Apolipoprotein E deposition and astrogliosis are associated with maturation of beta-amyloid plaques in betaAPPswe transgenic mouse: Implications for the pathogenesis of Alzheimer's disease

Characterization by Nano-Infrared Spectroscopy of Individual Aggregated Species of Amyloid Proteins

Amyloid fibrils are composed of aggregated peptides or proteins in a fibrillar structure with a higher β-sheet content than in their native structure. To characterize them, we used an innovative tool that coupled infrared spectroscopy with atomic force microscopy (AFM-IR). With this method, we show that we can detect different individual aggregated species from oligomers to fibrils and study their morphologies by AFM and their secondary structures based on their IR spectra. AFM-IR overcomes the weak spatial resolution of usual infrared spectroscopy and achieves a resolution of ten nanometers, the size of isolated fibrils. We characterized oligomers, amyloid fibrils of Aβ42 and fibrils of α-synuclein. To our surprise, we figured out that the nature of some surfaces (ZnSe) used to study the samples induces destructuring of amyloid samples, leading to amorphous aggregates. We strongly suggest taking this into consideration in future experiments with amyloid fibrils. More importantly, we demonstrate the advantages of AFM-IR, with a high spatial resolution (≤ 10 nm) allowing spectrum recording on individual aggregated supramolecular entities selected thanks to the AFM images or on thin layers of proteins.

Pyroglutamation of amyloid-βx-42 (Aβx-42) followed by Aβ1-40 deposition underlies plaque polymorphism in progressing Alzheimer's disease pathology

Amyloid-β (Aβ) pathology in Alzheimer's disease (AD) is characterized by the formation of polymorphic deposits comprising diffuse and cored plaques. Because diffuse plaques are predominantly observed in cognitively unaffected, amyloid-positive (CU-AP) individuals, pathogenic conversion into cored plaques appears to be critical to AD pathogenesis. Herein, we identified the distinct Aβ species associated with amyloid polymorphism in brain tissue from individuals with sporadic AD (s-AD) and CU-AP. To this end, we interrogated Aβ polymorphism with amyloid conformation–sensitive dyes and a novel in situ MS paradigm for chemical characterization of hyperspectrally delineated plaque morphotypes. We found that maturation of diffuse into cored plaques correlated with increased Aβ1–40 deposition. Using spatial in situ delineation with imaging MS (IMS), we show that Aβ1–40 aggregates at the core structure of mature plaques, whereas Aβ1–42 localizes to diffuse amyloid aggregates. Moreover, we observed that diffuse plaques have increased pyroglutamated Aβx-42 levels in s-AD but not CU-AP, suggesting an AD pathology–related, hydrophobic functionalization of diffuse plaques facilitating Aβ1–40 deposition. Experiments in tgAPP_Swe mice verified that, similar to what has been observed in human brain pathology, diffuse deposits display higher levels of Aβ1–42 and that Aβ plaque maturation over time is associated with increases in Aβ1–40. Finally, we found that Aβ1–40 deposition is characteristic for cerebral amyloid angiopathy deposition and maturation in both humans and mice. These results indicate that N-terminal Aβx-42 pyroglutamation and Aβ1–40 deposition are critical events in priming and maturation of pathogenic Aβ from diffuse into cored plaques, underlying neurotoxic plaque development in AD.

Astrocyte dysfunction in Alzheimer disease

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

Astrocytes and synaptic plasticity in health and disease

Activity-dependent synaptic plasticity phenomena such as long-term potentiation and long-term depression are candidate mechanisms for storing information in the brain. Regulation of synaptic plasticity is critical for healthy cognition and learning and this is provided in part by metaplasticity, which can act to maintain synaptic transmission within a dynamic range and potentially prevent excitotoxicity. Metaplasticity mechanisms also allow neurons to integrate plasticity-associated signals over time. Interestingly, astrocytes appear to be critical for certain forms of synaptic plasticity and metaplasticity mechanisms. Synaptic dysfunction is increasingly viewed as an early feature of AD that is correlated with the severity of cognitive decline, and the development of these pathologies is correlated with a rise in reactive astrocytes. This review focuses on the contributions of astrocytes to synaptic plasticity and metaplasticity in normal tissue, and addresses whether astroglial pathology may lead to aberrant engagement of these mechanisms in neurological diseases such as Alzheimer's disease.

