Overexpression of wild-type human APP in mice causes cognitive deficits and pathological features unrelated to Abeta levels

Pathogenesis of synaptic degeneration in Alzheimer's disease and Lewy body disease

Considerable progress has been made in the past few years in the fight against Alzheimer's disease (AD) and Parkinson's disease (PD). Neuropathological studies in human brains and experimental in vivo and in vitro models support the notion that synapses are affected even at the earliest stages of the neurodegenerative process. The objective of this manuscript is to review some of the mechanisms of synaptic damage in AD and PD. Some lines of evidence support the notion that oligomeric neurotoxic species of amyloid β, α-synuclein, and Tau might contribute to the pathogenesis of synaptic failure at early stages of the diseases. The mechanisms leading to synaptic damage by oligomers might involve dysregulation of glutamate receptors and scaffold molecules that results in alterations in the axonal transport of synaptic vesicles and mitochondria that later on lead to dendritic and spine alterations, axonal dystrophy, and eventually neuronal loss. However, while some studies support a role of oligomers, there is an ongoing debate as to the exact nature of the toxic species. Given the efforts toward earlier clinical and preclinical diagnosis of these disorders, understanding the molecular and cellular mechanisms of synaptic degeneration is crucial toward developing specific biomarkers and new therapies targeting the synaptic apparatus of vulnerable neurons.

Neuronal and glial alterations, increased anxiety, and cognitive impairment before hippocampal amyloid deposition in PDAPP mice, model of Alzheimer's disease

In the context of Alzheimer's disease (AD), hippocampal alterations have been well described in advanced stages of the pathology, when amyloid deposition, inflammation and glial activation occur, but less attention has been directed to studying early brain and behavioral changes. Using an animal model of AD, the transgenic PDAPP-J20 mouse at 5 months of age, when no amyloid plaques are present and low cerebral levels of amyloid peptides are detectable, we found structural, morphological, and cellular alterations in the hippocampus. Young transgenic mice showed a reduced hippocampal volume with less number of pyramidal and granular neurons, which additionally exhibited cell atrophy. The neurogenic capability in this zone, measured as DCX+ cells, was strongly diminished and associated to alterations in cell maturity. A decrease in presynaptic synaptophysin optical density was detected in mossy fibers reaching CA3 subfield but not in Golgi stained- CA1 dendritic spine density. Employing confocal microscopy and accurate stereological tools we also found a reduction in the number of GFAP+ cells, along with decreased astrocyte complexity, suggesting a potential detriment of neural support. According with untimely neuroglial alterations, young PDAPP mice failed in the novel location recognition test, that depends on hippocampal function. Moreover, multivariate statistical analysis of the behavioral outcome in the open-field test evidenced an elevated anxiety score in Tg mice compared with age-matched control mice. In line with this, the transgenic group showed a higher number of c-Fos+ nuclei in central and basolateral amygdala, a result that supports the early involvement of the emotionality factor in AD pathology. Applying an integrative approach, this work focuses on early structural, morphological and functional changes and provides new and compelling evidence of behavioral alterations that precede manifest AD.

Meserine, a novel carbamate AChE inhibitor, ameliorates scopolamine-induced dementia and alleviates amyloidogenesis of APP/PS1 transgenic mice

AIMS

To investigate whether Meserine, a novel phenylcarbamate derivative of (-)-meptazinol, possesses beneficial activities against cholinergic deficiency and amyloidogenesis, the two major pathological characteristics of Alzheimer's disease (AD).

METHODS

Ellman's assay and Morris water maze were used to detect acetylcholinesterase (AChE) activity and evaluate spatial learning and memory ability, respectively. Both high content screening and Western blotting were carried out to detect β-amyloid precursor protein (APP), while RT-PCR and ELISA were conducted to detect APP-mRNA and β-amyloid peptide (Aβ).

