Formation of neurofibrillary tangles in P301l tau transgenic mice induced by Abeta 42 fibrils

Selective cholinergic denervation, independent from oxidative stress, in a mouse model of Alzheimer’s disease

Alzheimer's disease (AD) is characterized by increases in amyloid-beta (Abeta) peptides, neurofibrillary tangles, oxidative stress and cholinergic deficits. However, the selectivity of these deficits and their relation with the Abeta pathology or oxidative stress remain unclear. We therefore investigated amyloidosis-related changes in acetylcholine (ACh) and serotonin (5-HT) innervations of hippocampus and parietal cortex by quantitative choline acetyltransferase (ChAT) and 5-HT immunocytochemistry, in 6, 12/14 and 18 month-old transgenic mice carrying familial AD-linked mutations (hAPP(Sw,Ind)). Further, using manganese superoxide dismutase (MnSOD) and nitrotyrosine immunoreactivity as markers, we evaluated the relationship between oxidative stress and the ACh deficit in 18 month-old mice. Thioflavin-positive Abeta plaques were seen in both regions at all ages; they were more numerous in hippocampus and increased in number (>15-fold) and size as a function of age. A majority of plaques exhibited or were surrounded by increased MnSOD immunoreactivity, and dystrophic ACh or 5-HT axons were seen in their immediate vicinity. Counts of immunoreactive axon varicosities revealed significant decreases in ACh innervation, with a sparing of the 5-HT, even in aged mice. First apparent in hippocampus, the loss of ACh terminals was in the order of 20% at 12/14 months, and not significantly greater (26%) at 18 months. In parietal cortex, the ACh denervation was significant at 18 months only, averaging 24% across the different layers. Despite increased perivascular MnSOD immunoreactivity, there was no evidence of dystrophic ACh varicosities or their accentuated loss in the perivascular area. Moreover, there was virtually no sign of tyrosine nitration in ChAT nerve terminals or neuronal cell bodies. These data suggest that aggregated Abeta exerts an early, non-selective and focal neurotoxic effect on both ACh and 5-HT axons, but that a selective, plaque- and oxidative stress-independent diffuse cholinotoxicity, most likely caused by soluble Abeta assemblies, is responsible for the hippocampal and cortical ACh denervation.

Enhanced delivery of γ-secretase inhibitor DAPT into the brain via an ascorbic acid mediated strategy

Inhibition of gamma-secretase, one of the enzymes responsible for the cleavage of the amyloid precursor protein (APP) to produce pathogenic Abeta peptides, is an attractive approach for the treatment of Alzheimer's disease. We designed a gamma-secretase inhibitor bearing an ascorbic acid moiety which allows a specific delivery of the drug to the brain. Through, on the one hand, Abeta peptide production measurements by specific in vitro assays (gamma-secretase cell free assay and cell based assay on HEK 293 APP transfected cells) and on the other hand through pharmacokinetic studies on animal models, the new inhibitor shows a good pharmacokinetic profile as well as a potent gamma-secretase inhibitory activity in vitro. From the obtained results, it is expected that drug will be mainly delivered to the CNS with a low diffusion in the peripheral tissues. Consequently the side effects of this gamma-secretase inhibitor on the immune cells could be reduced.

Substituted thiazolamide coupled to a redox delivery system: a new γ-secretase inhibitor with enhanced pharmacokinetic profile

Inhibition of gamma-secretase, one of the enzymes responsible for the cleavage of the amyloid precursor protein (APP) to produce pathogenic A beta peptides, is an attractive approach for the treatment of Alzheimer's disease. We have designed a new gamma-secretase thiazolamide inhibitor bearing a dihydronicotinoyl moiety as Redox Delivery System which allows specific delivery of the drug to the brain. Through, on the one hand, A beta peptide production measurements by specific in vitro assays (gamma-secretase Cell Free assay and Cell Based assay on HEK 293 APP transfected cells) and, on the other hand, pharmacokinetic studies on animal models, the new inhibitor shows a good pharmacokinetic profile as well as a potent gamma-secretase inhibitory activity in vitro. From the obtained results, it is expected that drug will be mainly delivered to the CNS with low diffusion in the peripheral tissues. Consequently the side effects of this gamma-secretase inhibitor on the immune cells could be reduced.

