total brain homogenates and synaptoneurosomal preparations failed to demonstrate significant differences between mismatches and demented cases. Together, these data suggest that amyloid-β plaques and tangles do not inevitably result in neural system derangement and dementia in all individuals. We identified distinct phenotypic characteristics in the profile of brain fibrillar and soluble amyloid-β and tau accrual and in the glial response that discriminated demented and non-demented individuals with high loads of Alzheimer's pathology. Amyloid-β deposition in the form of fibrillar plaques and intimately related oligomeric amyloid-β assemblies, hyperphosphorylated soluble tau species localized in synapses, and glial activation emerged in this series as likely mediators of neurotoxicity and altered cognition, providing further insight into factors and pathways potentially involved in human susceptibility or resilience to Alzheimer's pathological changes.

Differential relationships of reactive astrocytes and microglia to fibrillar amyloid deposits in Alzheimer disease

Although it is clear that astrocytes and microglia cluster around dense-core amyloid plaques in Alzheimer disease (AD), whether they are primarily attracted to amyloid deposits or are just reacting to plaque-associated neuritic damage remains elusive. We postulate that astrocytes and microglia may differentially respond to fibrillar amyloid β. Therefore, we quantified the size distribution of dense-core thioflavin-S (ThioS)-positive plaques in the temporal neocortex of 40 AD patients and the microglial and astrocyte responses in their vicinity (≤50 μm) and performed correlations between both measures. As expected, both astrocytes and microglia were clearly spatially associated with ThioS-positive plaques (p = 0.0001, ≤50 μm vs. >50 μm from their edge), but their relationship to ThioS-positive plaque size differed: larger ThioS-positive plaques were associated with more surrounding activated microglia (p = 0.0026), but this effect was not observed with reactive astrocytes. Microglial response to dense-core plaques seems to be proportional to their size, which we postulate reflects a chemotactic effect of amyloid β. By contrast, plaque-associated astrocytic response does not correlate with plaque size and seems to parallel the behavior of plaque-associated neuritic damage.

Evidence for miR-181 involvement in neuroinflammatory responses of astrocytes

Inflammation is a common component of acute injuries of the central nervous system (CNS) such as ischemia, and degenerative disorders such as Alzheimer's disease. Glial cells play important roles in local CNS inflammation, and an understanding of the roles for microRNAs in glial reactivity in injury and disease settings may therefore lead to the development of novel therapeutic interventions. Here, we show that the miR-181 family is developmentally regulated and present in high amounts in astrocytes compared to neurons. Overexpression of miR-181c in cultured astrocytes results in increased cell death when exposed to lipopolysaccharide (LPS). We show that miR-181 expression is altered by exposure to LPS, a model of inflammation, in both wild-type and transgenic mice lacking both receptors for the inflammatory cytokine TNF-α. Knockdown of miR-181 enhanced LPS-induced production of pro-inflammatory cytokines (TNF-α, IL-6, IL-1β, IL-8) and HMGB1, while overexpression of miR-181 resulted in a significant increase in the expression of the anti-inflammatory cytokine IL-10. To assess the effects of miR-181 on the astrocyte transcriptome, we performed gene array and pathway analysis on astrocytes with reduced levels of miR-181b/c. To examine the pool of potential miR-181 targets, we employed a biotin pull-down of miR-181c and gene array analysis. We validated the mRNAs encoding MeCP2 and X-linked inhibitor of apoptosis as targets of miR-181. These findings suggest that miR-181 plays important roles in the molecular responses of astrocytes in inflammatory settings. Further understanding of the role of miR-181 in inflammatory events and CNS injury could lead to novel approaches for the treatment of CNS disorders with an inflammatory component.

Alzheimer's disease risk gene CD33 inhibits microglial uptake of amyloid beta

The transmembrane protein CD33 is a sialic acid-binding immunoglobulin-like lectin that regulates innate immunity but has no known functions in the brain. We have previously shown that the CD33 gene is a risk factor for Alzheimer's disease (AD). Here, we observed increased expression of CD33 in microglial cells in AD brain. The minor allele of the CD33 SNP rs3865444, which confers protection against AD, was associated with reductions in both CD33 expression and insoluble amyloid beta 42 (Aβ42) levels in AD brain. Furthermore, the numbers of CD33-immunoreactive microglia were positively correlated with insoluble Aβ42 levels and plaque burden in AD brain. CD33 inhibited uptake and clearance of Aβ42 in microglial cell cultures. Finally, brain levels of insoluble Aβ42 as well as amyloid plaque burden were markedly reduced in APP(Swe)/PS1(ΔE9)/CD33(-/-) mice. Therefore, CD33 inactivation mitigates Aβ pathology and CD33 inhibition could represent a novel therapy for AD.