Ablation of Type-1 IFN Signaling in Hematopoietic Cells Confers Protection Following Traumatic Brain Injury

Type-1 interferons (IFNs) are pleiotropic cytokines that signal through the type-1 IFN receptor (IFNAR1). Recent literature has implicated the type-1 IFNs in disorders of the CNS. In this study, we have investigated the role of type-1 IFNs in neuroinflammation following traumatic brain injury (TBI). Using a controlled cortical impact model, TBI was induced in 8- to 10-week-old male C57BL/6J WT and IFNAR1^−/− mice and brains were excised to study infarct volume, inflammatory mediator release via quantitative PCR analysis and immune cell profile via immunohistochemistry. IFNAR1^−/− mice displayed smaller infarcts compared with WT mice after TBI. IFNAR1^−/− mice exhibited an altered anti-inflammatory environment compared with WT mice, with significantly reduced levels of the proinflammatory mediators TNFα, IL-1β and IL-6, an up-regulation of the anti-inflammatory mediator IL-10 and an increased activation of resident and peripheral immune cells after TBI. WT mice injected intravenously with an anti-IFNAR1 blocking monoclonal antibody (MAR1) 1 h before, 30 min after or 30 min and 2 d after TBI displayed significantly improved histological and behavioral outcome. Bone marrow chimeras demonstrated that the hematopoietic cells are a peripheral source of type-1 IFNs that drives neuroinflammation and a worsened TBI outcome. Type-1 IFN mRNA levels were confirmed to be significantly altered in human postmortem TBI brains. Together, these data demonstrate that type-1 IFN signaling is a critical pathway in the progression of neuroinflammation and presents a viable therapeutic target for the treatment of TBI.

Use of magnetization transfer contrast MRI to detect early molecular pathology in Alzheimer's disease

PURPOSE

The purpose of this study was to determine if magnetization transfer contrast (MTC) imaging could be used to detect early macromolecular accumulation in a mouse model of early Alzheimer's disease.

METHODS

We obtained MTC images at 9.4 T at three different age points in the Tg2576 mouse model of Alzheimer's disease. The Tg2576 mouse exhibits increased amyloid beta deposition that eventually progresses into amyloid beta plaque formation, increased hyper-phosphorylated tau but does not exhibit neurodegeneration.

RESULTS

Our results show an increase in the MTC signal that predates plaque formation and reported learning and memory deficits in the Tg2576 mouse. This increase in the MTC signal was reversed in a model of antioxidant therapy.

CONCLUSION

MTC magnetic resonance imaging can be used to detect early macromolecular changes in the Tg2576 mouse model of Alzheimer's disease. The source of the MTC contrast is likely complex and warrants further investigation in additional preclinical models that represent early and late stage Alzheimer's disease pathologies.

The contribution of activated astrocytes to Aβ production: implications for Alzheimer's disease pathogenesis

Background

β-Amyloid (Aβ) plays a central role in Alzheimer's disease (AD) pathogenesis. Neurons are major sources of Aβ in the brain. However, astrocytes outnumber neurons by at least five-fold. Thus, even a small level of astrocytic Aβ production could make a significant contribution to Aβ burden in AD. Moreover, activated astrocytes may increase Aβ generation. β-Site APP cleaving enzyme 1 (BACE1) cleavage of amyloid precursor protein (APP) initiates Aβ production. Here, we explored whether pro-inflammatory cytokines or Aβ42 would increase astrocytic levels of BACE1, APP, and β-secretase processing, implying a feed-forward mechanism of astrocytic Aβ production.

Methods

Mouse primary astrocytes were treated with combinations of LPS, TNF-α, IFN-γ, and IL-1β and analyzed by immunoblot and ELISA for endogenous BACE1, APP, and secreted Aβ40 levels. Inhibition of JAK and iNOS signaling in TNF-α+IFN-γ-stimulated astrocytes was also analyzed. In addition, C57BL/6J or Tg2576 mouse astrocytes were treated with oligomeric or fibrillar Aβ42 and analyzed by immunoblot for levels of BACE1, APP, and APPsβsw. Astrocytic BACE1 and APP mRNA levels were measured by TaqMan RT-PCR.