Increased tau phosphorylation on mitogen-activated protein kinase consensus sites and cognitive decline in transgenic models for Alzheimer's disease and FTDP-17: evidence for distinct molecular processes underlying tau abnormalities

Abnormal tau phosphorylation occurs in several neurodegenerative disorders, including Alzheimer's disease (AD) and frontotemporal dementia with Parkinsonism linked to chromosome 17 (FTDP-17). Here, we compare mechanisms of tau phosphorylation in mouse models of FTDP-17 and AD. Mice expressing a mutated form of human tau associated with FTDP-17 (tau(V337M)) showed age-related increases in exogenous tau phosphorylation in the absence of increased activation status of a number of kinases known to phosphorylate tau in vitro. In a "combined" model, expressing both tau(V337M) and the familial amyloid precursor protein AD mutation APP(V717I) in a CT100 fragment, age-dependent tau phosphorylation occurred at the same sites and was significantly augmented compared to "single" tau(V337M) mice. These effects were concomitant with increased activation status of mitogen-activated protein kinase (MAPK) family members (extracellular regulated kinases 1 and 2, p38, and c-Jun NH(2)-terminal kinase) but not glycogen synthase kinase-3alphabeta or cyclin-dependent kinase 5. The increase in MAPK activation was a discrete effect of APP(V717I)-CT100 transgene expression as near identical changes were observed in single APP(V717I)-CT100 mice. Age-dependent deficits in memory were also associated with tau(V337M) and APP(V717I)-CT100 expression. The data reveal distinct routes to abnormal tau phosphorylation in models of AD and FTDP-17 and suggest that in AD, tau irregularities may be linked to processing of APP C-terminal fragments via specific effects on MAPK activation status.

Tau alteration and neuronal degeneration in tauopathies: mechanisms and models

Tau becomes characteristically altered both functionally and structurally in several neurodegenerative diseases now collectively called tauopathies. Although increasing evidence supports that alterations of tau may directly cause neuronal degeneration and cell death, the mechanisms, which render tau to become a toxic agent are still unclear. In addition, it is obscure, whether neurodegeneration in tauopathies occurs via a common mechanism or specific differences exist. The aim of this review is to provide an overview about the different experimental models that currently exist, how they are used to determine the role of tau during degeneration and what has been learnt from them concerning the mechanistic role of tau in the disease process. The review begins with a discussion about similarities and differences in tau alteration in paradigmatic tauopathies such as frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17) and Alzheimer's disease (AD). The second part concentrates on major experimental models that have been used to address the mechanistic role of tau during degeneration. This will include a discussion of cell-free assays, culture models using cell lines or dissociated neurons, and animal models. How these models aid to understand (i) alterations in the function of tau as a microtubule-associated protein (MAP), (ii) direct cytotoxicity of altered tau protein, and (iii) the potential role of tau aggregation in neurodegenerative processes will be the central theme of this part. The review ends with concluding remarks about a general mechanistic model of the role of tau alteration and neuronal degeneration in tauopathies and future perspectives.

Endogenous β-amyloid peptide synthesis modulates cAMP response element-regulated gene expression in PC12 cells

Extracellular-regulated kinases play a fundamental role in several neuroplasticity processes. In order to test whether endogenous beta-amyloid peptides play a role in the activation of extracellular-regulated kinase, we investigated the Rap1-extracellular-regulated kinase pathway in PC12 cells expressing human beta-amyloid precursor protein containing familial Alzheimer's disease mutations. In PC12 cells transfected with mutant human beta-amyloid precursor proteins that lead to higher levels of endogenous beta-amyloid, we observed an up-regulation of phospho-extracellular-regulated kinase and higher levels of activity-induced cAMP response element-directed gene expression. These results suggest that moderate levels of endogenous beta-amyloid peptides stimulate cAMP response element-directed gene expression. This stimulation was via a Rap1/MEK/extracellular-regulated kinase signaling pathway, as it was blocked by inhibition of Rap1 and MEK activities, and it requires beta-amyloid precursor protein cleavage at the gamma-site as it was abolished by a gamma-secretase inhibitor. Interestingly, in agreement with the previous observations, micromolar levels of extracellular fibrillar beta-amyloid blocked the cAMP response element-regulated gene expression stimulated by potassium and forskolin. This indicates that beta-amyloid can provoke different responses on cAMP response element-directed gene expression, such that low beta-amyloid levels may play a physiological role favoring synaptic plasticity under normal conditions while it would inhibit this mechanism under pathological conditions.

