Cerebellar diffuse amyloid plaques are derived from dendritic Abeta42 accumulations in Purkinje cells

Consensus Paper: Cerebellum and Ageing

Given the key roles of the cerebellum in motor, cognitive, and affective operations and given the decline of brain functions with aging, cerebellar circuitry is attracting the attention of the scientific community. The cerebellum plays a key role in timing aspects of both motor and cognitive operations, including for complex tasks such as spatial navigation. Anatomically, the cerebellum is connected with the basal ganglia via disynaptic loops, and it receives inputs from nearly every region in the cerebral cortex. The current leading hypothesis is that the cerebellum builds internal models and facilitates automatic behaviors through multiple interactions with the cerebral cortex, basal ganglia and spinal cord. The cerebellum undergoes structural and functional changes with aging, being involved in mobility frailty and related cognitive impairment as observed in the physio-cognitive decline syndrome (PCDS) affecting older, functionally-preserved adults who show slowness and/or weakness. Reductions in cerebellar volume accompany aging and are at least correlated with cognitive decline. There is a strongly negative correlation between cerebellar volume and age in cross-sectional studies, often mirrored by a reduced performance in motor tasks. Still, predictive motor timing scores remain stable over various age groups despite marked cerebellar atrophy. The cerebello-frontal network could play a significant role in processing speed and impaired cerebellar function due to aging might be compensated by increasing frontal activity to optimize processing speed in the elderly. For cognitive operations, decreased functional connectivity of the default mode network (DMN) is correlated with lower performances. Neuroimaging studies highlight that the cerebellum might be involved in the cognitive decline occurring in Alzheimer's disease (AD), independently of contributions of the cerebral cortex. Grey matter volume loss in AD is distinct from that seen in normal aging, occurring initially in cerebellar posterior lobe regions, and is associated with neuronal, synaptic and beta-amyloid neuropathology. Regarding depression, structural imaging studies have identified a relationship between depressive symptoms and cerebellar gray matter volume. In particular, major depressive disorder (MDD) and higher depressive symptom burden are associated with smaller gray matter volumes in the total cerebellum as well as the posterior cerebellum, vermis, and posterior Crus I. From the genetic/epigenetic standpoint, prominent DNA methylation changes in the cerebellum with aging are both in the form of hypo- and hyper-methylation, and the presumably increased/decreased expression of certain genes might impact on motor coordination. Training influences motor skills and lifelong practice might contribute to structural maintenance of the cerebellum in old age, reducing loss of grey matter volume and therefore contributing to the maintenance of cerebellar reserve. Non-invasive cerebellar stimulation techniques are increasingly being applied to enhance cerebellar functions related to motor, cognitive, and affective operations. They might enhance cerebellar reserve in the elderly. In conclusion, macroscopic and microscopic changes occur in the cerebellum during the lifespan, with changes in structural and functional connectivity with both the cerebral cortex and basal ganglia. With the aging of the population and the impact of aging on quality of life, the panel of experts considers that there is a huge need to clarify how the effects of aging on the cerebellar circuitry modify specific motor, cognitive, and affective operations both in normal subjects and in brain disorders such as AD or MDD, with the goal of preventing symptoms or improving the motor, cognitive, and affective symptoms.

α7 nicotinic acetylcholine receptors are necessary for basal forebrain activation to increase expression of the nerve growth factor receptor TrkA

Activation of the basal forebrain leads to increases in the expression of the nerve growth factor receptor, Tropomyosin receptor kinase A (TrkA) and decreases in expression of the beta amyloid cleavage enzyme 1 (BACE1) in the cerebral cortex of both sexes of 5xFAD mice. The studies described in this report were designed to determine if these changes were dependent on acetylcholine receptors. Mice were stimulated unilaterally in the basal forebrain for two weeks. Animals were administered a cholinergic antagonist, or saline, 30 minutes prior to stimulation. Animals administered saline exhibited significant increases in TrkA expression and decreases in BACE1 in the stimulated hemisphere relative to the unstimulated. While both nonselective nicotinic and muscarinic acetylcholine receptor blockade attenuated the BACE1 decline, only the nicotinic receptor antagonism blocked the TrkA increase. Next, we applied selective nicotinic antagonists, and the α7 antagonist blocked the TrkA increases, but the α4β2 antagonist did not. BACE1 declines were not blocked by either intervention. Mice with a loxP conditional knockout of the gene for the α7 nicotinic receptor were also employed in these studies. Animals were either stimulated bilaterally for two weeks, or left unstimulated. With or without stimulation, the expression of TrkA receptors was lower in the cortical region with the α7 nicotinic receptor knockdown. We thus conclude that α7 nicotinic receptor activation is necessary for normal expression of TrkA and increases caused by basal forebrain activation, while BACE1 declines caused by stimulation have dependency on a broader array of receptor subtypes.

Intracellular Amyloid-β in the Normal Rat Brain and Human Subjects and Its relevance for Alzheimer's Disease

BACKGROUND

Amyloid-β (Aβ) is a normal product of neuronal activity, including that of the aggregation-prone Aβ42 variant that is thought to cause Alzheimer's disease (AD). Much knowledge about AD comes from studies of transgenic rodents expressing mutated human amyloid-β protein precursor (AβPP) to increase Aβ production or the Aβ42/40 ratio. Yet, little is known about the normal expression of Aβ42 in rodent brains.

OBJECTIVE

To characterize the brain-wide expression of Aβ42 throughout the life span of outbred Wistar rats, and to relate these findings to brains of human subjects without neurological disease.

METHODS

Aβ42 immunolabeling of 12 Wistar rat brains (3-18 months of age) and brain sections from six human subjects aged 20-88 years.

RESULTS

In healthy Wistar rats, we find intracellular Aβ42 (iAβ42) in neurons throughout the brain at all ages, but levels vary greatly between brain regions. The highest levels are in neurons of entorhinal cortex layer II, alongside hippocampal neurons at the CA1/subiculum border. Concerning entorhinal cortex layer II, we find similarly high levels of iAβ42 in the human subjects.

CONCLUSION

Expression of iAβ42 in healthy Wistar rats predominates in the same structures where iAβ accumulates and Aβ plaques initially form in the much used, Wistar based McGill-R-Thy1-APP rat model for AD. The difference between wild-type Wistar rats and these AD model rats, with respect to Aβ42, is therefore quantitative rather that qualitative. This, taken together with our human results, indicate that the McGill rat model in fact models the underlying wild-type neuronal population-specific vulnerability to Aβ42 accumulation.

Distinctive Association of the Functional Connectivity of the Posterior Cingulate Cortex on Memory Performances in Early and Late Amnestic Mild Cognitive Impairment Patients

Background

Attempts have been made to explore the biological basis of neurodegeneration in the amnestic mild cognitive impairment (MCI) stage, subdivided by memory performance. However, few studies have evaluated the differential impact of functional connectivity (FC) on memory performances in early- and late-MCI patients.

Objective

This study aims to explore the difference in FC of the posterior cingulate cortex (PCC) among healthy controls (HC) (n = 37), early-MCI patients (n = 30), and late-MCI patients (n = 35) and to evaluate a group-memory performance interaction against the FC of PCC.

Methods

The subjects underwent resting-state functional MRI scanning and a battery of neuropsychological tests.

Results

A significant difference among the three groups was found in FC between the PCC (seed region) and bilateral crus cerebellum, right superior medial frontal gyrus, superior temporal gyrus, and left middle cingulate gyrus (Monte Carlo simulation-corrected p < 0.01; cluster p < 0.05). Additionally, the early-MCI patients displayed higher FC values than the HC and late-MCI patients in the right superior medial frontal gyrus, cerebellum crus 1, and left cerebellum crus 2 (Bonferroni-corrected p < 0.05). Furthermore, there was a significant group-memory performance interaction (HC vs. early MCI vs. late MCI) for the FC between PCC and bilateral crus cerebellum, right superior medial frontal gyrus, superior temporal gyrus, and left middle cingulate gyrus (Bonferroni-corrected p < 0.05).