A phenotypic change but not proliferation underlies glial responses in Alzheimer disease

Classical immunohistochemical studies in the Alzheimer disease (AD) brain reveal prominent glial reactions, but whether this pathological feature is due primarily to cell proliferation or to a phenotypic change of existing resting cells remains controversial. We performed double-fluorescence immunohistochemical studies of astrocytes and microglia, followed by unbiased stereology-based quantitation in temporal cortex of 40 AD patients and 32 age-matched nondemented subjects. Glial fibrillary acidic protein (GFAP) and major histocompatibility complex II (MHC2) were used as markers of astrocytic and microglial activation, respectively. Aldehyde dehydrogenase 1 L1 and glutamine synthetase were used as constitutive astrocytic markers, and ionized calcium-binding adaptor molecule 1 (IBA1) as a constitutive microglial marker. As expected, AD patients had higher numbers of GFAP(+) astrocytes and MHC2(+) microglia than the nondemented subjects. However, both groups had similar numbers of total astrocytes and microglia and, in the AD group, these total numbers remained essentially constant over the clinical course of the disease. The GFAP immunoreactivity of astrocytes, but not the MHC2 immunoreactivity of microglia, increased in parallel with the duration of the clinical illness in the AD group. Cortical atrophy contributed to the perception of increased glia density. We conclude that a phenotypic change of existing glial cells, rather than a marked proliferation of glial precursors, accounts for the majority of the glial responses observed in the AD brain.

Chronic neuron- and age-selective down-regulation of TNF receptor expression in triple-transgenic Alzheimer disease mice leads to significant modulation of amyloid- and Tau-related pathologies

Neuroinflammation, through production of proinflammatory molecules and activated glial cells, is implicated in Alzheimer's disease (AD) pathogenesis. One such proinflammatory mediator is tumor necrosis factor α (TNF-α), a multifunctional cytokine produced in excess and associated with amyloid β-driven inflammation and cognitive decline. Long-term global inhibition of TNF receptor type I (TNF-RI) and TNF-RII signaling without cell or stage specificity in triple-transgenic AD mice exacerbates hallmark amyloid and neurofibrillary tangle pathology. These observations revealed that long-term pan anti-TNF-α inhibition accelerates disease, cautions against long-term use of anti-TNF-α therapeutics for AD, and urges more selective regulation of TNF signaling. We used adeno-associated virus vector-delivered siRNAs to selectively knock down neuronal TNF-R signaling. We demonstrate divergent roles for neuronal TNF-RI and TNF-RII where loss of opposing TNF-RII leads to TNF-RI-mediated exacerbation of amyloid β and Tau pathology in aged triple-transgenic AD mice. Dampening of TNF-RII or TNF-RI+RII leads to a stage-independent increase in Iba-1-positive microglial staining, implying that neuronal TNF-RII may act nonautonomously on the microglial cell population. These results reveal that TNF-R signaling is complex, and it is unlikely that all cells and both receptors will respond positively to broad anti-TNF-α treatments at various stages of disease. In aggregate, these data further support the development of cell-, stage-, and/or receptor-specific anti-TNF-α therapeutics for AD.

Neuroinflammation and neuronal loss precede Aβ plaque deposition in the hAPP-J20 mouse model of Alzheimer's disease

Recent human trials of treatments for Alzheimer's disease (AD) have been largely unsuccessful, raising the idea that treatment may need to be started earlier in the disease, well before cognitive symptoms appear. An early marker of AD pathology is therefore needed and it is debated as to whether amyloid-βAβ? plaque load may serve this purpose. We investigated this in the hAPP-J20 AD mouse model by studying disease pathology at 6, 12, 24 and 36 weeks. Using robust stereological methods, we found there is no neuron loss in the hippocampal CA3 region at any age. However loss of neurons from the hippocampal CA1 region begins as early as 12 weeks of age. The extent of neuron loss increases with age, correlating with the number of activated microglia. Gliosis was also present, but plateaued during aging. Increased hyperactivity and spatial memory deficits occurred at 16 and 24 weeks. Meanwhile, the appearance of plaques and oligomeric Aβ were essentially the last pathological changes, with significant changes only observed at 36 weeks of age. This is surprising given that the hAPP-J20 AD mouse model is engineered to over-expresses Aβ. Our data raises the possibility that plaque load may not be the best marker for early AD and suggests that activated microglia could be a valuable marker to track disease progression.