Results

TNF-α+IFN-γ stimulation significantly increased levels of astrocytic BACE1, APP, and secreted Aβ40. BACE1 and APP elevations were post-transcriptional at early time-points, but became transcriptional with longer TNF-α+IFN-γ treatment. Despite a ~4-fold increase in astrocytic BACE1 protein level following TNF-α+IFN-γ stimulation, BACE1 mRNA level was significantly decreased suggesting a post-transcriptional mechanism. Inhibition of iNOS and JAK did not reduce TNF-α+IFN-γ-stimulated elevation of astrocytic BACE1, APP, and Aβ40, except that JAK inhibition blocked the APP increase. Finally, oligomeric and fibrillar Aβ42 dramatically increased levels of astrocytic BACE1, APP, and APPsβsw through transcriptional mechanisms, at least in part.

Conclusions

Cytokines including TNF-α+IFN-γ increase levels of endogenous BACE1, APP, and Aβ and stimulate amyloidogenic APP processing in astrocytes. Oligomeric and fibrillar Aβ42 also increase levels of astrocytic BACE1, APP, and β-secretase processing. Together, our results suggest a cytokine- and Aβ42-driven feed-forward mechanism that promotes astrocytic Aβ production. Given that astrocytes greatly outnumber neurons, activated astrocytes may represent significant sources of Aβ during neuroinflammation in AD.

Cholesterol metabolism in neurons and astrocytes

Cells in the mammalian body must accurately maintain their content of cholesterol, which is an essential membrane component and precursor for vital signalling molecules. Outside the brain, cholesterol homeostasis is guaranteed by a lipoprotein shuttle between the liver, intestine and other organs via the blood circulation. Cells inside the brain are cut off from this circuit by the blood-brain barrier and must regulate their cholesterol content in a different manner. Here, we review how this is accomplished by neurons and astrocytes, two cell types of the central nervous system, whose cooperation is essential for normal brain development and function. The key observation is a remarkable cell-specific distribution of proteins that mediate different steps of cholesterol metabolism. This form of metabolic compartmentalization identifies astrocytes as net producers of cholesterol and neurons as consumers with unique means to prevent cholesterol overload. The idea that cholesterol turnover in neurons depends on close cooperation with astrocytes raises new questions that need to be addressed by new experimental approaches to monitor and manipulate cholesterol homeostasis in a cell-specific manner. We conclude that an understanding of cholesterol metabolism in the brain and its role in disease requires a close look at individual cell types.

Inhibition of EGF-induced ERK/MAP kinase-mediated astrocyte proliferation by mu opioids: integration of G protein and beta-arrestin 2-dependent pathways

Although micro, kappa, and delta opioids activate extracellular signal-regulated kinase (ERK)/mitogen-activated protein (MAP) kinase, the mechanisms involved in their signaling pathways and the cellular responses that ensue differ. Here we focused on the mechanisms by which micro opioids rapidly (min) activate ERK and their slower (h) actions to inhibit epidermal growth factor (EGF)-induced ERK-mediated astrocyte proliferation. The micro-opioid agonists ([d-ala(2), mephe(4), gly-ol(5)] enkephalin and morphine) promoted the phosphorylation of ERK/MAP kinase within 5 min via G(i/o) protein, calmodulin (CaM), and beta-arrestin2-dependent signaling pathways in immortalized and primary astrocytes. This was based on the attenuation of the micro-opioid activation of ERK by pertussis toxin (PTX), the CaM antagonist, W-7, and siRNA silencing of beta-arrestin2. All three pathways were shown to activate ERK via an EGF receptor transactivation-mediated mechanism. This was disclosed by abolishment of micro-opioid-induced ERK phosphorylation with the EGF receptor-specific tyrosine phosphorylation inhibitor, AG1478, and micro-opioid-induced reduction of EGF receptor tyrosine phosphorylation by PTX, and beta-arrestin2 targeting siRNA in the present studies and formerly by CaM antisense. Long-term (h) treatment of primary astrocytes with [d-ala(2),mephe(4),gly-ol(5)] enkephalin or morphine, attenuated EGF-induced ERK phosphorylation and proliferation (as measured by 5'-bromo-2'-deoxy-uridine labeling). PTX and beta-arrestin2 siRNA but not W-7 reversed the micro-opioid inhibition. Unexpectedly, beta-arrestin-2 siRNA diminished both EGF-induced ERK activation and primary astrocyte proliferation suggesting that this adaptor protein plays a novel role in EGF signaling as well as in the opioid receptor phase of this pathway. The results lend insight into the integration of the different micro-opioid signaling pathways to ERK and their cellular responses.