RESULTS

In scopolamine-induced dementia mice, Meserine (1 mg/kg, i.p.) significantly ameliorated spatial learning and memory deficits, which was consistent with its in vitro inhibitory ability against AChE (recombinant human AChE, IC50 = 274 ± 49 nM). Furthermore, Meserine (7.5 mg/kg) injected intraperitoneally once daily for 3 weeks lowered APP level by 28% and Aβ42 level by 42% in APP/PS1 transgenic mouse cerebrum. This APP modulation action might be posttranscriptional, as Meserine reduced APP by about 30% in SH-SY5Y-APP695 cells but did not alter APP-mRNA level. And both APP and Aβ42 lowering action of Meserine maintained longer than that of rivastigmine.

CONCLUSION

Meserine executes dual actions against cholinergic deficiency and amyloidogenesis and provides a promising lead compound for symptomatic and modifying therapy of AD.

The airbag problem-a potential culprit for bench-to-bedside translational efforts: relevance for Alzheimer's disease

For the last 20 years, the "amyloid cascade hypothesis" has dominated research aimed at understanding, preventing, and curing Alzheimer's disease (AD). During that time researchers have acquired an enormous amount of data and have been successful, more than 300 times, in curing the disease in animal model systems by treatments aimed at clearing amyloid deposits. However, to date similar strategies have not been successful in human AD patients. Hence, before rushing into further clinical trials with compounds that aim at lowering amyloid-beta (Aβ) levels in increasingly younger people, it would be of highest priority to re-assess the initial assumption that accumulation of Aβ in the brain is the primary pathological event driving AD. Here we question this assumption by highlighting experimental evidence in support of the alternative hypothesis suggesting that APP and Aβ are part of a neuronal stress/injury system, which is up-regulated to counteract inflammation/oxidative stress-associated neurodegeneration that could be triggered by a brain injury, chronic infections, or a systemic disease. In AD, this protective program may be overridden by genetic and other risk factors, or its maintenance may become dysregulated during aging. Here, we provide a hypothetical example of a hypothesis-driven correlation between car accidents and airbag release in analogy to the evolution of the amyloid focus and as a way to offer a potential explanation for the failure of the AD field to translate the success of amyloid-related therapeutic strategies in experimental models to the clinic.

Comprehensive behavioral characterization of an APP/PS-1 double knock-in mouse model of Alzheimer's disease

INTRODUCTION

Despite the extensive mechanistic and pathological characterization of the amyloid precursor protein (APP)/presenilin-1 (PS-1) knock-in mouse model of Alzheimer's disease (AD), very little is known about the AD-relevant behavioral deficits in this model. Characterization of the baseline behavioral performance in a variety of functional tasks and identification of the temporal onset of behavioral impairments are important to provide a foundation for future preclinical testing of AD therapeutics. Here we perform a comprehensive behavioral characterization of this model, discuss how the observed behavior correlates with the mechanistic and pathological observations of others, and compare this model with other commonly used AD mouse models.

METHODS

FOUR DIFFERENT GROUPS OF MICE RANGING ACROSS THE LIFESPAN OF THIS MODEL (TEST GROUPS: 7, 11, 15, and 24 months old) were run in a behavioral test battery consisting of tasks to assess motor function (grip strength, rotor rod, beam walk, open field ambulatory movement), anxiety-related behavior (open field time spent in peripheral zone vs. center zone, elevated plus maze), and cognitive function (novel object recognition, radial arm water maze).

RESULTS

There were no differences in motor function or anxiety-related behavior between APP/PS-1 knock-in mice and wild-type counterpart mice for any age group. Cognitive deficits in both recognition memory (novel object recognition) and spatial reference memory (radial arm water maze) became apparent for the knock-in animals as the disease progressed.

CONCLUSION

This is the first reported comprehensive behavioral analysis of the APP/PS1 knock-in mouse model of AD. The lack of motor/coordination deficits or abnormal anxiety levels, coupled with the age/disease-related cognitive decline and high physiological relevance of this model, make it well suited for utilization in preclinical testing of AD-relevant therapeutics.