Neuropathological alterations in Alzheimer disease

The neuropathological hallmarks of Alzheimer disease (AD) include "positive" lesions such as amyloid plaques and cerebral amyloid angiopathy, neurofibrillary tangles, and glial responses, and "negative" lesions such as neuronal and synaptic loss. Despite their inherently cross-sectional nature, postmortem studies have enabled the staging of the progression of both amyloid and tangle pathologies, and, consequently, the development of diagnostic criteria that are now used worldwide. In addition, clinicopathological correlation studies have been crucial to generate hypotheses about the pathophysiology of the disease, by establishing that there is a continuum between "normal" aging and AD dementia, and that the amyloid plaque build-up occurs primarily before the onset of cognitive deficits, while neurofibrillary tangles, neuron loss, and particularly synaptic loss, parallel the progression of cognitive decline. Importantly, these cross-sectional neuropathological data have been largely validated by longitudinal in vivo studies using modern imaging biomarkers such as amyloid PET and volumetric MRI.

Systems analysis of human brain gene expression: mechanisms for HIV-associated neurocognitive impairment and common pathways with Alzheimer's disease

Background

Human Immunodeficiency Virus-1 (HIV) infection frequently results in neurocognitive impairment. While the cause remains unclear, recent gene expression studies have identified genes whose transcription is dysregulated in individuals with HIV-association neurocognitive disorder (HAND). However, the methods for interpretation of such data have lagged behind the technical advances allowing the decoding genetic material. Here, we employ systems biology methods novel to the field of NeuroAIDS to further interrogate extant transcriptome data derived from brains of HIV + patients in order to further elucidate the neuropathogenesis of HAND. Additionally, we compare these data to those derived from brains of individuals with Alzheimer’s disease (AD) in order to identify common pathways of neuropathogenesis.

Methods

In Study 1, using data from three brain regions in 6 HIV-seronegative and 15 HIV + cases, we first employed weighted gene co-expression network analysis (WGCNA) to further explore transcriptome networks specific to HAND with HIV-encephalitis (HIVE) and HAND without HIVE. We then used a symptomatic approach, employing standard expression analysis and WGCNA to identify networks associated with neurocognitive impairment (NCI), regardless of HIVE or HAND diagnosis. Finally, we examined the association between the CNS penetration effectiveness (CPE) of antiretroviral regimens and brain transcriptome. In Study 2, we identified common gene networks associated with NCI in both HIV and AD by correlating gene expression with pre-mortem neurocognitive functioning.

Results

Study 1: WGCNA largely corroborated findings from standard differential gene expression analyses, but also identified possible meta-networks composed of multiple gene ontology categories and oligodendrocyte dysfunction. Differential expression analysis identified hub genes highly correlated with NCI, including genes implicated in gliosis, inflammation, and dopaminergic tone. Enrichment analysis identified gene ontology categories that varied across the three brain regions, the most notable being downregulation of genes involved in mitochondrial functioning. Finally, WGCNA identified dysregulated networks associated with NCI, including oligodendrocyte and mitochondrial functioning. Study 2: Common gene networks dysregulated in relation to NCI in AD and HIV included mitochondrial genes, whereas upregulation of various cancer-related genes was found.

Conclusions

While under-powered, this study identified possible biologically-relevant networks correlated with NCI in HIV, and common networks shared with AD, opening new avenues for inquiry in the investigation of HAND neuropathogenesis. These results suggest that further interrogation of existing transcriptome data using systems biology methods can yield important information.

Presenilin-1 adopts pathogenic conformation in normal aging and in sporadic Alzheimer's disease

Accumulation of amyloid-β (Aβ) and neurofibrillary tangles in the brain, inflammation and synaptic and neuronal loss are some of the major neuropathological hallmarks of Alzheimer's disease (AD). While genetic mutations in amyloid precursor protein and presenilin-1 and -2 (PS1 and PS2) genes cause early-onset familial AD, the etiology of sporadic AD is not fully understood. Our current study shows that changes in conformation of endogenous wild-type PS1, similar to those found with mutant PS1, occur in sporadic AD brain and during normal aging. Using a mouse model of Alzheimer's disease (Tg2576) that overexpresses the Swedish mutation of amyloid precursor protein but has normal levels of endogenous wild-type presenilin, we report that the percentage of PS1 in a pathogenic conformation increases with age. Importantly, we found that this PS1 conformational shift is associated with amyloid pathology and precedes amyloid-β deposition in the brain. Furthermore, we found that oxidative stress, a common stress characteristic of aging and AD, causes pathogenic PS1 conformational change in neurons in vitro, which is accompanied by increased Aβ42/40 ratio. The results of this study provide important information about the timeline of pathogenic changes in PS1 conformation during aging and suggest that structural changes in PS1/γ-secretase may represent a molecular mechanism by which oxidative stress triggers amyloid-β accumulation in aging and in sporadic AD brain.