Kappa opioids promote the proliferation of astrocytes via Gbetagamma and beta-arrestin 2-dependent MAPK-mediated pathways

GTP binding regulatory protein (G protein)-coupled receptors can activate MAPK pathways via G protein-dependent and -independent mechanisms. However, the physiological outcomes correlated with the cellular signaling events are not as well characterized. In this study, we examine the involvement of G protein and beta-arrestin 2 pathways in kappa opioid receptor-induced, extracellular signal-regulated kinase 1/2 (ERK1/2)-mediated proliferation of both immortalized and primary astrocyte cultures. As different agonists induce different cellular signaling pathways, we tested the prototypic kappa agonist, U69593 as well as the structurally distinct, non-nitrogenous agonist, C(2)-methoxymethyl salvinorin B (MOM-Sal-B). In immortalized astrocytes, U69593, activated ERK1/2 by a rapid (min) initial stimulation that was sustained over 2 h and increased proliferation. Sequestration of activated Gbetagamma subunits attenuated U69593 stimulation of ERK1/2 and suppressed proliferation in these cells. Furthermore, small interfering RNA silencing of beta-arrestin 2 diminished sustained ERK activation induced by U69593. In contrast, MOM-Sal-B induced only the early phase of ERK1/2 phosphorylation and did not affect proliferation of immortalized astrocytes. In primary astrocytes, U69593 produced the same effects as seen in immortalized astrocytes. MOM-Sal-B elicited sustained ERK1/2 activation which was correlated with increased primary astrocyte proliferation. Proliferative actions of both agonists were abolished by either inhibition of ERK1/2, Gbetagamma subunits or beta-arrestin 2, suggesting that both G protein-dependent and -independent ERK pathways are required for this outcome.

Loss of astrocytic domain organization in the epileptic brain

Gliosis is a pathological hallmark of posttraumatic epileptic foci, but little is known about these reactive astrocytes beyond their high glial fibrillary acidic protein (GFAP) expression. Using diolistic labeling, we show that cortical astrocytes lost their nonoverlapping domain organization in three mouse models of epilepsy: posttraumatic injury, genetic susceptibility, and systemic kainate exposure. Neighboring astrocytes in epileptic mice showed a 10-fold increase in overlap of processes. Concurrently, spine density was increased on dendrites of excitatory neurons. Suppression of seizures by the common antiepileptic, valproate, reduced the overlap of astrocytic processes. Astrocytic domain organization was also preserved in APP transgenic mice expressing a mutant variant of human amyloid precursor protein despite a marked upregulation of GFAP. Our data suggest that loss of astrocytic domains was not universally associated with gliosis, but restricted to seizure pathologies. Reorganization of astrocytes may, in concert with dendritic sprouting and new synapse formation, form the structural basis for recurrent excitation in the epileptic brain.

Decreased nicotinic receptors and cognitive deficit in rats intracerebroventricularly injected with beta-amyloid peptide(1-42) and fed a high-cholesterol diet

To investigate whether the changes in nicotinic receptors (nAChRs) and in learning and memory associated with Alzheimer's disease (AD) are influenced by both beta-amyloid peptide (Abeta) and cholesterol in vivo, we examined the effects of intracerebroventricular injection of Abeta(1-42) and/or a high-cholesterol diet on brain levels of nAChRs and learning and memory in rats. The levels of nAChR subunit proteins and the corresponding mRNA were measured by Western blotting and RT-PCR, respectively; and learning and memory were evaluated with the Morris Water Maze examination. Injection of Abeta(1-42) resulted in deposition of this peptide, activation of astrocytes, decreased levels of the alpha7 and alpha4 protein subunits of the nAChR, and elevated expression of alpha7 mRNA, as well as impaired learning and spatial memory. A high-cholesterol diet activated astrocytes and, more importantly, potentiated the toxic effects of Abeta on nAChR subunit levels and on learning and memory. These findings may be highly relevant to the mechanisms underlying the cognitive deficits associated with AD.