Conclusion

These findings contribute to the biological implications of early- and late-MCI stages, categorized by evaluating the impairment of memory performance. Additionally, comprehensively analyzing the structural differences in the subdivided amnestic MCI (aMCI) stages could deepen our understanding of these biological meanings.

Cerebellar Calcium-Binding Protein and Neurotrophin Receptor Defects in Down Syndrome and Alzheimer's Disease

Cerebellar hypoplasia is a major characteristic of the Down syndrome (DS) brain. However, the consequences of trisomy upon cerebellar Purkinje cells (PC) and interneurons in DS are unclear. The present study performed a quantitative and qualitative analysis of cerebellar neurons immunostained with antibodies against calbindin D-28k (Calb), parvalbumin (Parv), and calretinin (Calr), phosphorylated and non-phosphorylated intermediate neurofilaments (SMI-34 and SMI-32), and high (TrkA) and low (p75NTR) affinity nerve growth factor (NGF) receptors as well as tau and amyloid in DS (n = 12), Alzheimer's disease (AD) (n = 10), and healthy non-dementia control (HC) (n = 8) cases. Our findings revealed higher Aβ42 plaque load in DS compared to AD and HC but no differences in APP/Aβ plaque load between HC, AD, and DS. The cerebellar cortex neither displayed Aβ40 containing plaques nor pathologic phosphorylated tau in any of the cases examined. The number and optical density (OD) measurements of Calb immunoreactive (-ir) PC soma and dendrites were similar between groups, while the number of PCs positive for Parv and SMI-32 were significantly reduced in AD and DS compared to HC. By contrast, the number of SMI-34-ir PC dystrophic axonal swellings, termed torpedoes, was significantly greater in AD compared to DS. No differences in SMI-32- and Parv-ir PC OD measurements were observed between groups. Conversely, total number of Parv- (stellate/basket) and Calr (Lugaro, brush, and Golgi)-positive interneurons were significantly reduced in DS compared to AD and HC. A strong negative correlation was found between counts for Parv-ir interneurons, Calr-ir Golgi and brush cells, and Aβ42 plaque load. Number of TrkA and p75NTR positive PCs were reduced in AD compared to HC. These findings suggest that disturbances in calcium binding proteins play a critical role in cerebellar neuronal dysfunction in adults with DS.

Cerebellar Volume Is Associated with Cognitive Decline in Mild Cognitive Impairment: Results from ADNI

Alzheimer's disease (AD) is a disease with dysfunctional brain network. Previous studies found the cerebellar volume changes over the course of AD disease progression; however, whether cerebellar volume change contributes to the cognitive decline in AD, or its earlier disease stage (i.e., mild cognitive impairment [MCI]) remains unclear. In ADNI, cognitive function was assessed using Alzheimer's Disease Assessment Scale-Cognitive Behavior section (ADAS-Cog). We used linear regression and linear mixed effects models to examine whether cerebellar volume is associated with either baseline cognition or with cognitive changes over time in MCI or in AD. We used logistic regression to assess the relationship between cerebellar volume and disease progression to MCI and AD. We found that cerebellar volume is associated with cognition in patients with MCI, after adjusting for age, gender, education, hippocampal volume, and APOE4 status. Consistently, cerebellar volume is associated with increased odds of the disease stages of MCI and AD when compared to controls. However, cerebellar volume is not associated with cognitive changes over time in either MCI or AD. In summary, cerebellar volume may contribute to cognition level in MCI, but not in AD, indicating that the cerebellar network might modulate the cognitive function in the early stage of the disease. The cerebellum may be a potential target for neuromodulation in treating MCI.

Altered microglia and neurovasculature in the Alzheimer's disease cerebellum

Traditionally regarded to coordinate movement, the cerebellum also exerts non-motor functions including the regulation of cognitive and behavioral processing, suggesting a potential role in neurodegenerative conditions affecting cognition, such as Alzheimer's disease (AD). This study aims to investigate neuropathology and AD-related molecular changes within the neocerebellum using post-mortem human brain tissue microarrays (TMAs). Immunohistochemistry was conducted on neocerebellar paraffin-embedded TMAs from 24 AD and 24 matched control cases, and free-floating neocerebellar sections from 6 AD and 6 controls. Immunoreactivity was compared between control and AD groups for neuropathological hallmarks (amyloid-β, tau, ubiquitin), Purkinje cells (calbindin), microglia (IBA1, HLA-DR), astrocytes (GFAP) basement-membrane associated molecules (fibronectin, collagen IV), endothelial cells (CD31/PECAM-1) and mural cells (PDGFRβ, αSMA). Amyloid-β expression (total immunolabel intensity) and load (area of immunolabel) was increased by >4-fold within the AD cerebellum. Purkinje cell counts, ubiquitin and tau immunoreactivity were unchanged in AD. IBA1 expression and load was increased by 91% and 69%, respectively, in AD, with no change in IBA1-positive cell number. IBA1-positive cell process length and branching was reduced by 22% and 41%, respectively, in AD. HLA-DR and GFAP immunoreactivity was unchanged in AD. HLA-DR-positive cell process length and branching was reduced by 33% and 49%, respectively, in AD. Fibronectin expression was increased by 27% in AD. Collagen IV, PDGFRβ and αSMA immunoreactivity was unchanged in AD. The number of CD31-positive vessels was increased by 98% in AD, suggesting the increase in CD31 expression and load in AD is due to greater vessel number. The PDGFRβ/CD31 load ratio was reduced by 59% in AD. These findings provide evidence of molecular changes affecting microglia and the neurovasculature within the AD neocerebellum. These changes, occurring without overt neuropathology, support the hypothesis of microglial and neurovascular dysfunction as drivers of AD, which has implications on the neocerebellar contribution to AD symptomatology and pathophysiology.

The cerebellum in Alzheimer's disease: evaluating its role in cognitive decline

The cerebellum has long been regarded as essential only for the coordination of voluntary motor activity and motor learning. Anatomical, clinical and neuroimaging studies have led to a paradigm shift in the understanding of the cerebellar role in nervous system function, demonstrating that the cerebellum appears integral also to the modulation of cognition and emotion. The search to understand the cerebellar contribution to cognitive processing has increased interest in exploring the role of the cerebellum in neurodegenerative and neuropsychiatric disorders. Principal among these is Alzheimer's disease. Here we review an already sizeable existing literature on the neuropathological, structural and functional neuroimaging studies of the cerebellum in Alzheimer's disease. We consider these observations in the light of the cognitive deficits that characterize Alzheimer's disease and in so doing we introduce a new perspective on its pathophysiology and manifestations. We propose an integrative hypothesis that there is a cerebellar contribution to the cognitive and neuropsychiatric deficits in Alzheimer's disease. We draw on the dysmetria of thought theory to suggest that this cerebellar component manifests as deficits in modulation of the neurobehavioural deficits. We provide suggestions for future studies to investigate this hypothesis and, ultimately, to establish a comprehensive, causal clinicopathological disease model.

Data-driven modeling of mitochondrial dysfunction in Alzheimer's disease

Intracellular accumulation of oligomeric forms of β amyloid (Aβ) are now believed to play a key role in the earliest phase of Alzheimer's disease (AD) as their rise correlates well with the early symptoms of the disease. Extensive evidence points to impaired neuronal Ca²⁺ homeostasis as a direct consequence of the intracellular Aβ oligomers. However, little is known about the downstream effects of the resulting Ca²⁺ rise on the many intracellular Ca²⁺-dependent pathways. Here we use multiscale modeling in conjunction with patch-clamp electrophysiology of single inositol 1,4,5-trisphosphate (IP₃) receptor (IP₃R) and fluorescence imaging of whole-cell Ca²⁺ response, induced by exogenously applied intracellular Aβ₄₂ oligomers to show that Aβ₄₂ inflicts cytotoxicity by impairing mitochondrial function. Driven by patch-clamp experiments, we first model the kinetics of IP₃R, which is then extended to build a model for the whole-cell Ca²⁺ signals. The whole-cell model is then fitted to fluorescence signals to quantify the overall Ca²⁺ release from the endoplasmic reticulum by intracellular Aβ₄₂ oligomers through G-protein-mediated stimulation of IP₃ production. The estimated IP₃ concentration as a function of intracellular Aβ₄₂ content together with the whole-cell model allows us to show that Aβ₄₂ oligomers impair mitochondrial function through pathological Ca²⁺ uptake and the resulting reduced mitochondrial inner membrane potential, leading to an overall lower ATP and increased production of reactive oxygen species and H₂O₂. We further show that mitochondrial function can be restored by the addition of Ca²⁺ buffer EGTA, in accordance with the observed abrogation of Aβ₄₂ cytotoxicity by EGTA in our live cells experiments.