Mutations causing neurodegenerative tauopathies

Tau is the major component of the intracellular filamentous deposits that define a number of neurodegenerative diseases. They include the largely sporadic Alzheimer's disease (AD), progressive supranuclear palsy, corticobasal degeneration, Pick's disease and argyrophilic grain disease, as well as the inherited frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). For a long time, it was unclear whether the dysfunction of tau protein follows disease or whether disease follows tau dysfunction. This was resolved when mutations in Tau were found to cause FTDP-17. Currently, 32 different mutations have been identified in over 100 families. About half of the known mutations have their primary effect at the protein level. They reduce the ability of tau protein to interact with microtubules and increase its propensity to assemble into abnormal filaments. The other mutations have their primary effect at the RNA level and perturb the normal ratio of three-repeat to four-repeat tau isoforms. Where studied, this resulted in a relative overproduction of tau protein with four microtubule-binding domains in the brain. Individual Tau mutations give rise to diseases that resemble progressive supranuclear palsy, corticobasal degeneration or Pick's disease. Moreover, the H1 haplotype of Tau has been identified as a significant risk factor for progressive supranuclear palsy and corticobasal degeneration. At a practical level, the new work is leading to the production of experimental animal models that reproduce the essential molecular and cellular features of the human tauopathies, including the formation of abundant filaments made of hyperphosphorylated tau protein and nerve cell degeneration.

Metabolic/signal transduction hypothesis of Alzheimer's disease and other tauopathies

Alzheimer's disease (AD), the major cause of dementia in middle- to old-aged individuals, is multifactorial. Independent of the etiology, whether genetic or non-genetic, this disease is characterized by extracellular beta-amyloid plaques and intraneuronal neurofibrillary tangles of abnormally hyperphosphorylated tau. However, the molecular mechanisms of neither AD nor other tauopathies are completely understood. To date, the most popular hypothesis of AD is the "Amyloid cascade hypothesis", according to which beta-amyloid, the cleavage product of beta-amyloid precursor protein (APP), is neurotoxic and causes neurodegeneration and dementia. However, this hypothesis is inconsistent with the presence in normal aged human brain of the beta-amyloid plaque burden similar to that in AD, and the absence of neurofibrillary pathology and neurodegeneration in mutated APP, presenilin-1 and presenilin-2 transgenic mice that show extensive beta-amyloid plaque pathology. Here we propose an alternate hypothesis, the "Metabolic/signal transduction hypothesis", which is consistent both with the pathology seen in AD and other tauopathies and as well as all experimental animal conditions. In this hypothesis, with increasing age, the fluidity of neuronal membranes is progressively reduced, which makes it less resistant to environmental/metabolic insults affecting one or more signal transduction pathways, which lead to a protein phosphorylation/dephosphorylation imbalance and abnormal hyperphosphorylation of tau. The hyperphosphorylated tau sequesters normal tau, MAP1 and MAP2, which results in breakdown of the microtubule network and, consequently, a progressive retrograde degeneration of the affected neurons and, ultimately, dementia.

Oligomerization of amyloid ?-protein occurs during the isolation of lipid rafts

Cholesterol- and glycosphingolipid-rich microdomains, called "lipid rafts," are suggested to initiate and promote the pathophysiology of Alzheimer's disease by serving as a platform for generation, aggregation, or degradation of amyloid-beta protein (Abeta). However, methods for biochemical isolation of these microdomains may produce artifacts. In this study, when synthetic Abeta1- 40 monomers were added to the brain fragment at a final concentration of 2.1 microM, followed by homogenization and isolation of lipid rafts by an established method, Abeta1- 40 accumulated as oligomers in the lipid raft fraction. However, in the absence of a brain homogenate, synthetic Abeta1- 40 did not accumulate in the lipid raft fraction. When fractionation was performed in the absence of synthetic Abeta1-40 and synthetic Abeta1-40 was incubated in an aliquot of each fraction, a marked oligomerization of Abeta1- 40 was observed in the lipid raft aliquot. These results indicate that exogenous Abeta associates with lipid rafts, and Abeta bound to rafts forms oligomers during the isolation of lipid rafts. In addition, endogenous Abeta1-40 in a Triton X-100-insoluble fraction of a brain homogenate of the Tg2576 transgenic mouse model of Alzheimer's disease formed oligomers when the fraction was incubated at 4 degrees C for 20 hr. Thus, one should be careful when one discusses the role of lipid rafts in amyloid precursor protein processing and in the generation, aggregation, and degradation of Abeta.