Pan-neuronal expression of APL-1, an APP-related protein, disrupts olfactory, gustatory, and touch plasticity in Caenorhabditis elegans

Patients with Alzheimer's disease show age-related cognitive decline. Postmortem autopsy of their brains shows the presence of large numbers of senile plaques, whose major component is the β-amyloid peptide. The β-amyloid peptide is a cleavage product of the amyloid precursor protein (APP). In addition to the neurodegeneration associated with β-amyloid aggregation in Alzheimer's disease patients, mutations in APP in mammalian model organisms have also been shown to disrupt several behaviors independent of visible amyloid plaque formation. However, the pathways in which APP function are unknown and difficult to unravel in mammals. Here we show that pan-neuronal expression of APL-1, the Caenorhabditis elegans ortholog of APP, disrupts several behaviors, such as olfactory and gustatory learning behavior and touch habituation. These behaviors are mediated by distinct neural circuits, suggesting a broad impact of APL-1 on sensory plasticity in C. elegans. Furthermore, we found that disruption of these three behaviors requires activity of the TGFβ pathway and reduced activity of the insulin pathway. These results suggest pathways and molecular components that may underlie behavioral plasticity in mammals and in patients with Alzheimer's disease.

Overexpression of amyloid-β protein precursor induces mitochondrial oxidative stress and activates the intrinsic apoptotic cascade

Aberrant processing of amyloid-β protein precursor (AβPP) into amyloid-β (Aβ) fragments underlies the formation of senile plaques in Alzheimer's disease (AD). Moreover, Aβ fragments, particularly Aβ(42), exert direct toxic effects within neurons including the induction of mitochondrial oxidative stress (MOS). Interestingly, individuals with Down syndrome (DS) frequently develop early onset AD as a major co-morbid phenotype. One hypothesis for AD associated with DS involves the overexpression of wild type (WT) AβPP protein, due to its location on chromosome 21. However, the mechanism by which the overexpression of WT AβPP might trigger MOS and induce cell death is presently unclear. Here we show that transient overexpression of DsRed2-tagged AβPP (WT) in CHO cells induces caspase-3 activation and nuclear fragmentation indicative of apoptosis. AβPP localizes to the mitochondrial fraction of transfected CHO cells and induces glutathione-sensitive opening of the mitochondrial permeability transition pore (mPTP) and cytochrome c release. MOS and intrinsic apoptosis induced by AβPP are significantly inhibited by co-expression of Bcl-2 or treatment with either glutathione or a pan-caspase inhibitor. The mPTP inhibitor, cyclosporin A, also significantly attenuates AβPP-induced apoptosis. AβPP-induced apoptosis is unaffected by a β-secretase inhibitor and is independent of detectable Aβ(42); however, a γ-secretase inhibitor significantly protects against AβPP overexpression, suggesting a possible role of the AβPP intracellular domain in cell death. These data indicate that overexpression of WT AβPP is sufficient to induce MOS and intrinsic apoptosis, suggesting a novel pro-oxidant role for AβPP at mitochondria which may be relevant in AD and DS disease pathologies.

Detection of peri-synaptic amyloid-β pyroglutamate aggregates in early stages of Alzheimer's disease and in AβPP transgenic mice using a novel monoclonal antibody

The neurodegenerative pathology in patients with Alzheimer's disease (AD) has been associated with the progressive accumulation of aggregated and post-translationally modified amyloid-β (Aβ) species. Among them, recent studies indicate that the pyroglutamate modification of Aβ (pE(3)Aβ) catalyzed by glutaminyl cyclase might play an important role in the pathogenesis of AD. Although the effects of the pyroglutamate modification on Aβ aggregation and toxicity have been investigated, less is known about the distribution of pE(3)Aβ across the spectrum of AD and in the brains of amyloid-β protein precursor (AβPP) transgenic (tg) animals. For this purpose, we generated a novel monoclonal antibody (denominated D129) that specifically recognizes pE(3)Aβ and characterized the patterns of distribution in the postmortem brain samples from AD patients divided by disease stage (Braak stage) and in AβPP tg mice. We found that in early stages of AD and young AβPP tg mice pE(3)Aβ was found in discrete linear and granular aggregates in the neuropil that co-localized with the pre-synaptic protein synaptophysin and was in close opposition to dendrites labeled with MAP2. In later stages of AD and in older AβPP tg mice, pE(3)Aβ was abundant in diffuse and mature plaques. In conclusion, this study suggests that peri-synaptic accumulation of pE(3)Aβ might contribute to early cognitive dysfunction in AD.