A Novel Early Diagnosis System for Mild Cognitive Impairment Based on Local Region Analysis: A Pilot Study

Alzheimer's disease (AD) is an irreversible neurodegenerative disorder that accounts for 60–70% of cases of dementia in the elderly. An early diagnosis of AD is usually hampered for many reasons including the variable clinical and pathological features exhibited among affected individuals. This paper presents a computer-aided diagnosis (CAD) system with the primary goal of improving the accuracy, specificity, and sensitivity of diagnosis. In this system, PiB-PET scans, which were obtained from the ADNI database, underwent five essential stages. First, the scans were standardized and de-noised. Second, an Automated Anatomical Labeling (AAL) atlas was utilized to partition the brain into 116 regions or labels that served for local (region-based) diagnosis. Third, scale-invariant Laplacian of Gaussian (LoG) was used, per brain label, to detect the discriminant features. Fourth, the regions' features were analyzed using a general linear model in the form of a two-sample t-test. Fifth, the support vector machines (SVM) and their probabilistic variant (pSVM) were constructed to provide local, followed by global diagnosis. The system was evaluated on scans of normal control (NC) vs. mild cognitive impairment (MCI) (19 NC and 65 MCI scans). The proposed system showed superior accuracy, specificity, and sensitivity as compared to other related work.

Sparse temporally dynamic resting-state functional connectivity networks for early MCI identification

In conventional resting-state functional MRI (R-fMRI) analysis, functional connectivity is assumed to be temporally stationary, overlooking neural activities or interactions that may happen within the scan duration. Dynamic changes of neural interactions can be reflected by variations of topology and correlation strength in temporally correlated functional connectivity networks. These connectivity networks may potentially capture subtle yet short neural connectivity disruptions induced by disease pathologies. Accordingly, we are motivated to utilize disrupted temporal network properties for improving control-patient classification performance. Specifically, a sliding window approach is firstly employed to generate a sequence of overlapping R-fMRI sub-series. Based on these sub-series, sliding window correlations, which characterize the neural interactions between brain regions, are then computed to construct a series of temporal networks. Individual estimation of these temporal networks using conventional network construction approaches fails to take into consideration intrinsic temporal smoothness among successive overlapping R-fMRI sub-series. To preserve temporal smoothness of R-fMRI sub-series, we suggest to jointly estimate the temporal networks by maximizing a penalized log likelihood using a fused sparse learning algorithm. This sparse learning algorithm encourages temporally correlated networks to have similar network topology and correlation strengths. We design a disease identification framework based on the estimated temporal networks, and group level network property differences and classification results demonstrate the importance of including temporally dynamic R-fMRI scan information to improve diagnosis accuracy of mild cognitive impairment patients.

Selective Transgenic Expression of Mutant Ubiquitin in Purkinje Cell Stripes in the Cerebellum

The ubiquitin-proteasome system (UPS) is one of the major mechanisms for protein breakdown in cells, targeting proteins for degradation by enzymatically conjugating them to ubiquitin molecules. Intracellular accumulation of ubiquitin-B+1 (UBB+1), a frameshift mutant of ubiquitin-B, is indicative of a dysfunctional UPS and has been implicated in several disorders, including neurodegenerative disease. UBB+1-expressing transgenic mice display widespread labeling for UBB+1 in brain and exhibit behavioral deficits. Here, we show that UBB+1 is specifically expressed in a subset of parasagittal stripes of Purkinje cells in the cerebellar cortex of a UBB+1-expressing mouse model. This expression pattern is reminiscent of that of the constitutively expressed Purkinje cell antigen HSP25, a small heat shock protein with neuroprotective properties.

The cerebellum shrinks faster than normal ageing in Alzheimer's disease but not in mild cognitive impairment

BACKGROUND

While acceleration in age-related cerebral atrophy has been well documented in Alzheimer's disease, the cerebellar contributions to this effect have not been thoroughly investigated.

OBJECTIVE

This study investigated cerebellar volume and atrophy rate using magnetic resonance imaging in individuals with normal cognition (CN), mild cognitive impairment (MCI), and Alzheimer's disease (AD).

METHODS

Two hundred twenty-nine CN, 398 MCI and 191 AD participants of stage I ADNI database with screening scans were evaluated for cerebellar volume. Of those, 758 individuals with two or more follow-up scans were categorized into stable, converted, and reverted CN, MCI and AD and evaluated for cerebellar atrophy rate.

RESULTS

Cerebellar volume was 2.5% larger in CN than in those with AD but there were no differences between CN and MCI and MCI and AD in cross-sectional analysis. Similarly, the atrophy rate was 49% larger in AD and 64% larger in MCI who converted to AD but no difference was detected between CN and MCI. There were no association between education and APOEe4 and cerebellar volume or cerebellar atrophy across the diagnostic groups.

CONCLUSION

Cerebellar atrophy contributes to Alzheimer's clinical progression but mostly at the late stage of the disease. However, even in the late stage shrinkage rate is less than the average of the shrinkage in the cerebrum and is not associated with AD moderators. This suggests that cerebellar involvement is secondary to cerebral involvement and can be due to network connection spread regardless of the primary pathology. Hum Brain Mapp, 2017. © 2017 Wiley Periodicals, Inc. Hum Brain Mapp 38:3141-3150, 2017. © 2017 The Authors Human Brain Mapping Published by Wiley Periodicals, Inc. Hum Brain Mapp 38:3141-3150, 2017. © 2017 Wiley Periodicals, Inc.

Accumulation of amyloid-β in the cerebellar cortex of essential tremor patients

The accumulation of insoluble amyloid-beta (Aβ) peptides is associated with neurodegenerative disorders, such as Alzheimer's disease (AD). As essential tremor (ET) could involve neurodegenerative processes in the cerebellum, we quantified soluble and insoluble Aβ in cerebellar cortices from patients diagnosed with ET (n=9), compared to Controls (n=16) or individuals with Parkinson's disease (n=10). Although ante-mortem cognitive performance was not documented, all individuals included had the diagnosis of AD ruled out by a neuropathologist. ELISA-determined concentrations of insoluble Aβ42 in ET patients displayed a bimodal distribution, with a median 246-fold higher than in Controls (P<0.01, Kruskal-Wallis). Higher Aβ42 concentrations were measured in the parietal cortex of the same ET patients, compared to Controls (107-fold median increase, P<0.01, Kruskal-Wallis), but similar phosphorylated tau levels were detected. The rise in cerebellar insoluble Aβ42 concentrations is not associated to APP expression and processing or the ApoE4 status. However, Aβ42 levels in ET individuals were correlated with cerebellar insoluble phosphorylated tau (r(2)=0.71, P=0.005), unphosphorylated neurofilament heavy chain (NF-H; r(2)=0.50, P=0.030) and Lingo-1 (r(2)=0.73, P=0.007), indicative of a generalized neurodegenerative process involving the cerebellum. Our results suggest prevalent accumulations of insoluble Aβ42 in the cerebellum of ET, but not in age-matched PD. Whether this anomaly plays a role in ET symptoms warrants further investigations.