Modeling tauopathy: a range of complementary approaches

The large group of neurodegenerative diseases which feature abnormal metabolism and accumulation of tau protein (tauopathies) characteristically produce a multiplicity of cellular and systemic abnormalities in human patients. Understanding the complex pathogenetic mechanisms by which abnormalities in tau lead to systemic neurofibrillary degenerative disease requires the construction and use of model experimental systems in which the behavior of human tau can be analyzed under controlled conditions. In this paper, we survey the ways in which in vitro, cellular and whole-animal models of human tauopathy are being used to add to our knowledge of the pathogenetic mechanisms underlying these conditions. In particular, we focus on the complementary advantages and limitations of various approaches to constructing tauopathy models presently in use with respect to those of murine transgenic tauopathy models.

The critical role of methionine 35 in Alzheimer's amyloid beta-peptide (1-42)-induced oxidative stress and neurotoxicity

Amyloid beta-peptide (1-42) [Abeta(1-42)] has been proposed to play a central role in the pathogenesis of Alzheimer's disease, a neurodegenerative disorder associated with cognitive decline and aging. AD brain is under extensive oxidative stress, and Abeta(1-42) has been shown to induce protein oxidation, lipid peroxidation, and reactive oxygen species formation in neurons and synaptosomes, all of which are inhibited by the antioxidant vitamin E. Additional studies have shown that Abeta(1-42) induces oxidative stress when expressed in vivo in Caenorhabditis elegans, but when methionine 35 is replaced by cysteine, the oxidative stress is attenuated. This finding coupled with in vitro studies using mutant peptides have demonstrated a critical role for methionine 35 in the oxidative stress and neurotoxic properties of Abeta(1-42). In this review, we discuss the role of methionine 35 in the oxidative stress and neurotoxicity induced by Abeta(1-42) and the implications of these findings in the pathogenesis of AD.

Assessment of the Bioactivity of Antibodies against β-Amyloid Peptide in vitro and in vivo

The accumulation of the beta-amyloid peptide (Abeta) is a central event in the pathogenesis of Alzheimer's disease (AD). Abeta removal from the brain by immune therapy shows promising potential for the treatment of patients with AD, although the mechanisms of the antibody action are incompletely understood. In this study we compared the biological activities of antibodies raised against various Abeta fragments for Abeta reduction in vitro and in vivo. Antibodies against Abeta enhanced the uptake of Abeta42 aggregates up to 6-fold by primary microglial cells in vitro. The kinetics of Abeta42 uptake varied considerably among antibodies. Based on the activity to mediate Abeta42 uptake by microglial cells, we identified a bioactive antibody that significantly reduced Abeta42 levels in the brains of transgenic mice with neuronal expression of an AD-related mutated amyloid precursor protein. This effect depended on the epitopes recognized by the antibody. Our data suggest that the ability to facilitate Abeta42 uptake by primary microglia cells in vitro can be used to predict the biological activity of the antibody by passive immunization in vivo. This protocol may prove useful for the rapid validation of the activity of antibodies designed to be used in immune therapy of AD.

Clearing tau pathology with Abeta immunotherapy--reversible and irreversible stages revealed

The report by Oddo and colleagues in this issue of Neuron demonstrates for the first time that clearance of amyloid also results in the removal of early-stage tau pathology in mice that develop both amyloid plaques and neurofibrillary tangles (NFT), the two hallmark lesions of Alzheimer's disease (AD). This result supports a primary role for Abeta in AD etiology.

Abeta immunotherapy leads to clearance of early, but not late, hyperphosphorylated tau aggregates via the proteasome

Amyloid-beta (Abeta) plaques and neurofibrillary tangles are the hallmark neuropathological lesions of Alzheimer's disease (AD). Using a triple transgenic model (3xTg-AD) that develops both lesions in AD-relevant brain regions, we determined the consequence of Abeta clearance on the development of tau pathology. Here we show that Abeta immunotherapy reduces not only extracellular Abeta plaques but also intracellular Abeta accumulation and most notably leads to the clearance of early tau pathology. We find that Abeta deposits are cleared first and subsequently reemerge prior to the tau pathology, indicative of a hierarchical and direct relationship between Abeta and tau. The clearance of the tau pathology is mediated by the proteasome and is dependent on the phosphorylation state of tau, as hyperphosphorylated tau aggregates are unaffected by the Abeta antibody treatment. These findings indicate that Abeta immunization may be useful for clearing both hallmark lesions of AD, provided that intervention occurs early in the disease course.