All in the Family: How the APPs Regulate Neurogenesis

Recent intriguing evidence suggests that metabolites of amyloid precursor protein (APP), mutated in familial forms of Alzheimer’s disease (AD), play critical roles in developmental and postnatal neurogenesis. Of note is soluble APPα (sAPPα) that regulates neural progenitor cell proliferation. The APP family encompasses a group of ubiquitously expressed and evolutionarily conserved, type I transmembrane glycoproteins, whose functions have yet to be fully elucidated. APP can undergo proteolytic cleavage by mutually exclusive pathways. The subtle structural differences between metabolites generated in the different pathways, as well as their equilibrium, may be crucial for neuronal function. The implications of this new body of evidence are significant. Miscleavage of APP would readily impact developmental and postnatal neurogenesis, which might contribute to cognitive deficits characterizing Alzheimer’s disease. This review will discuss the implications of the role of the APP family in neurogenesis for neuronal development, cognitive function, and brain disorders that compromise learning and memory, such as AD.

α-Synuclein knockout mice have cognitive impairments

α-Synuclein is a member of the synuclein family of cytoplasmic, predominantly neuron-specific proteins. Considerable amount of α-synuclein is found in axons and presynaptic terminals of neurons located in brain areas responsible for emotions and memory. In the present study we have carried out behavioral evaluation of spatial and working long-term memory of α-synuclein knockout mice. Our data shows that α-synuclein knockout mice have reduced learning ability in tests requiring both working and spatial memory. For the first time we have demonstrated that α-synuclein is necessary for these types of learning.

NMDA receptor/amyloid precursor protein interactions: a comparison between wild-type and amyloid precursor protein mutations associated with familial Alzheimer's disease

Two recent reports showed that amyloid precursor protein (APP) may contribute to postsynaptic mechanisms via the regulation of the surface trafficking of excitatory N-methyl-D-aspartate (NMDA) receptors. Here we have investigated the interactions and surface trafficking of NR1-1a/NR2A and NR1-1a/NR2B NMDA receptor subtypes with three APP mutations linked to familial Alzheimer's disease, APP695(Indiana), APP695(London) and APP695(Swedish). Flag-tagged mutated APP695s were generated and shown to be expressed at equivalent levels to wild-type APP695 in mammalian cells. Each APP mutant co-precipitated with NR1-1a/NR2A and NR1-1a/NR2B receptors following co-expression in mammalian cells. Further, as found for wild-type APP695, each enhanced NMDA receptor surface expression with no concomitant increase in total NR1-1a, NR2A or NR2B subunit expression. Thus these three familial APP mutations behave as wild-type APP695 with respect to their association with assembled NMDA receptors and their APP695-enhanced receptor cell surface trafficking.

Alzheimer mechanisms and therapeutic strategies

There are still no effective treatments to prevent, halt, or reverse Alzheimer's disease, but research advances over the past three decades could change this gloomy picture. Genetic studies demonstrate that the disease has multiple causes. Interdisciplinary approaches combining biochemistry, molecular and cell biology, and transgenic modeling have revealed some of its molecular mechanisms. Progress in chemistry, radiology, and systems biology is beginning to provide useful biomarkers, and the emergence of personalized medicine is poised to transform pharmaceutical development and clinical trials. However, investigative and drug development efforts should be diversified to fully address the multifactoriality of the disease.