Oxidative stress in Alzheimer's disease: Primary villain or physiological by-product?

The prevalence of Alzheimer's disease (AD) is increasing rapidly worldwide due to an ageing population and largely ineffective treatments. In AD cognitive decline is due to progressive neuron loss that begins in the medial temporal lobe and spreads through many brain regions. Despite intense research the pathogenesis of the common sporadic form of AD remains largely unknown. The popular amyloid cascade hypothesis suggests that the accumulation of soluble oligomers of beta amyloid peptides (Aβ) initiates a series of events that cause neuronal loss. Among their putative toxic effects, Aβ oligomers are thought to act as pro-oxidants combining with redox-active metals to produce excessive reactive oxygen and nitrogen species. However, to date the experimental therapies that reduce Aβ load in AD have failed to halt cognitive decline. Another hypothesis proposed by the late Mark Smith and colleagues is that oxidative stress, rather than Aβ, precipitates the pathogenesis of AD. That is, Aβ and microtubule-associated protein tau are upregulated to address the redox imbalance in the AD brain. As the disease progresses, excess Aβ and tau oligomerise to further accelerate the disease process. Here, we discuss redox balance in the human brain and how this balance is affected by ageing. We then discuss where oxidative stress is most likely to act in the disease process and the potential for intervention to reduce its effects.

The neurobiology of Alzheimer disease defined by neuroimaging

PURPOSE OF REVIEW

In 2011, a new set of new guidelines for the research diagnosis of three stages of Alzheimer disease was promulgated by the US National Institute of Aging and the Alzheimer Association. For the first time, they include the diagnosis of presymptomatic Alzheimer disease, recognizing that the disease process begins years before cognitive impairment develops. Awareness of this fact has largely been driven by neuroimaging, and particularly by imaging amyloid β (abeta) deposition in the brain, a procedure approved by the US Food and Drug Administration for clinical use in April 2012.

RECENT FINDINGS

In Alzheimer disease, abeta deposition antecedes, probably by decades, the onset of cognitive impairment. In brain regions with greatest abeta deposition, synaptic dysfunction can be imaged beginning at preclinical stages. In regions that are not identical with the ones with greatest abeta deposition but heavily connected with them, regional atrophy and loss of white-matter anisotropy can be detected later in the course of the disease, near the time when mild cognitive impairment supervenes. Together with neuropsychological testing, imaging can improve the prediction of worsening to Alzheimer disease among patients with mild cognitive impairment.

SUMMARY

These findings have huge implications for research on therapeutic approaches to Alzheimer disease. For instance, while so far only patients with the clinical diagnosis have been treated with immunotherapy targeting abeta removal, a consensus is building that to be effective, this therapy should be given in the preclinical stages of the disease, which are assessed most advantageously by means of neuroimaging.

Stable size distribution of amyloid plaques over the course of Alzheimer disease

Amyloid β plaques are a key pathologic feature of Alzheimer disease (AD), but whether plaque sizes increase or stabilize over the course of AD is unknown. We measured the size distribution of total immunoreactive (10D5-positive) and dense-core (Thioflavin S-positive) plaques in the temporal neocortex of a large group of subjects with AD and age-matched plaque-bearing subjects without dementia to test the hypothesis that amyloid plaques continue to grow along with the progression of the disease. The size of amyloid β (10D5)-positive plaques did not differ between groups, whereas dense-core plaques from the group with AD were slightly larger than those from the group without dementia (∼25%-30%, p = 0.01). Within the group with AD, dense-core plaque size did not independently correlate with duration of clinical disease (from 4 to 21 years, p = 0.68), whereas 10D5-positive plaque size correlated negatively with disease duration (p = 0.01). By contrast, an earlier age of symptom onset strongly predicted a larger postmortem plaque size; this effect was independent of disease duration and the presence of the APOE[Latin Small Letter Open E]4 allele (p = 0.0001). We conclude that plaques vary in size among patients, with larger size distributions correlating with an earlier age of onset, but plaques do not substantially increase in size over the clinical course of the disease.