Cell biology of protein misfolding: The examples of Alzheimer's and Parkinson's diseases

The salutary intersection of fundamental cell biology with the study of disease is well illustrated by the emerging elucidation of neurodegenerative disorders. Novel mechanisms in cell biology have been uncovered through disease-orientated research; for example, the discovery of presenilin as an intramembrane aspartyl protease that processes many diverse proteins within the lipid bilayer. A common theme has arisen in this field: normally-soluble proteins accumulate, misfold and oligomerize, inducing cytotoxic effects that are particularly devastating in the post-mitotic milieu of the neuron.

Tau protein and neurodegeneration

Tau protein is the major component of the intracellular filamentous deposits that define a number of neurodegenerative diseases. They include the largely sporadic Alzheimer's disease, progressive supranuclear palsy (PSP), corticobasal degeneration (CBD), Pick's disease (PiD), argyrophilic grain disease, as well as the inherited frontotemporal dementia and parkinsonism linked to chromosome 17 (FTDP-17). The identification of mutations in Tau as the cause of FTDP-17 established that dysfunction or misregulation of tau protein is sufficient to cause neurodegeneration and dementia. At an experimental level, the new understanding is leading to the development of good transgenic animal models of the tauopathies.

Amyloid-beta precursor protein processing in neurodegeneration

The amyloid-beta precursor protein is proteolytically cleaved by secretases, resulting in a series of fragments, including the amyloid-beta peptide of Alzheimer's disease. The amyloid precursor protein, when membrane anchored, could operate as a receptor. After cleavage, the soluble ectodomain exerts a trophic function in the subventricular zone. The amyloid-beta peptide itself has a depressant role in synaptic transmission, with both physiological and pathological implications. During the past two years, much time has been invested in determining the molecular pathways that regulate the processing and the signal transduction of the amyloid precursor protein. However, the absence of consistent and informative phenotypes in different loss of function animal models make elucidating the molecular actions of the amyloid-beta precursor protein an ongoing challenge.

Transient cerebral ischemia induces site-specific hyperphosphorylation of tau protein

Neurofibrillary tangles (NFTs) are a pathological hallmark of many neurodegenerative diseases including Alzheimer's disease (AD), Parkinson's disease (PD), and frontotemporal dementia with parkinsonism linked to chromosome 17 (FTDP17). However, the cellular origin and the consequence of the NFT formation are poorly understood. Epidemiological evidence suggests a much higher occurrence of dementia in stroke patients. This may represent the pathogenesis of sporadic AD, which accounts for the majority of AD occurrence. Here we show that after a transient cerebral ischemia, hyperphosphorylated tau accumulates in cortical neurons in a site-specific manner. The hyperphosphorylated tau presents a conformation similar to those present in human tauopathies, and colocalizes largely with signs of apoptosis. Our current study suggests that tau hyperphosphorylation may contribute to the brain damage induced by transient cerebral ischemia, and may be involved in the pathogenesis of neurodegenerative disorders in patients after stroke. Further, these results indicate that ischemic neuronal damage and apoptosis associates with tau hyperphosphorylation, and potentially NFTs formation. Finally, our results also suggest that neuronal apoptosis may be a therapeutic target in preventing tauopathy-related neurodegenerative diseases.

Calcium dyshomeostasis in beta-amyloid and tau-bearing skeletal myotubes

The relative scarcity of inclusion-affected muscle cells or markers of cell death in inclusion body myositis (IBM) is in distinction to the specific and early intracellular deposition of several Alzheimer's Disease (AD)-related proteins. The current study examined the possible correlation between myotube beta-amyloid and/or Tau accumulations and a widespread mishandling of intracellular muscle calcium concentration that could potentially account for the unrelenting weakness in affected patients. Cultured myogenic cells (C(2)C(12)) expressed beta-amyloid-42 (Abeta(42)) and fetal Tau peptides, as human transgenes encoded by herpes simplex virus, either individually or concurrently. Co-expression of Abeta(42) in C(2)C(12) myotubes resulted in hyperphosphorylation of Tau protein that was not observed when Tau was expressed alone. Resting calcium concentration and agonist-induced RyR-mediated Ca(2+) release were examined using calcium-specific microelectrodes and Fluo-4 epifluorescence, respectively. Co-expression of Abeta(42) and Tau cooperatively elevated basal levels of myoplasmic-free calcium, an effect that was accompanied by depolarization of the plasma membrane. Sarcoplasmic reticulum (SR) calcium release, induced by KCl depolarization, was not affected by Abeta(42) or Tau. In contrast, expression of Abeta(42), Tau, or Abeta(42) together with Tau resulted in enhanced sensitivity of ryanodine receptors to activation by caffeine. Notably, expression of beta-amyloid, alone, was sufficient to result in an increased sensitivity to direct activation by caffeine. Current results indicate that amyloid proteins cooperate to raise resting calcium levels and that these effects are associated with a passive SR Ca(2+) leak and Tau hyperphosphorylation in skeletal muscle.