Omega-3 fatty acid docosahexaenoic acid increases SorLA/LR11, a sorting protein with reduced expression in sporadic Alzheimer's disease (AD): relevance to AD prevention

Environmental and genetic factors, notably ApoE4, contribute to the etiology of late-onset Alzheimer's disease (LOAD). Reduced mRNA and protein for an apolipoprotein E (ApoE) receptor family member, SorLA (LR11) has been found in LOAD but not early-onset AD, suggesting that LR11 loss is not secondary to pathology. LR11 is a neuronal sorting protein that reduces amyloid precursor protein (APP) trafficking to secretases that generate beta-amyloid (Abeta). Genetic polymorphisms that reduce LR11 expression are associated with increased AD risk. However these polymorphisms account for only a fraction of cases with LR11 deficits, suggesting involvement of environmental factors. Because lipoprotein receptors are typically lipid-regulated, we postulated that LR11 is regulated by docosahexaenoic acid (DHA), an essential omega-3 fatty acid related to reduced AD risk and reduced Abeta accumulation. In this study, we report that DHA significantly increases LR11 in multiple systems, including primary rat neurons, aged non-Tg mice and an aged DHA-depleted APPsw AD mouse model. DHA also increased LR11 in a human neuronal line. In vivo elevation of LR11 was also observed with dietary fish oil in young rats with insulin resistance, a model for type II diabetes, another AD risk factor. These data argue that DHA induction of LR11 does not require DHA-depleting diets and is not age dependent. Because reduced LR11 is known to increase Abeta production and may be a significant genetic cause of LOAD, our results indicate that DHA increases in SorLA/LR11 levels may play an important role in preventing LOAD.

Role of apolipoprotein E4 in protecting children against early childhood diarrhea outcomes and implications for later development

Our group and others have reported a series of studies showing that heavy burdens of diarrheal diseases in the formative first two years of life in children in urban shantytowns have profound consequences of impaired physical and cognitive development lasting into later childhood and schooling. Based on these previous studies showing that apolipoprotein E4 (APOE4) is relatively common in favela children, we review recent data suggesting a protective role for the APOE4 allele in the cognitive and physical development of children with heavy burdens of diarrhea in early childhood. Despite being a marker for cognitive decline with Alzheimer's and cardiovascular diseases later in life, APOE4 appears to be important for cognitive development under the stress of heavy diarrhea. The reviewed findings provide a potential explanation for the survival advantage in evolution of the thrifty APOE4 allele and raise questions about its implications for human development under life-style changes and environmental challenges.

Alzheimer's disease-like neuropathology of gene-targeted APP-SLxPS1mut mice expressing the amyloid precursor protein at endogenous levels

Most transgenic mice used for preclinical evaluation of potential disease-modifying treatments of Alzheimer's disease develop major histopathological features of this disease by several-fold overexpression of the human amyloid precursor protein. We studied the phenotype of three different strains of gene-targeted mice which express the amyloid precursor protein at endogenous levels. Only further crossing with transgenic mice overexpressing mutant human presenilin1 led to the deposition of extracellular amyloid, accompanied by the deposition of apolipoprotein E, an astrocyte and microglia reaction, and the occurrence of dilated cholinergic terminals in the cortex. Features of neurodegeneration, however, were absent. The pattern of plaque development and deposition in these mice was similar to that of amyloid precursor protein overproducing strains if crossed to presenilin1-transgenics. However, plaque development started much later and developed slowly until the age of 18 months but then increased more rapidly.

Sleep and circadian abnormalities in a transgenic mouse model of Alzheimer's disease: a role for cholinergic transmission

The Tg2576 mouse model of Alzheimer's disease (AD) exhibits age-dependent amyloid beta (Abeta) deposition in the brain. We studied electroencephalographically defined sleep and the circadian regulation of waking activities in Tg2576 mice to determine whether these animals exhibit sleep abnormalities akin to those in AD. In Tg2576 mice at all ages studied, the circadian period of wheel running rhythms in constant darkness was significantly longer than that of wild type mice. In addition, the increase in electroencephalographic delta (1-4 Hz) power that occurs during non-rapid eye movement sleep after sleep deprivation was blunted in Tg2576 mice relative to controls at all ages studied. Electroencephalographic power during non-rapid eye movement sleep was shifted to higher frequencies in plaque-bearing mice relative to controls. The wake-promoting efficacy of the acetylcholinesterase inhibitor donepezil was lower in plaque-bearing Tg2576 mice than in controls. Sleep abnormalities in Tg2576 mice may be due in part to a cholinergic deficit in these mice. At 22 months of age, two additional deficits emerged in female Tg2576 mice: time of day-dependent modulation of sleep was blunted relative to controls and rapid eye movement sleep as a percentage of time was lower in Tg2576 than in wild type controls. The rapid eye movement sleep deficit in 22 month-old female Tg2576 mice was abolished by brief passive immunization with an N-terminal antibody to Abeta. The Tg2576 model provides a uniquely powerful tool for studies on the pathophysiology of and treatments for sleep deficits and associated cholinergic abnormalities in AD.