Somatostatin receptor subtype-4 agonist NNC 26-9100 decreases extracellular and intracellular Aβ₁₋₄₂ trimers

Soluble amyloid β-protein (Aβ) oligomers are primary mediators of synaptic dysfunction associated with the progression of Alzheimer's disease. Such Aβ oligomers exist dependent on their rates of aggregation and metabolism. Use of selective somatostatin receptor-subtype agonists have been identified as a potential means to mitigate Aβ accumulation in the brain, via regulation of the enzyme neprilysin. Herein, we first evaluated the impact of the somatostatin receptor subtype-4 agonist 1-[3-[N-(5-Bromopyridin-2-yl)-N-(3,4-dichlorobenzyl)amino]propyl]-3-[3-(1H-imidazol-4-yl)propyl]thiourea (NNC 26-9100) on learning and memory in 12-month SAMP8 mice (i.c.v. injection). NNC 26-9100 (0.2 μg-dose) was shown to enhance both learning (T-maze) and memory (object recognition) compared to vehicle controls. Cortical and hippocampal tissues were evaluated subsequent to NNC 26-9100 (0.2 μg) or vehicle administration for changes in neprilysin activity, along with protein expression of amyloid-precursor protein (APP), neprilysin, and Aβ₁₋₄₂ oligomers within respective cellular fractions (extracellular, intracellular and membrane). NNC 26-9100 increased neprilysin activity in cortical tissue, with an associated protein expression increase in the extracellular fraction and decreased in the intracellular fraction. A decrease in intracellular APP expression was found with treatment in both cortical and hippocampal tissues. NNC 26-9100 also significantly decreased expression of Aβ₁₋₄₂ trimers within both the extracellular and intracellular cortical fractions. No expression changes were found in membrane fractions for any protein. These finding suggest the potential use of selective SSTR4 agonists to mitigate toxic oligomeric forms of Aβ₁₋₄₂ in critical regions of the brain identified with learning and memory decline.

The use of mouse models for understanding the biology of down syndrome and aging

Down syndrome is a complex condition caused by trisomy of human chromosome 21. The biology of aging may be different in individuals with Down syndrome; this is not well understood in any organism. Because of its complexity, many aspects of Down syndrome must be studied either in humans or in animal models. Studies in humans are essential but are limited for ethical and practical reasons. Fortunately, genetically altered mice can serve as extremely useful models of Down syndrome, and progress in their production and analysis has been remarkable. Here, we describe various mouse models that have been used to study Down syndrome. We focus on segmental trisomies of mouse chromosome regions syntenic to human chromosome 21, mice in which individual genes have been introduced, or mice in which genes have been silenced by targeted mutagenesis. We selected a limited number of genes for which considerable evidence links them to aspects of Down syndrome, and about which much is known regarding their function. We focused on genes important for brain and cognitive function, and for the altered cancer spectrum seen in individuals with Down syndrome. We conclude with observations on the usefulness of mouse models and speculation on future directions.

The amyloid precursor protein intracellular domain-fe65 multiprotein complexes: a challenge to the amyloid hypothesis for Alzheimer's disease?

Since its proposal in 1994, the amyloid cascade hypothesis has prevailed as the mainstream research subject on the molecular mechanisms leading to the Alzheimer's disease (AD). Most of the field had been historically based on the role of the different forms of aggregation of β-amyloid peptide (Aβ). However, a soluble intracellular fragment termed amyloid precursor protein (APP) intracellular domain (AICD) is produced in conjunction with Aβ fragments. This peptide had been shown to be highly toxic in both culture neurons and transgenic mice models. With the advent of this new toxic fragment, the centerpiece for the ethiology of the disease may be changed. This paper discusses the potential role of multiprotein complexes between the AICD and its adapter protein Fe65 and how this could be a potentially important new agent in the neurodegeneration observed in the AD.

Oxidative Stress and Down Syndrome: A Route toward Alzheimer-Like Dementia

Down syndrome (DS) is one of the most frequent genetic abnormalities characterized by multiple pathological phenotypes. Indeed, currently life expectancy and quality of life for DS patients have improved, although with increasing age pathological dysfunctions are exacerbated and intellectual disability may lead to the development of Alzheimer's type dementia (AD). The neuropathology of DS is complex and includes the development of AD by middle age, altered free radical metabolism, and impaired mitochondrial function, both of which contribute to neuronal degeneration. Understanding the molecular basis that drives the development of AD is an intense field of research. Our laboratories are interested in understanding the role of oxidative stress as link between DS and AD. This review examines the current literature that showed oxidative damage in DS by identifying putative molecular pathways that play a central role in the neurodegenerative processes. In addition, considering the role of mitochondrial dysfunction in neurodegenerative phenomena, results demonstrating the involvement of impaired mitochondria in DS pathology could contribute a direct link between normal aging and development of AD-like dementia in DS patients.