Targeting the α7 nicotinic acetylcholine receptor to prevent progressive dementia and improve cognition in adults with Down's syndrome

As persons with Down's syndrome (DS) age into the third decade and beyond, they develop Alzheimer's disease (AD)-like histopathological changes in brain and may manifest progressive worsening of adaptive functions. Increasingly, persons with DS have near-normal to normal life spans; thus, it has become a therapeutic imperative to preserve adaptive functions and ability to live as independently as possible in the least restrictive environment throughout adulthood. Data suggest that these histopathological changes and worsening adaptive functions result, at least in part, from the binding of the amyloidogenic Aβ1-42 peptide to α7 nicotinic acetylcholine receptors (α7nAChRs) on the surface of neurons, which can lead to the internalization of the tightly-bound complex and cell lysis. Pharmacotherapeutic targeting of the α7nAChR may inhibit the creation of the Aβ1-42-α7nAChR complex, which has been observed both intraneuronally and as a component of the amyloid plaque seen in AD. Additionally, selective α7nAChR agonists may improve memory and cognition independently of their potential ability to attenuate the cytotoxicity of Aβ1-42 and retard the deposition of amyloid plaques in adults with DS. However, there are conflicting data supporting an antagonist strategy to improve cognition in the presence of elevated levels of Aβ amyloidogenic peptides, as well as to prevent emergence of pyramidal neuron hyperexcitability. A major challenge to the implementation of clinical trials of targeted α7nAChR interventions in adults with DS will be the ability to detect medication-induced changes in cognition in the context of intellectual disability. The Review will consider some of the current evidence supporting both the role of the Aβ1-42-α7nAChR complex in the pathogenesis of the AD-like histopathology in adult persons with DS, and pharmacotherapeutic interventions with α7nAChR agonists.

Mitogen-activated protein kinase signaling pathways are involved in regulating α7 nicotinic acetylcholine receptor-mediated amyloid-β uptake in SH-SY5Y cells

Intraneuronal accumulation of beta-amyloid protein (Aβ) is an early pathological change in Alzheimer's disease (AD). Recent studies demonstrate that α7 nicotinic acetylcholine receptor (α7nAChR) binds to soluble Aβ with a high affinity. In vitro and in vivo experiments also show that Aβ activates p38 MAPK and ERK1/2 signaling pathways via the α7nAChR. Interestingly, it has been reported that p38 MAPK and ERK1/2 signaling pathways affect the regulation of receptor-mediated endocytosis. These data suggest that MAPK signaling pathways maybe involved in the regulation of α7nAChR-mediated Aβ uptake. However, the evidence for this hypothesis is lacking. In the present study, we examined whether Aβ1-42 oligomers activate MAPK signaling pathways via α7nAChR, and assessed the role of MAPK signaling pathways in the regulation of Aβ1-42 uptake by α7nAChR. We confirm that undifferentiated SH-SY5Y cells are capable of taking up extracellular Aβ1-42. The internalization of Aβ1-42 accumulates in the endosomes/lysosomes and mitochondria. MAPK signaling pathways are activated by Aβ1-42 via α7nAChR. Aβ1-42 and α7nAChR are co-localized in SH-SY5Y cells and the expression of α7nAChR involves in Aβ1-42 uptake and accumulation in SH-SY5Y cells. Our data demonstrate that Aβ1-42 induces an α7nAChR-dependent pathway that relates to the activation of p38 MAPK and ERK1/2, resulting in internalization of Aβ1-42. Our findings suggest that α7nAChR and MAPK signaling pathways play an important role in the uptake and accumulation of Aβ1-42 in SH-SY5Y cells. Blockade of α7nAChR may have a beneficial effect by limiting intracellular accumulation of amyloid in AD brain and serves a potential therapeutic target for AD.

Reactive oxidative species enhance amyloid toxicity in APP/PS1 mouse neurons

OBJECTIVE

To investigate whether intracellular amyloid β (iAβ) induces toxicity in wild type (WT) and APP/PS1 mice, a mouse model of Alzheimer's disease.

METHODS

Different forms of Aβ aggregates were microinjected into cultured WT or APP/PS1 mouse hippocampal neurons. TUNEL staining was performed to examine neuronal cell death. Reactive oxidative species (ROS) were measured by MitoSOXRed mitochondrial superoxide indicator.

RESULTS

Crude, monomer and protofibril Aβ induced more toxicity in APP/PS1 neurons than in WT neurons. ROS are involved in mediating the vulnerability of APP/PS1 neurons to iAβ toxicity.

CONCLUSION

Oxidative stress may mediate cell death induced by iAβ in neurons.

Elevated plasma endothelial microparticles in Alzheimer's disease

BACKGROUND/AIMS

Endothelial microparticles (EMPs) in plasma are elevated in several vascular diseases. Alzheimer's disease (AD) is associated with microcirculatory injury, capillary blocking and disruption of the blood-brain barrier. We wanted to test the hypothesis that EMPs would be increased in AD patients and would correlate with a cognitive decline, and to determine if EMPs are released as a result of activation or apoptosis/necrosis in AD.

METHODS

EMP levels in plasma of AD patients and controls were quantified by flow cytometry. EMP markers for apoptosis/necrosis [platelet/endothelial cell adhesion molecule-1 (PECAM-1)/CD31] and for activation (E-selectin/CD62e) were evaluated. The EMP CD62E/CD31 populations ratio of ≤1.0 was used to differentiate activation from apoptosis.

RESULTS

Significantly higher CD31+/CD42- and CD62e+/CD42- counts were observed in the AD group relative to the controls (p < 0.05). There was no difference between the moderate- to-severe AD group and the mild AD group. Significant correlations were found between circulating EMP counts and Mini-Mental State Examination and AD Assessment cognition (ADAS-cog) score. Multivariate regression analysis demonstrated the persistence of significant correlations between ADAS-cog score and CD31+/CD42- EMPs.

CONCLUSION

The (PECAM-1)/CD31 ratio demonstrated that EMPs were generated via apoptosis/necrosis and not by activation. Certain circulating EMP phenotypes may be associated with a cognitive decline of AD patients. EMP analysis shows a promising contribution to understanding vascular pathophysiology in AD.

Existence of different types of senile plaques between brain and spinal cord of TgCRND8 mice

Conflicting findings exist regarding the formation of diffuse and dense-core β-amyloid (Aβ) plaques in Alzheimer's disease (AD). In the present study, we characterized Aβ plaque types in the brain and spinal cord of TgCRND8 mice, which express a transgene incorporating both the Indiana mutation (V717F) and the Swedish mutations (K670N/M671L) in the human amyloid-β protein precursor (APP) gene. By combining immunohistochemistry and thioflavin S staining, we were able to define dense-core and diffuse plaques in neocortex of the brain and spinal cord of 9 week-, 5 month-, 10 month- and 20-month-old TgCRND8 mice. The senile plaques in the neocortex were predominantly dense-core plaques, even in the youngest mice. However, diffuse plaques were instead detected in spinal cord of the mice, regardless of age. Our results that relative predominance of dense-core plaques in the neocortex and diffuse plaques in the spinal cord of TgCNRD8 mice of all disease durations argue against the notion that diffuse plaques may represent an early stage in the evolution of dense-core plaques. Furthermore, we also found that the ratio of Aβ42/Aβ40 of the brain was much higher than that of the spinal cord by Aβ ELISA assay. Our findings strongly indicate that diffuse and dense-core plaques may form via independent processes in AD and Aβ42 is more prone to form dense-core plaques than is Aβ40.

Astrocyte senescence as a component of Alzheimer's disease

Aging is the main risk factor for Alzheimer’s disease (AD); however, the aspects of the aging process that predispose the brain to the development of AD are largely unknown. Astrocytes perform a myriad of functions in the central nervous system to maintain homeostasis and support neuronal function. In vitro, human astrocytes are highly sensitive to oxidative stress and trigger a senescence program when faced with multiple types of stress. In order to determine whether senescent astrocytes appear in vivo, brain tissue from aged individuals and patients with AD was examined for the presence of senescent astrocytes using p16^INK4a and matrix metalloproteinase-1 (MMP-1) expression as markers of senescence. Compared with fetal tissue samples (n = 4), a significant increase in p16^INK4a-positive astrocytes was observed in subjects aged 35 to 50 years (n = 6; P = 0.02) and 78 to 90 years (n = 11; P<10⁻⁶). In addition, the frontal cortex of AD patients (n = 15) harbored a significantly greater burden of p16^INK4a-positive astrocytes compared with non-AD adult control subjects of similar ages (n = 25; P = 0.02) and fetal controls (n = 4; P<10⁻⁷). Consistent with the senescent nature of the p16^INK4a-positive astrocytes, increased metalloproteinase MMP-1 correlated with p16^INK4a. In vitro, beta-amyloid 1–42 (Aβ_1–42) triggered senescence, driving the expression of p16^INK4a and senescence-associated beta-galactosidase. In addition, we found that senescent astrocytes produce a number of inflammatory cytokines including interleukin-6 (IL-6), which seems to be regulated by p38MAPK. We propose that an accumulation of p16^INK4a-positive senescent astrocytes may link increased age and increased risk for sporadic AD.