High selective expression of alpha7 nicotinic receptors on astrocytes in the brains of patients with sporadic Alzheimer's disease and patients carrying Swedish APP 670/671 mutation: a possible association with neuritic plaques

In the present study, we have investigated the expression of nicotinic acetylcholine receptors (nAChRs) on astrocytes and neurons in the hippocampus and temporal cortex of subjects carrying the Swedish amyloid precursor protein (APP) 670/671 mutation (APPswe), patients with sporadic Alzheimer's disease (AD), and age-matched control subjects. Significant increases in the total numbers of astrocytes and of astrocytes expressing the alpha7 nAChR subunit, along with significant decreases in the levels of alpha7 and alpha4 nAChR subunits on neurons, were observed in the hippocampus and temporal cortex of both APPswe and sporadic AD brains. Both of these phenomena were more pronounced in APPswe than sporadic AD cases. Furthermore, the number of [(125)I]alpha-BTX binding sites (alpha7 nAChR) in the temporal cortex of the APPswe brain was significant lower than in the younger control group, reflecting the lower neuronal level of alpha7 nAChR. The increase in the level of expression of alpha7 nAChR on astrocytes was positively correlated with the extent of neuropathological alternations, especially the number of neuritic plaques, in the AD brain. The elevated expression of alpha7 nAChR on astrocytes might participate in Abeta cascade and formation of neuritic plaques, thereby playing an important role in the pathogenesis of AD.

Nicotinic acetylcholine receptor immunohistochemistry in Alzheimer's disease and dementia with Lewy bodies: differential neuronal and astroglial pathology

Alzheimer's disease (AD) and dementia with Lewy bodies (DLB) are common forms of dementia in the elderly. The neuropathology of AD and DLB is related to cholinergic dysfunctions, and both alpha4 and alpha7 nicotinic acetylcholine receptor (nAChR) subunits are decreased in several brain areas in both diseases. In this immunohistochemical study, we compared neuronal and astroglial alpha4 and alpha7 subunits in AD, DLB and age-matched controls in the hippocampal formation. The numbers of alpha4 reactive neurons were decreased in layer 3 of the entorhinal cortex of AD and DLB, whereas those of alpha7 reactive neurons were decreased in layer 2 of the subiculum of AD and DLB and in layer 3 of the entorhinal cortex of DLB. In contrast, the intensity of alpha7 reactive neuropil was significantly higher in AD than in controls or DLB in a number of areas of the hippocampus (CA3/4 and stratum granulosum), subiculum and entorhinal cortex. An increase in alpha7 immunoreactivity in AD was also associated with astrocytes. The number of astrocytes double-labelled with alpha7 and glial fibrillary acidic protein (GFAP) antibodies was increased in most areas of the hippocampus and entorhinal cortex in AD compared with controls and DLB. Increased astrocyte alpha7 nAChRs in AD may be associated with inflammatory mechanisms related to degenerative processes specific to this disease.

Cognitive impairment in PDAPP mice depends on ApoE and ACT-catalyzed amyloid formation

Biochemical and genetic studies indicate that the inflammatory proteins, apolipoprotein E (ApoE) and alpha(1)-antichymotrypsin (ACT) are important in the pathogenesis of Alzheimer's disease (AD). Using several lines of multiply transgenic/knockout mice we show here that murine ApoE and human ACT separately and synergistically facilitate both diffuse A beta immunoreactive and fibrillar amyloid deposition and thus also promote cognitive impairment in aged PDAPP(V717F) mice. The degree of cognitive impairment is highly correlated with the ApoE- and ACT-dependent hippocampal amyloid burden, with PDAPP mice lacking ApoE and ACT having little amyloid and little learning disability. A analysis of young mice before the onset of amyloid formation shows that steady-state levels of monomeric A beta peptide are unchanged by ApoE or ACT. These data suggest that the process or product of amyloid formation is more critical than monomeric A beta for the neurological decline in AD, and that the risk factors ApoE and ACT participate primarily in disease processes downstream of APP processing.