Role of APP for dendritic spine formation and stability

The amyloid precursor protein (APP) is transported in high amounts to the presynaptic endings where its function is still unknown. Several studies indicate that lack of APP or its overexpression affects the number of dendritic spines, the postsynaptic compartment of excitatory synapses. Since synapse loss has been identified as one of the most important structural correlates of cognitive decline in Alzheimer's diseases (AD), the physiological function of APP at synapses, specifically at dendritic spines, has come into focus in AD research. This review intends to give an overview of the very controversial results on APP expression on dendritic spine number in the mouse brain.

Insulin suppresses the expression of amyloid precursor protein, presenilins, and glycogen synthase kinase-3beta in peripheral blood mononuclear cells

OBJECTIVE

Our objective was to determine whether peripheral blood mononuclear cells express amyloid precursor protein (APP) and other mediators involved in the pathogenesis of Alzheimer's disease and whether their expression is suppressed by insulin.

RESEARCH DESIGN AND METHODS

Ten obese type 2 diabetic patients were infused with insulin (2 U/h with 100 ml 5% dextrose/h) for 4 h. Patients were also infused with 5% dextrose/h or normal physiological saline for 4 h, respectively, on two other days as controls. Blood samples were obtained at 0, 2, 4, and 6 h.

RESULTS

Insulin infusion significantly suppressed the expression of APP, presenilin-1, presenilin-2, and glycogen synthase kinase-3β in peripheral blood mononuclear cells. Dextrose and saline infusions did not alter these indices. Insulin infusion also caused significant parallel reductions in nuclear factor-κB binding activity and plasma concentrations of serum amyloid A and intercellular adhesion molecule-1.

CONCLUSIONS

A low dose infusion of insulin suppresses APP, presenilin-1, presenilin-2, and glycogen synthase kinase-3β, key proteins involved in the pathogenesis of Alzheimer's disease, in parallel with exerting its other antiinflammatory effects.

Rabs, SNAREs and α-synuclein--membrane trafficking defects in synucleinopathies

Neuronal dysfunctions and neurodegeneration are often associated with defects in membrane transport. Synucleinopathies are a diverse group of neurodegenerative disorders that share a common pathological feature--insoluble aggregates composed largely of the protein α-synuclein in certain populations of neurons and glia. The actual physiological function of the brain-enriched α-synuclein is still not particularly clear. What is obvious is that when the protein is present in pathologically high amounts, or in mutant forms with enhanced membrane association and oligomerization, it causes neuronal demise with manifestations of impaired neuronal traffic, heightened oxidative stress, mitochondrial degeneration and defects in lipid metabolism. α-synuclein's direct association with the activities of key components of the eukaryotic membrane traffic machinery, namely Rabs and the soluble N-ethylmaleimide sensitive factor (NSF) attachment protein receptors (SNAREs), has highlighted a key role for membrane transport defects in α-synuclein-mediated pathology. Here, we summarize and discuss recent findings in this regard, and their implications in the molecular aspects of synucleinopathy.

Activity-induced dendrite and dendritic spine development in human amyloid precursor protein transgenic mice

The amyloid precursor protein is essential for proper neuronal function but an imbalance in processing or metabolism or its overexpression lead to severe malfunction of the brain. The present study focused on dendritic morphology of hippocampal neurons in mice overexpressing the wild-type human amyloid precursor protein (hAPP). In addition, we examined whether enhanced physical activity may affect hAPP-related morphological changes. Overexpression of hAPP resulted in significant enlargement of dendrites, especially within the basal dendritic field but had no effect on spine density. Enhanced physical activity only moderately potentiated hAPP induced changes in dendritic size. Physical activity dependent increases in spine density were, however, augmented by hAPP overexpression. The results suggest that enhanced levels of wild-type hAPP do not result in degenerative changes of neuronal morphology, but rather promote dendritic growth.