Cerebellar volume in patients with dementia

OBJECTIVE

The aim of this study was to examine the cerebellar volume of subjects at different stages of Alzheimer's disease and to investigate whether volume reductions in this structure are related to cognitive decline.

METHOD

Ninety-six subjects from an epidemiological study were submitted to a magnetic resonance imaging scan and evaluated using the Mini-Mental State Examination and the Functional Activities Questionnaire. Subjects were divided into five groups according to the Clinical Dementia Rating scale. Twenty-six subjects from the original group who had no dementia diagnosis at baseline were re-evaluated for the onset of dementia after two years.

RESULTS

The volumes of the cerebellar hemispheres, posterior cerebellar lobe, vermis and temporal lobe were found to be reduced as a function of the severity of the disease. There were significant positive correlations between the volume of the temporal lobe and cerebellum and the language, attention, and total scores in the Mini-Mental State Examination and the Functional Activities Questionnaire. A logistic regression analysis demonstrated that reduced temporal lobe, posterior cerebellar lobe and vermal volume at baseline is a risk factor for the onset of dementia.

CONCLUSION

This is the first study demonstrating that reduced cerebellar volume is already apparent at the predementia stage. The results of this study support the involvement of the cerebellum in the progression of dementia. Whereas the cerebellum might not be directly associated with the origin of Alzheimer's disease, it may provide useful information related to its prognosis.

Intraneuronal APP, not free Aβ peptides in 3xTg-AD mice: implications for tau versus Aβ-mediated Alzheimer neurodegeneration

Alzheimer's disease (AD) is characterized by the accumulation of intraneuronal tau and extracellular amyloid-β (Aβ) peptide. A triple transgenic (Tg) mouse (3xTg-AD) was reported to develop Aβ plaques and tau inclusions as well as remarkable accumulations of intracellular Aβ that were suggested to be the initiators of AD pathogenesis. However, it was unclear whether the anti-Aβ antibodies were able to distinguish Aβ peptide from the same Aβ epitopes within the amyloid precursor protein (APP). To further elucidate the identity of the immunoreactive intraneuronal material in 3xTg-AD mice, we conducted immunohistochemical, biochemical, and ultrastructural studies using a well characterized panel of antibodies that distinguish Aβ within APP from cleaved Aβ peptides. We found that the intraneuronal material shared epitopes with full-length APP but not free Aβ. To demonstrate unequivocally that this intraneuronal material was not free Aβ peptide, we generated 3xTg-AD mice deficient for β-secretase (BACE), the protease required for Aβ generation from APP. In the absence of Aβ production, robust intraneuronal APP immunostaining was detected in the 3xTg-AD/BACE(-/-) mice. Finally, we found that the formation of tau lesions was not different between 3xTg-AD versus 3xTg-AD/BACE(-/-) mice, thereby demonstrating that tau pathology forms independently from Aβ peptide generation in this mouse model. Although we cannot corroborate the presence of intraneuronal Aβ peptide in 3xTg-AD mice, our findings warrant further study as to the role of aberrant APP accumulation in this unique model of AD.

Deposition of hyperphosphorylated tau in cerebellum of PS1 E280A Alzheimer's disease

Early-onset familial Alzheimer's disease (AD) caused by presenilin-1 mutation E280A (PS1-E280A) presents wide clinical and neuropathological variabilities. We characterized clinically and neuropathologically PS1-E280A focusing in cerebellar involvement and compared it with early-onset sporadic Alzheimer's disease (EOSAD). Twelve E280A brains and 12 matched EOSAD brains were analyzed for beta-amyloid and hyperphosphorylated tau (pTau) morphology, beta-amyloid subspecies 1-40, 1-42 levels, pTau levels, and expression of stress kinases in frontal cortex and cerebellum. The data were correlated to clinical and genetic findings. We observed higher beta-amyloid load, beta-amyloid 1-42 and pTau concentrations in frontal cortex of PS1-E280A compared with EOSAD. High beta-amyloid load was found in the cerebellum of PS1-E280A and EOSAD patients. In PS1-E280A, beta-amyloid localized to the molecular and Purkinje cell layers, whereas EOSAD showed them in Purkinje and granular cell layers. Surprisingly, 11 out of 12 PS1-E280A patients showed deposition of pTau in the cerebellum. Also, seven out of 12 PS1-E280A patients presented cerebellar ataxia. We conclude that deposition of beta-amyloid in the cerebellum is prominent in early-onset AD irrespective of genetic or sporadic origin. The presence of pTau in cerebellum in PS1-E280A underscores the relevance of cerebellar involvement in AD and might be correlated to clinical phenotype.

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

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

Analysis of microdissected human neurons by a sensitive ELISA reveals a correlation between elevated intracellular concentrations of Abeta42 and Alzheimer's disease neuropathology

In Alzheimer's disease (AD), Purkinje neurons in the cerebellum are spared, while, for instance, pyramidal neurons in the hippocampus are neuropathologically affected. Several lines of evidence suggest that the pathogenesis could be induced by the concentration-dependent polymerization of the amyloid beta-peptide (Abeta) into extracellular oligomers. The role of intracellular Abeta is not fully investigated, but recent data indicate that also this pool could be of importance. Here, we use laser capture microdissection microscopy for isolation of Purkinje neurons from AD cases and controls, and quantify the low levels of intracellular Abeta using a novel and highly sensitive ELISA. Similar to Cornu Ammonis 1 pyramidal neurons, the intracellular levels of the most toxic variant, Abeta42, as well as the Abeta42/Abeta40 ratio, were increased in Purkinje neurons from sporadic AD cases as compared to controls. However, the levels of Abeta42 as well as Abeta40 were clearly lower in Purkinje neurons than in pyramidal neurons. Based on the volume of the captured Purkinje neurons, the intraneuronal concentrations of Abeta42 were calculated to be 200 nM in sporadic AD cases and 90 nM in controls. The corresponding concentrations in pyramidal neurons from hippocampus were 3 muM and 660 nM, respectively. The Abeta40 concentration was not significantly altered in AD cases compared to controls. However, we found ten times higher concentration of Abeta40 in pyramidal neurons (10 muM) compared to Purkinje neurons (1 muM). Finally, we suggest that high concentration of intracellular Abeta42 correlates with vulnerability to AD neuropathology.

Intracellular amyloid induces impairments on electrophysiological properties of cultured human neurons

The role of intracellular amyloid beta (iAbeta) in Alzheimer's disease (AD) initiation and progression attracts more and more attention in recent years. To address whether iAbeta induces early alterations of electrophysiological properties in cultured human primary neurons, we delivered iAbeta with adeno-virus and measured the electrophysiological properties of infected neurons with whole-cell recordings. Our results show that iAbeta induces an increase in neuronal resting membrane potentials, a decrease in K(+) currents and a hyperpolarizing shift in voltage-dependent activation of K(+) currents. These results suggest the electrophysiological impairments induced by iAbeta may be responsible for its neuronal toxicity.

The cerebellum in mild cognitive impairment and Alzheimer's disease - a structural MRI study

Neuropathological research consistently revealed the cerebellum to undergo degenerative changes in Alzheimer's disease (AD). Whether these alterations affect cerebellar morphology in vivo has not yet been investigated in a comprehensive way. Magnetic resonance imaging was performed in 20 patients with AD, 20 with mild cognitive impairment (MCI), and 20 healthy controls. By manual tracing the cerebellum was divided in four substructures (anterior lobe, superior posterior lobe, inferior posterior lobe and corpus medullare, respectively) on each hemisphere. Posterior cerebellar lobes were significantly smaller in AD patients when compared to healthy controls. In the AD group, atrophy of the posterior cerebellar regions was associated with poorer cognitive performance. Our findings lend further support for cerebellar involvement in AD.