The epidemiological neuropathology of dementia and the implications for drug development

SUMMARY Identification of the determinants and risk factors for dementia, in addition to the underlying brain changes associated with disease, is fundamental to the development of dementia therapeutics. The gold standard to achieving this is through population-based (epidemiological) studies of the biology of aging, cognitive decline and dementia. In this paper, the main findings from epidemiological cohorts on dementia pathology are compared to case–control and convenience samples, and findings on the different neuropathological features of dementia, its risk factors and clinical course are summarized. The strengths and weakness of different research designs, the keys aspects of disease these have identified and the targets that have been subsequently developed will be discussed. It is highlighted that dementia within epidemiological frameworks is found to be a heterogeneous disease. For treatment to be effective it will therefore need to reflect not only population variation in patterns of pathology, but also intra-individual reactions to these treatments.

The challenges of tau imaging

In vivo imaging of tau pathology will provide new insights into tau deposition in the human brain, thus facilitating research into causes, diagnosis and treatment of major dementias, such as Alzheimer’s disease, or some variants of frontotemporal lobar degeneration, in which tau plays a role. Tau imaging poses several challenges, some related to the singularities of tau aggregation, and others related to radiotracer design. Several groups around the world are working on the development of imaging agents that will allow the in vivo assessment of tau deposition in aging and in neurodegeneration. Development of a tau imaging tracer will enable researchers to noninvasively examine the degree and extent of tau pathology in the brain, quantify changes in tau deposition over time, evaluate its relation to cognition and assess the efficacy of anti-tau therapy.

Apolipoprotein E4 effects in Alzheimer's disease are mediated by synaptotoxic oligomeric amyloid-β

The apolipoprotein E ε4 gene is the most important genetic risk factor for sporadic Alzheimer's disease, but the link between this gene and neurodegeneration remains unclear. Using array tomography, we analysed >50000 synapses in brains of 11 patients with Alzheimer's disease and five non-demented control subjects and found that synapse loss around senile plaques in Alzheimer's disease correlates with the burden of oligomeric amyloid-β in the neuropil and that this synaptotoxic oligomerized peptide is present at a subset of synapses. Further analysis reveals apolipoprotein E ε4 patients with Alzheimer's disease have significantly higher oligomeric amyloid-β burden and exacerbated synapse loss around plaques compared with apolipoprotein E ε3 patients. Apolipoprotein E4 protein colocalizes with oligomeric amyloid-β and enhances synaptic localization of oligomeric amyloid-β by >5-fold. Biochemical characterization shows that the amyloid-β enriched at synapses by apolipoprotein E4 includes sodium dodecyl sulphate-stable dimers and trimers. In mouse primary neuronal culture, lipidated apolipoprotein E4 enhances oligomeric amyloid-β association with synapses via a mechanism involving apolipoprotein E receptors. Together, these data suggest that apolipoprotein E4 is a co-factor that enhances the toxicity of oligomeric amyloid-β both by increasing its levels and directing it to synapses, providing a link between apolipoprotein E ε4 genotype and synapse loss, a major correlate of cognitive decline in Alzheimer's disease.

Apoptotic and non-apoptotic roles of caspases in neuronal physiology and pathophysiology

Caspases are cysteine proteases that mediate apoptosis, which is a form of regulated cell death that effectively and efficiently removes extra and unnecessary cells during development. In the mature nervous system, caspases are not only involved in mediating cell death but also regulatory events that are important for neural functions, such as axon pruning and synapse elimination, which are necessary to refine mature neuronal circuits. Furthermore, caspases can be reactivated to cause cell death as well as non-lethal changes in neurons during numerous pathological processes. Thus, although a global activation of caspases leads to apoptosis, restricted and localized activation may control normal physiology and pathophysiology in living neurons. This Review explores the multiple roles of caspase activity in neurons.

PET applications in animal models of neurodegenerative and neuroinflammatory disorders

Studies on hereditary neurological disorders such as familial Alzheimer's disease (AD) have revealed abnormalities of pathogenic proteins causative of neurodegeneration, while molecular initiators of sporadic neuropsychiatric conditions remain unidentified. Such disorders are characterized by collections of molecular abnormalities that may be critically involved in synaptic dysfunctions and other deteriorations in neurons. Diverse classes of radiochemicals designed for positron emission tomographic (PET) imaging facilitate delineation of mechanistic links among key molecules in these processes by tracking their spatiotemporal correlations. This assay technique is of particular utility when applied to rodent and nonhuman primate models given their suitability for invasive genetic and pharmacological interventions. In addition, the detection of neurochemical and neuropathological changes by PET can be examined in laboratory animals when combined with invasive antemortem and postmortem investigations such as in vivo microdialysis, electrophysiological and histopathological techniques. This review primarily covers the use of small animal models of brain disorders using PET to elucidate etiological molecular cascades to facilitate in turn the search for diagnostic and therapeutic agents applicable to AD and related disorders in humans.