Familial amyloid precursor protein mutants cause caspase-6-dependent but amyloid β-peptide-independent neuronal degeneration in primary human neuron cultures

Although familial Alzheimer disease (AD)-associated autosomal dominant mutants have been extensively studied, little is known about the underlying molecular mechanisms of neurodegeneration induced by these mutants in AD. Wild-type, Swedish or London amyloid precursor protein (APP) transfection in primary human neurons induced neuritic beading, in which several co-expressed proteins, such as enhanced green fluorescent protein, red fluorescent protein (RFP)-tau and RFP-ubiquitin, accumulated. APP-induced neuritic beading was dependent on caspase-6 (Casp6), because it was inhibited with 5 μM z-VEID-fmk or with dominant-negative Casp6. Neuritic beading was independent from APP-mediated amyloid β-peptide (Aβ) production, because the APPM596V (APP(MV)) mutant, which cannot generate Aβ, still induced Casp6-dependent neuritic beading. However, the beaded neurons underwent Casp6- and Aβ-dependent cell death. These results indicate that overexpression of wild-type or mutant APP causes Casp6-dependent but Aβ-independent neuritic degeneration in human neurons. Because Casp6 is activated early in AD and is involved in axonal degeneration, these results suggest that the inhibition of Casp6 may represent an efficient early intervention against familial forms of AD. Furthermore, these results indicate that removing Aβ without inhibiting Casp6 may have little effect in preventing the progressive dementia associated with sporadic or familial AD.

Progressive accumulation of amyloid-beta oligomers in Alzheimer's disease and in amyloid precursor protein transgenic mice is accompanied by selective alterations in synaptic scaffold proteins

The cognitive impairment in patients with Alzheimer's disease is closely associated with synaptic loss in the neocortex and limbic system. Although the neurotoxic effects of aggregated amyloid-beta oligomers in Alzheimer's disease have been studied extensively in experimental models, less is known about the characteristics of these aggregates across the spectrum of Alzheimer's disease. In this study, postmortem frontal cortex samples from controls and patients with Alzheimer's disease were fractionated and analyzed for levels of oligomers and synaptic proteins. We found that the levels of oligomers correlated with the severity of cognitive impairment (blessed information-memory-concentration score and mini-mental state examination) and with the loss of synaptic markers. Reduced levels of the synaptic vesicle protein, vesicle-associated membrane protein-2, and the postsynaptic protein, postsynaptic density-95, correlated with the levels of oligomers in the various fractions analyzed. The strongest associations were found with amyloid-beta dimers and pentamers. Co-immunoprecipitation and double-labeling experiments supported the possibility that amyloid-beta and postsynaptic density-95 interact at synaptic sites. Similarly, in transgenic mice expressing high levels of neuronal amyloid precursor protein, amyloid-beta co-immunoprecipitated with postsynaptic density-95. This was accompanied by a decrease in the levels of the postsynaptic proteins Shank1 and Shank3 in patients with Alzheimer's disease and in the brains of amyloid precursor protein transgenic mice. In conclusion, this study suggests that the presence of a subpopulation of amyloid-beta oligomers in the brains of patients with Alzheimer's disease might be related to alterations in selected synaptic proteins and cognitive impairment.

Aging and excitotoxic stress exacerbate neural circuit reorganization in amyloid precursor protein intracellular domain transgenic mice

The cleavage of amyloid precursor protein (APP) by presenilins simultaneously generates amyloid-β (Aβ) and APP intracellular Domain (AICD) peptides. Aβ plays a pivotal role in Alzheimer's disease (AD) pathology and recently AICD was also shown to contribute to AD. Transgenic mice overexpressing AICD show age-dependent tau phosphorylation and aggregation, memory deficits, and neurodegeneration. Moreover, these mice show aberrant electrical activity and silent seizures beginning at 3-4 months of age. Here we show that AICD mice also displayed abnormal mossy fiber sprouting beginning about the same time and that this sprouting intensified as the animals aged. Expression of neuropeptide Y was increased in mossy fiber terminals in aged but not young AICD mice. Importantly, young AICD mice injected with kainic acid showed similar pathology to that observed in aged AICD mice. These data show that elevated levels of AICD render neurons hypersensitive to stress and induce hippocampal circuit reorganization, which can further exacerbate hyperexcitability. These results further demonstrate that AICD, in addition to Aβ, can play a significant role in AD pathogenesis.