Co-localization and distribution of cerebral APP and SP1 and its relationship to amyloidogenesis

Alzheimer's disease is characterized by amyloid-beta peptide (Abeta)-loaded plaques in the brain. Abeta is a cleavage fragment of amyloid-beta protein precursor (APP) and over production of APP may lead to amyloidogenesis. The regulatory region of the APP gene contains consensus sites recognized by the transcription factor, specificity protein 1 (SP1), which has been shown to be required for the regulation of APP and Abeta. To understand the role of SP1 in APP biogenesis, herein we have characterized the relative distribution and localization of SP1, APP, and Abeta in various brain regions of rodent and primate models using immunohistochemistry. We observed that overall distribution and cellular localization of SP1, APP, and Abeta are similar and neuronal in origin. Their distribution is abundant in various layers of neocortex, but restricted to the Purkinje cell layer of the cerebellum, and the pyramidal cell layer of hippocampus. These findings suggest that overproduction of Abeta in vivo may be associated with transcriptional pathways involving SP1 and the APP gene.

Mechanisms of amyloid plaque pathogenesis

The first ultrastructural investigations of Alzheimer's disease noted the prominence of degenerating mitochondria in the dystrophic neurites of amyloid plaques, and speculated that this degeneration might be a major contributor to plaque pathogenesis. However, the fate of these organelles has received scant consideration in the intervening decades. A number of hypotheses for the formation and progression of amyloid plaques have since been suggested, including glial secretion of amyloid, somal and synaptic secretion of amyloid-beta protein from neurons, and endosomal-lysosomal aggregation of amyloid-beta protein in the cell bodies of neurons, but none of these hypotheses fully account for the focal accumulation of amyloid in plaques. In addition to Alzheimer's disease, amyloid plaques occur in a variety of conditions, and these conditions are all accompanied by dystrophic neurites characteristic of disrupted axonal transport. The disruption of axonal transport results in the autophagocytosis of mitochondria without normal lysosomal degradation, and recent evidence from aging, traumatic injury, Alzheimer's disease and transgenic mice models of Alzheimer's disease, suggests that the degeneration of these autophagosomes may lead to amyloid production within dystrophic neurites. The theory of amyloid plaque pathogenesis has thus come full circle, back to the intuitions of the very first researchers in the field.

β-Amyloid(1–42) induces a reduction in the parallel fiber responses of Purkinje cells: Possible involvement of pro-inflammatory processes

In Alzheimer's disease there is an increased production of the toxic beta-amyloid peptides (Abeta), especially the longer forms such as Abeta(1-42). Using the patch-clamp technique we have studied the contribution of early pro-inflammatory processes to the acute effects of 1 microM Abeta(1-42) on the parallel fiber EPSC (PF-EPSC) of Purkinje cells in cerebellar slices. Abeta(1-42) induces a decrease in the PF-EPSC amplitude. This decrease is accompanied by a decrease in the frequency and amplitude of the miniature EPSCs, suggesting that Abeta acts at both pre- and post-synaptic sites. In the presence of L-NAME, a nitric oxide synthase inhibitor, the effects of Abeta were partially blocked. The frequency of mEPSCs was unchanged while Abeta still reduced the mEPSCs amplitude. The anti-inflammatory agent flurbiprofen blocked the depressant action of Abeta on the mEPSCs amplitude but not its effect on mEPSCs frequency. Both a p38 inhibitor (SB203580) and a JNK inhibitor (SP600125) reverse the effects of Abeta as an increase in the mEPSCs frequency and amplitude was observed. This study provides evidence that the Abeta-induced depression of the PF-EPSCs was mediated via an activation of JNK and p38 and by the action of NO and raises the possibility of the involvement of an early pro-inflammatory process.

The role of intracellular amyloid beta in Alzheimer's disease

Extracellular amyloid beta (Abeta) that confers neurotoxicity and modulates synaptic plasticity and memory function has been central to the amyloid hypothesis of Alzheimer's disease (AD) pathology. Like many other misfolded proteins identified in neurodegenerative disorders, Abeta also accumulates inside the AD neurons. This intracellular Abeta affects a variety of cellular physiology from protein degradation, axonal transport, autophagy to apoptosis, further documenting the role of Abeta in AD. Therapeutics targeting intracellular Abeta could be effective treatment for AD.

The ?-amyloid protein of Alzheimer?s disease binds to membrane lipids but does not bind to the ?7 nicotinic acetylcholine receptor

Accumulation of the amyloid protein (Abeta) in the brain is an important step in the pathogenesis of Alzheimer's disease. However, the mechanism by which Abeta exerts its neurotoxic effect is largely unknown. It has been suggested that the peptide can bind to the alpha7 nicotinic acetylcholine receptor (alpha7nAChR). In this study, we examined the binding of Abeta1-42 to endogenous and recombinantly expressed alpha7nAChRs. Abeta1-42 did neither inhibit the specific binding of alpha7nAChR ligands to rat brain homogenate or slice preparations, nor did it influence the activity of alpha7nAChRs expressed in Xenopus oocytes. Similarly, Abeta1-42 did not compete for alpha-bungarotoxin-binding sites on SH-SY5Y cells stably expressing alpha7nAChRs. The effect of the Abeta1-42 on tau phosphorylation was also examined. Although Abeta1-42 altered tau phosphorylation in alpha7nAChR-transfected SH-SY5Y cells, the effect of the peptide was unrelated to alpha7nAChR expression or activity. Binding studies using surface plasmon resonance indicated that the majority of the Abeta bound to membrane lipid, rather than to a protein component. Fluorescence anisotropy experiments indicated that Abeta may disrupt membrane lipid structure or fluidity. We conclude that the effects of Abeta are unlikely to be mediated by direct binding to the alpha7nAChR. Instead, we speculate that Abeta may exert its effects by altering the packing of lipids within the plasma membrane, which could, in turn, influence the function of a variety of receptors and channels on the cell surface.

Expression changes of the mRNA of Alzheimer's disease related factors in the permanent ischemic rat brain

The rat with permanent occlusion of the bilateral common carotid arteries (2VO) is useful model for the study of dementia. The expression changes of amyloid precursor protein (APP), secretase, alpha7 nicotinic acetylcholine receptor (alpha7NicR) and acetylcholine esterase (AChE), which are involved in Alzheimer's disease, were examined by quantitative RT/PCR in this model rat brain. The expression of APP, alpha7NicR and secretase were increased 4 d after 2VO. The alpha7NicR level at 2 d after operation already tended to increase. These result suggest that alpha7NicR expression was enhanced at early stage of brain ischemia. Using this model to find drugs which regulate the alpha7NicR expression will be useful to assay the materials with anti-dementive effect.

Age-related changes of intracellular Abeta in cynomolgus monkey brains

To confirm the intracellular accumulation of amyloid beta-protein (Abeta), we carefully performed immunohistochemistry using brains of cynomolgus monkeys of various ages. Cortical neurones and their large neurites were immunostained with antibodies against Abeta in young monkey brains. In aged monkey brains, intracellular Abeta localized within cortical neurones; no clear association was found between the presence of intracellular Abeta and senile plaques (SPs). Interestingly, we did not observe Abeta-immunoreactive cortical neurones in brains fixed with neutral buffered formalin. Western blot analyses of microsomal and nerve ending fractions derived from the brains of young to aged monkeys revealed that intracellular Abeta generation changed with age. In the microsomal fraction, the amount of Abeta42 significantly increased in brains from older monkeys (>30 years of age), and the amount of Abeta43 significantly decreased with age in the microsomal fraction. The amount of Abeta40 remained the same regardless of age. Biochemical analyses also showed that intracellular levels of each of these Abeta molecules significantly increased with age in nerve ending fractions. As we previously observed that a similar accumulation of presenilin1, beta-amyloid precursor protein (APP) and APP C-terminal fragment cleaved by beta-secretase in the nerve ending fractions obtained from brains with SPs, the accumulation of intracellular Abeta in this fraction may be closely related to formation of spontaneous SPs with age. Taken together, these results suggest that intensive investigation of age-related changes in the nerve ending will contribute to a better understanding of the pathogenesis of age-related neurodegenerative disorders such as sporadic Alzheimer's disease.