Exclusive association and simultaneous appearance of congophilic plaques and AT8-positive dystrophic neurites in Tg2576 mice suggest a mechanism of senile plaque formation and progression of neuritic dystrophy in Alzheimer's disease

Progression of neuritic dystrophy is a histological hallmark of Alzheimer's disease (AD) in addition to amyloid deposition and neurofibrillary tangle formation. Dystrophic neurites (DNs) are abnormal neurites, and are closely associated with amyloid deposits. To clarify the process of DN formation, we immunohistochemically investigated phosphorylated tau (AT8 and Ser396)-positive DNs and plaques in Tg2576 mice overexpressing human beta-amyloid precursor protein (APP) with the Swedish type mutation (K670N/M671L). AT8-positive DNs were exclusively associated with the Congo red-positive plaques examined, and all Abeta(1-40)-positive plaques appeared to be associated with AT8-positive DNs, whereas there were no AT8-positive DNs with Abeta(1-42)-positive/Abeta(1-40)-negative plaques. Since we have previously shown that Abeta(1-42)-positive plaque precede Abeta(1-40) deposition, the appearance of congophilic structures is also late. Quantitative analyses were performed on AT8-positive DNs that were associated with congophilic plaques in the cerebral cortex and hippocampus (more than 1,000 plaques). The number of congophilic plaques increased dramatically with age. The area of DNs in the cerebral cortex and hippocampus increased 120- and 60-fold from 11-13 to 20.5 months of age, respectively. Interestingly, the mean ratio of DN area to congophilic plaque area in every plaque was unchanged, approximately 10%, through the ages examined. The mean plaque size was stable with age in both the cortex and hippocampus. These data suggest that the formation of AT8-positive DNs is simultaneous with Congo red-positive plaque development, and that the event may be closely related in the pathological progression of AD.

Inflammation occurs early during the Abeta deposition process in TgCRND8 mice

Alzheimer's disease (AD) is characterized by a progressive cognitive decline leading to dementia and involves the deposition of amyloid-beta (Abeta) peptides into senile plaques. Other neuropathological features that accompany progression of the disease include a decrease in synaptic density, neurofibrillary tangles, dystrophic neurites, inflammation, and neuronal cell loss. In this study, we report the early kinetics of brain amyloid deposition and its associated inflammation in an early onset transgenic mouse model of AD (TgCRND8) harboring the human amyloid precursor protein gene with the Indiana and Swedish mutations. Both diffuse and compact plaques were detected as early as 9-10 weeks of age. Abeta-immunoreactive (Abeta-IR) plaques (4G8-positive) appeared first in the neocortex and amygdala, then in the hippocampal formation, and lastly in the thalamus. Compact plaques (ThioS-positive) with an amyloid core were observed as early as diffuse plaques were detected, but in lower numbers. Amyloid deposition increased progressively with age. The formation of plaques was concurrent with the appearance of activated microglial cells and shortly followed by the clustering of activated astrocytes around plaques at 13-14 weeks of age. This TgCRND8 mouse model allows for a rapid, time-dependent study of the relationship between the fibrillogenic process and the inflammatory response during the brain amyloidogenic process.

Transgenic mice overexpressing amyloid beta protein are an incomplete model of Alzheimer disease

We compared lesions in elderly transgenic (tg) mice carrying the Swedish double mutation KM670/671NL with lesions in Alzheimer disease (AD) by histochemical and immunohistochemical techniques. Highly similar staining for beta-amyloid protein (Abeta) was observed in AD and the mouse models. The abundant amyloid deposits in tg mice were in a consolidated state as revealed by strong Congo red birefringence. In both tg mice and AD, amyloid deposits were ApoE-positive and were surrounded by activated astrocytes. However, Bielschowsky silver staining and immunostaining with tau antibodies revealed no neurofibrillary tangles (NFTs) in the mice as opposed to abundant NFTs in AD. The microglial pattern was also distinctly different. Tg mice had only weakly activated microglia, which expressed low levels of the complement receptor CD11b. They were gathered around the periphery of the deposits. In contrast, AD lesions had strongly activated microglia, which expressed high levels of CD11b. They were associated with the plaque core. Immunostaining for complement proteins was weak in tg mice but very strong in AD deposits. We conclude that the weak inflammatory response and absence of NFTs indicate that tg mice are only a limited model of AD. Therapeutic strategies for the treatment of AD based on tg mouse models that overexpress Abeta may be limited in their application.