Molecular mechanisms of neurodegeneration in Alzheimer's disease

Alzheimer's disease (AD) is characterized by cognitive impairment, progressive neurodegeneration and formation of amyloid-beta (Abeta)-containing plaques and neurofibrillary tangles composed of hyperphosphorylated tau. The neurodegenerative process in AD is initially characterized by synaptic damage accompanied by neuronal loss. In addition, recent evidence suggests that alterations in adult neurogenesis in the hippocampus might play a role. Synaptic loss is one of the strongest correlates to the cognitive impairment in patients with AD. Several lines of investigation support the notion that the synaptic pathology and defective neurogenesis in AD are related to progressive accumulation of Abeta oligomers rather than fibrils. Abnormal accumulation of Abeta resulting in the formation of toxic oligomers is the result of an imbalance between the levels of Abeta production, aggregation and clearance. Abeta oligomers might lead to synaptic damage by forming pore-like structures with channel activity; alterations in glutamate receptors; circuitry hyper-excitability; mitochondrial dysfunction; lysosomal failure and alterations in signaling pathways related to synaptic plasticity, neuronal cell and neurogenesis. A number of signaling proteins, including fyn kinase; glycogen synthase kinase-3beta (GSK3beta) and cyclin-dependent kinase-5 (CDK5), are involved in the neurodegenerative progression of AD. Therapies for AD might require the development of anti-aggregation compounds, pro-clearance pathways and blockers of hyperactive signaling pathways.

Alzheimer's disease-like pathological features in transgenic mice expressing the APP intracellular domain

The hypothesis that amyloid-beta (Abeta) peptides are the primary cause of Alzheimer's disease (AD) remains the best supported theory of AD pathogenesis. Yet, many observations are inconsistent with the hypothesis. Abeta peptides are generated when amyloid precursor protein (APP) is cleaved by presenilins, a process that also produces APP intracellular domain (AICD). We previously generated AICD-overexpressing transgenic mice that showed abnormal activation of GSK-3beta, a pathological feature of AD. We now report that these mice exhibit additional AD-like characteristics, including hyperphosphorylation and aggregation of tau, neurodegeneration and working memory deficits that are prevented by treatment with lithium, a GSK-3beta inhibitor. Consistent with its potential role in AD pathogenesis, we find AICD levels to be elevated in brains from AD patients. The in vivo findings that AICD can contribute to AD pathology independently of Abeta have important therapeutic implications and may explain some observations that are discordant with the amyloid hypothesis.

Time for a change in the research paradigm for Alzheimer's disease: the value of a chaotic matrix modeling approach

The amyloid cascade hypothesis, based on the genetic data from early onset, familial forms of the disease, has been the dominant model for many years and involves over production and deposition of the beta amyloid protein as causal in the disease process. However, it does not apply very well to the more common, later onset, sporadic form of the disease, where a wider range of factors appear to be involved in disease progression. Over recent years, data illustrating reciprocal interactions between the amyloid precursor protein (APP) and its various metabolites with many factors involved in normal synaptic plasticity have emerged. These feedback relationships have the potential to affect the complex kinase cascades involved in every aspect of neuronal function. Further, data regarding the multiple roles of the presenilins have the potential to allow the over expression and deposition of the amyloid beta protein to be both a cause and consequence of disease progression, with relevance in both sporadic and familial of Alzheimer's disease (AD). Disease progression might be better explained by a chaotic matrix of factors and raises the question again whether AD should be approached as a single entity or as a syndrome, with important consequences for disease identification and treatment.