The contribution of microscopy to the study of Alzheimer's disease, amyloid plaques and Abeta fibrillogenesis

A broad survey is presented in this chapter, dealing with the impact that microscopy has made to the study of Alzheimer's disease, amyloid plaques and amyloid-beta fibrillogenesis. This includes classical light microscopy and the modem immunolabelling and confocal microscopies, together with the contribution of transmission electron microscopy and atomic force microscopy. Whilst usefully standing alone, the individual microscopies often contribute most effectively when they are integrated with cellular, biophysical and molecular approaches.

The microglial phagocytic role with specific plaque types in the Alzheimer disease brain

Alzheimer disease (AD) involves glial inflammation associated with amyloid plaques. The role of the microglial cells in the AD brain is controversial, as it remains unclear if the microglia form the amyloid fibrils of plaques or react to them in a macrophage-phagocytic role. Also, it is not known why microglia are preferentially associated with some amyloid plaque types. This review will provide substantial evidence to support the phagocytic role of microglia in the brain as well as explain why microglia are generally associated with specific plaque types that may be explained through their unique mechanisms of formation. In summary, the data presented suggests that plaque associated microglial activation is typically subsequent to specific amyloid plaque formations in the AD brain.

Contribution of glial cells to the development of amyloid plaques in Alzheimer's disease

Amyloid plaques appear early during Alzheimer's disease (AD), and their development is intimately linked to activated astrocytes and microglia. Astrocytes are capable of accumulating substantial amounts of neuron-derived, amyloid beta(1-42) (Abeta42)-positive material and other neuron-specific proteins as a consequence of their debris-clearing role in response to local neurodegeneration. Immunohistochemical analyses have suggested that astrocytes overburdened with these internalized materials can eventually undergo lysis, and radial dispersal of their cytoplasmic contents, including Abeta42, can lead to the deposition of a persistent residue in the form of small, GFAP-rich, astrocytic amyloid plaques, first appearing in the molecular layer of the cerebral cortex. Microglia, most of which appear to be derived from blood monocytes and recruited from local blood vessels, rapidly migrate into and congregate within neuritic and dense-core plaques, but not diffuse plaques. Instead of internalizing and removing Abeta from plaques, microglia appear to contribute to their morphological and chemical evolution by facilitating the conversion of existing soluble and oligomeric Abeta within plaques to the fibrillar form. Abeta fibrillogenesis may occur largely within tiny, tube-like invaginations in the surface plasma membrane of microglia. These results highlight the therapeutic potential of blocking the initial intracellular accumulation of Abeta42 in neurons and astrocytes and inhibiting microglia-mediated assembly of fibrillar Abeta, which is particularly resistant to degradation in Alzheimer brain.

Evidence linking neuronal cell death to autoimmunity in Alzheimer's disease

The catastrophic loss of cerebral neurons in Alzheimer's disease (AD) is not fully understood. Since serum proteins are known to extravasate into the brain parenchyma in AD due to blood-brain barrier (BBB) dysfunction, this study was designed to explore the possibility that neuronal cell death may be the consequence of the anomalous presence of serum proteins in the brain. As compared to age-matched, non-demented 'control' brain tissues, highly significant increases of immunoglobulins (Igs) were detected in parenchyma, which were associated with vessels in the AD brain tissues. Also, there were dramatic increases of +Ig-neurons in areas with greater parenchymal Ig reactivity. The Ig labeling extended throughout the cell, which showed neurodegenerative apoptotic features that were not observed in -Ig-neurons. Thus, the presence of +Ig-neurons in AD brains implies a critical link between the faulty BBB and neuronal death through an autoimmune mechanism.

Presenilin-1, nicastrin, amyloid precursor protein, and gamma-secretase activity are co-localized in the lysosomal membrane

Alzheimer's disease (AD) is caused by the cerebral deposition of beta-amyloid (Abeta), a 38-43-amino acid peptide derived by proteolytic cleavage of the amyloid precursor protein (APP). Initial studies indicated that final cleavage of APP by the gamma-secretase (a complex containing presenilin and nicastrin) to produce Abeta occurred in the endosomal/lysosomal system. However, other studies showing a predominant endoplasmic reticulum localization of the gamma-secretase proteins and a neutral pH optimum of in vitro gamma-secretase assays have challenged this conclusion. We have recently identified nicastrin as a major lysosomal membrane protein. In the present work, we use Western blotting and immunogold electron microscopy to demonstrate that significant amounts of mature nicastrin, presenilin-1, and APP are co-localized with lysosomal associated membrane protein-1 (cAMP-1) in the outer membranes of lysosomes. Furthermore, we demonstrate that these membranes contain an acidic gamma-secretase activity, which is immunoprecipitable with an antibody to nicastrin. These experiments establish APP, nicastrin, and presenilin-1 as resident lysosomal membrane proteins and indicate that gamma-secretase is a lysosomal protease. These data reassert the importance of the lysosomal/endosomal system in the generation of Abeta and suggest a role for lysosomes in the pathophysiology of AD.

The use of formic acid to embellish amyloid plaque detection in Alzheimer's disease tissues misguides key observations

We compared our heat pretreatment method to the widely used formic acid pretreatment technique to immunohistochemically detect amyloid in control and Alzheimer's disease brain tissues. Both methods detected amyloid in plaques, neurons, ependymal cells, circulating monocytes, vascular smooth muscle and endothelial cells. Although there were no observable differences in the intensity of the amyloid labeling in these cell types using both pretreatment methods, there were considerable differences in the intensity of amyloid immunolabeling in the plaques. The formic acid produced much more intense amyloid labeling in the plaques than the heat method. With the heat method, the intensity of the amyloid labeling in the plaques was similar to that detected in nearby neurons suggesting a neuronal origin of plaques. Conversely, the intensity of the amyloid in nearby neurons and plaques was drastically different using the formic acid suggesting unique origins of amyloid. The obvious benefits of formic acid for increasing the sensitivity of amyloid plaque immunolabeling may artifactually emphasize plaques over amyloid-containing cells during analyses.

Astrocytes accumulate A beta 42 and give rise to astrocytic amyloid plaques in Alzheimer disease brains

beta-Amyloid(1-42) (A beta 42), a major component of amyloid plaques, accumulates within pyramidal neurons in the brains of individuals with Alzheimer's disease (AD) and Down syndrome. In brain areas exhibiting AD pathology, A beta 42-immunopositive material is observed in astrocytes. In the present study, single- and double-label immunohistochemistry were used to reveal the origin and fate of this material in astrocytes. Our findings suggest that astrocytes throughout the entorhinal cortex of AD patients gradually accumulate A beta 42-positive material and that the amount of this material correlates positively with the extent of local AD pathology. A beta 42-positive material within astrocytes appears to be of neuronal origin, most likely accumulated via phagocytosis of local degenerated dendrites and synapses, especially in the cortical molecular layer. The co-localization of neuron-specific proteins, alpha 7 nicotinic acetylcholine receptor and choline acetyltransferase, in A beta 42-burdened, activated astrocytes supports this possibility. Our results also suggest that some astrocytes containing A beta 42-positive deposits undergo lysis, resulting in the formation of astrocyte-derived amyloid plaques in the cortical molecular layer in brain regions showing moderate to advanced AD pathology. These astrocytic plaques can be distinguished from those arising from neuronal lysis by virtue of their smaller size, their nearly exclusive localization in the subpial portion of the molecular layer of the cerebrocortex, and by their intense glial fibrillary acidic protein immunoreactivity. Overall, A beta 42 accumulation and the selective lysis of A beta 42-burdened neurons and astrocytes appear to make a major contribution to the observed amyloid plaques in AD brains.