Co-localization of cholesterol, apolipoprotein E and fibrillar Abeta in amyloid plaques

Recent evidence strongly suggests a role for cholesterol and apolipoprotein E in the etiology of Alzheimer's disease. We have demonstrated the co-localization of cholesterol and apolipoprotein E with beta-amyloid immunoreactivity and thioflavin S immunofluorescence in AD type plaques of a transgenic mouse model. Cholesterol and apolipoprotein E co-localized to the core of thioflavin S-positive (fibrillar) plaques, but not thioflavin S-negative (diffuse) plaques from an early age. By 18 months of age, there was extensive coverage of fibrillar plaques immunopositive for apolipoprotein E and cholesterol oxidase. These findings support evidence that cholesterol and apolipoprotein E are involved in fibrillar plaque formation or maintenance, and suggest that cholesterol may impact amyloid formation extracellularly, as well as through an intracellular effect.

Alzheimer's disease is associated with a selective increase in alpha7 nicotinic acetylcholine receptor immunoreactivity in astrocytes

Alzheimer's disease (AD) and dementia with Lewy bodies (DLB) are common forms of dementia in the elderly associated with cholinergic dysfunction, including reductions in nicotinic acetylcholine receptors (nAChRs). In AD, astrocytes are implicated in the formation of senile plaques, one of the core pathological features. Using immunohistochemistry, we have investigated astrocytic expression of the two major nicotinic receptor alpha subunits in the human hippocampus and entorhinal cortex. alpha7, but not alpha4, subunit immunoreactivity was associated with astrocytes. An increase in the proportion of astrocytes expressing alpha7 immunoreactivity was observed in AD compared with age-matched controls. A similar increase was not evident in DLB. Elevated alpha7 nAChRs on astrocytes in AD may contribute to alterations in calcium homeostasis and nitric oxide production, which in turn could affect beta-amyloid-mediated inflammatory processes in AD.

Distinct binding sites in the structure of alpha 2-macroglobulin mediate the interaction with beta-amyloid peptide and growth factors

Alpha(2)-macroglobulin (alpha(2)M) and its receptor, low density lipoprotein receptor-related protein (LRP), function together to facilitate the cellular uptake and degradation of beta-amyloid peptide (Abeta). In this study, we demonstrate that Abeta binds selectively to alpha(2)M that has been induced to undergo conformational change by reaction with methylamine. Denatured alpha(2)M subunits, which were immobilized on polyvinylidene difluoride membranes, bound Abeta, suggesting that alpha(2)M tertiary and quaternary structure are not necessary. To determine whether a specific sequence in alpha(2)M is responsible for Abeta binding, we prepared and analyzed defined alpha(2)M fragments and glutathione S-transferase-alpha(2)M peptide fusion proteins. A single sequence, centered at amino acids (aa) 1314-1365, was identified as the only major Abeta-binding site. Importantly, Abeta did not bind to the previously characterized growth factor-binding site (aa 718-734). Although the Abeta binding sequence is adjacent to the binding site for LRP, the results of experiments with mutated fusion proteins indicate that the two sites are distinct. Furthermore, a saturating concentration of Abeta did not inhibit LRP-mediated clearance of alpha(2)M-MA in mice. Using various methods, we determined that the K(D) for the interaction of Abeta with its binding site in the individual alpha(2)M subunit is 0.7-2.4 microm. The capacity of alpha(2)M to bind Abeta and deliver it to LRP may be greater than that predicted by the K(D), because each alpha(2)M subunit may bind Abeta and the bound Abeta may multimerize. These studies suggest a model in which alpha(2)M has three protein interaction sites with distinct specificities, mediating the interaction with Abeta, growth factors, and LRP.