Beta-amyloid and cholinergic neurons

It is generally accepted that the crucial events in the pathogeny of Alzheimer's disease (AD) are the increased accumulation of amyloidogenic peptides derived from amyloid precursor protein and the harmful actions of these peptides on neurons, which bring about neurodegeneration. The enhanced beta-amyloid accumulation is known to be caused by mutations of specific genes in patients who suffer from the familial (hereditary) form of AD but who represent just a minor group within the total population of AD patients. The reasons for beta-amyloid accumulation are not known in the much larger group of patients with the sporadic form of the disease. A biochemical feature common to either form of the disease is the preferential atrophy and degeneration of cholinergic neurons, which is probably responsible for much of the cognitive decline characteristic of the disease. We present an overview of recent investigations on the interactions between beta-amyloid and cholinergic neurons.

Amyloid beta(1-42) peptide alters the gating of human and mouse alpha-bungarotoxin-sensitive nicotinic receptors

The beta-amyloid(1-42) peptide (Abeta(1-42)), a major constituent of the Alzheimer's disease amyloid plaque, specifically binds to the neuronal alpha-bungarotoxin (alpha-BuTx)-sensitive alpha7 nicotinic acetylcholine receptor (alpha7 nAChR). Accordingly, Abeta1-42 interferes with the function of alpha7 nAChRs in chick and rodent neurons. To gain insights into the human disease, we studied the action of Abeta(1-42) on human alpha7 nAChRs expressed in Xenopus oocytes. In voltage-clamped oocytes expressing the wild-type receptor, Abeta(1-42) blocked ACh-evoked currents. The block was non-competitive, required over 100 s to develop and was partially reversible. In oocytes expressing the mutant L248T receptor, Abeta(1-42) activated methyllycaconitine-sensitive currents in a dose-dependent manner. Peptide-evoked unitary events, recorded in outside-out patches, showed single-channel conductances and open duration comparable to ACh-evoked events. Abeta(1-42) had no effect on the currents evoked by glutamate, GABA or glycine in oocytes expressing human or mouse receptors for these transmitters. Muscle nAChRs are also alpha-BuTx-sensitive and we therefore investigated whether they respond to Abeta(1-42). In human kidney BOSC 23 cells expressing the fetal or adult mouse muscle nAChRs, Abeta(1-42) blocked ACh-evoked whole-cell currents, accelerating their decay. Outside-out single-channel recordings showed that the block was due to a reduced channel open probability and enhanced block upon ACh application. We also report that the inverse peptide Abeta(42-1), but not Abeta(40-1), partially mimicked the effects of the physiological Abeta(1-42) peptide. Possible implications for degenerative neuronal and muscular diseases are discussed.

BetaAPP and furin mRNA concentrates in immature senile plaques in the brain of Alzheimer patients

This study examined the possibility that in Alzheimer disease (AD) beta-amyloid precursor protein (betaAPP) mRNA is delivered to senile plaques (SPs) via dendritic processes. BetaAPP mRNA was detected in SPs by in situ hybridization, using a 1.4-kb cRNA in which both [35S]-UTP and [35S]-CTP were incorporated together. The betaAPP mRNA was compared with that of furin, a proteolytic enzyme putatively involved in betaAPP processing, and its orthologue proprotein convertase PCI served as a control. Human presenile AD cases with mostly immature SPs and AD cases generally with mature SPs were analyzed. To decrypt SPs after hybridization, brain sections were stained with thioflavin S. To establish relationships between the density of dystrophic fibers, the degree of plaque maturation, and the concentration of mRNA in SPs, the plaque maturity markers Abeta(1-42) and Abeta(1-40) peptides were co-localized with neurofilament protein 200 and compared with microtubule-associated protein 2 (MAP 2). The results suggest that immature, Abeta(1-42)- and dystrophic dendrite-containing SPs (but not mature SPs containing Abeta(1-40) and missing dystrophic dendrites) are capable of concentrating specific mRNAs. Dystrophic dendrites may thus serve as a route for the transport of specific mRNAs from the cell bodies to SPs.

Interaction of intracellular beta amyloid peptide with chaperone proteins

Expression of the human beta amyloid peptide (A beta) in transgenic Caenorhabditis elegans animals can lead to the formation of intracellular immunoreactive deposits as well as the formation of intracellular amyloid. We have used this model to identify proteins that interact with intracellular A beta in vivo. Mass spectrometry analysis of proteins that specifically coimmunoprecipitate with A beta has identified six likely chaperone proteins: two members of the HSP70 family, three alpha B-crystallin-related small heat shock proteins (HSP-16s), and a putative ortholog of a mammalian small glutamine-rich tetratricopeptide repeat-containing protein proposed to regulate HSP70 function. Quantitative reverse transcription-PCR analysis shows that the small heat shock proteins are also transcriptionally induced by A beta expression. Immunohistochemistry demonstrates that HSP-16 protein closely colocalizes with intracellular A beta in this model. Transgenic animals expressing a nonaggregating A beta variant, a single-chain A beta dimer, show an altered pattern of coimmunoprecipitating proteins and an altered cellular distribution of HSP-16. Double-stranded RNA inhibition of R05F9.10, the putative C. elegans ortholog of the human small glutamine-rich tetratricopeptide-repeat-containing protein (SGT), results in suppression of toxicity associated with A beta expression. These results suggest that chaperone function can play a role in modulating intracellular A beta metabolism and toxicity.

Intracellular accumulation of beta-amyloid(1-42) in neurons is facilitated by the alpha 7 nicotinic acetylcholine receptor in Alzheimer's disease

Amyloid beta(1-42), a major component of amyloid plaques, binds with exceptionally high affinity to the alpha 7 nicotinic acetylcholine receptor and accumulates intracellularly in neurons of Alzheimer's disease brains. In this study, we investigated the possibility that this binding plays a key role in facilitating intraneuronal accumulation of amyloid beta(1-42). Consecutive section immunohistochemistry and digital imaging were used to reveal the spatial relationship between amyloid beta(1-42) and the alpha 7 receptor in affected neurons of Alzheimer's disease brains. Results showed that neurons containing substantial intracellular accumulations of amyloid beta(1-42) invariably express relatively high levels of the alpha 7 receptor. Furthermore, this receptor is highly co-localized with amyloid beta(1-42) within neurons of Alzheimer's disease brains. To experimentally test the possibility that the binding interaction between exogenous amyloid beta(1-42) and the alpha 7 receptor facilitates internalization and intracellular accumulation of amyloid beta(1-42) in Alzheimer's disease brains, we studied the fate of exogenous amyloid beta(1-42) and its interaction with the alpha 7 receptor in vitro using cultured, transfected neuroblastoma cells that express elevated levels of this receptor. Transfected cells exhibited rapid binding, internalization and accumulation of exogenous amyloid beta(1-42), but not amyloid beta(1-40). Furthermore, the rate and extent of amyloid beta(1-42) internalization was related directly to the alpha 7 receptor protein level, since (1) the rate of amyloid beta(1-42) accumulation was much lower in untransfected cells that express much lower levels of this receptor and (2) internalization was effectively blocked by alpha-bungarotoxin, an alpha 7 receptor antagonist. As in neurons of Alzheimer's disease brains, the alpha 7 receptor in transfected cells was precisely co-localized with amyloid beta(1-42) in prominent intracellular aggregates. Internalization of amyloid beta(1-42) in transfected cells was blocked by phenylarsine oxide, an inhibitor of endocytosis. We suggest that the intraneuronal accumulation of amyloid beta(1-42) in Alzheimer's disease brains occurs predominantly in neurons that express the alpha 7 receptor. In addition, internalization of amyloid beta(1-42) may be facilitated by the high-affinity binding of amyloid beta(1-42) to the alpha 7 receptor on neuronal cell surfaces, followed by endocytosis of the resulting complex. This provides a plausible explanation for the selective vulnerability of neurons expressing the alpha 7 receptor in Alzheimer's disease brains and for the fact that amyloid beta(1-42) is the dominant amyloid beta peptide species in intracellular accumulations and amyloid plaques.