Spatial and temporal relationships between plaques and tangles in Alzheimer-pathology

One histological hallmark in Alzheimer's disease is the tangle. The other is the plaque. A widely discussed hypothesis is the "amyloid cascade" assuming that tangle formation is a direct consequence of amyloid plaque formation. The aim of this study was to examine plaques and tangles in a highly defined neuronal circuitry in order to determine their detailed spatial and temporal relationships. We investigated serial sections of the whole hippocampal formation of brains with early Braak-stages (0-III) for tangles only, i.e. one case at stage 0, six at stage I, six at stage II, and nine at stage III. Most cases displayed both plaques and tangles. Four cases of stages 0 and I, three cases with stage II, and even one with stage III, however, did not display plaques. In turn, no plaque was found in the absence of tangles. The spatial relationship indicates that plaques lay in the terminal fields of tangle-bearing neurons. Our analysis suggests that tangles either antecede plaques or--less likely--are independently formed.

The APP intracellular domain forms nuclear multiprotein complexes and regulates the transcription of its own precursor

The physiological functions of the beta-amyloid precursor protein (APP) may include nuclear signaling. To characterize the role of the APP adaptor proteins Fe65, Jip1b, X11α (MINT1) and the chromatin-associated protein Tip60, we analyzed their interactions by confocal microscopy and co-immunoprecipitations. AICD corresponding to S3-cleaved APP bound to Fe65 that transported it to nuclei and docked it to Tip60. These proteins formed AICD-Fe65-Tip60 (AFT) complexes that were concentrated in spherical nuclear spots. γ-Secretase inhibitors prevented AFT-complex formation with AICD derived from full-length APP. The APP adaptor protein Jip1b also transported AICD to nuclei and docked it to Tip60, but AICD-Jip1b-Tip60 (AJT) complexes had different, speckle-like morphology. By contrast, X11α trapped AICD in the cytosol. Induced AICD expression identified the APP-effector genes APP, BACE, Tip60, GSK3β and KAI1, but not the Notch-effector gene Hes1 as transcriptional targets. These data establish a role for APP in nuclear signaling, and they suggest that therapeutic strategies designed to modulate the cleavage of APP affect AICD-dependent signaling.

Amyloid‐induced neurofibrillary tangle formation in Alzheimer's disease: insight from transgenic mouse and tissue‐culture models

Of all forms of dementia, Alzheimer's disease is the most prevalent. It is histopathologically characterized by beta-amyloid-containing plaques, tau-containing neurofibrillary tangles, reduced synaptic density and neuronal loss in selected brain areas. For the rare familial forms of Alzheimer's disease, pathogenic mutations have been identified in both the gene encoding the precursor of the Abeta peptide, APP, itself and in the presenilin genes which encode part of the APP-protease complex. For the more frequent sporadic forms of Alzheimer's disease, the pathogenic trigger has not been unambiguously identified. Whether Abeta is again the main cause remains to be heavily discussed. In a related disorder termed frontotemporal dementia, which is characterized by tangles in the absence of beta-amyloid deposition, mutations have been identified in tau which also lead to neurodegeneration and dementia. For Alzheimer's disease the existence of familial forms lead to the proposition of the amyloid cascade hypothesis, which claims that beta-amyloid causes or enhances the tangle pathology. In this review, we describe tau transgenic mouse models in which aspects of the tau-associated pathology, including tangle formation, has been achieved. Moreover, tau transgenic mouse and tissue-culture models were used to test the amyloid cascade hypothesis. In addition, we discuss alternative hypotheses to explain the sporadic forms. The animal and tissue-culture models will provide insight into the underlying biochemical mechanisms of tau aggregation and nerve cell degeneration. These mechanisms may be partially shared between sporadic Alzheimer's disease, the familial forms and frontotemporal dementia. Eventually, Alzheimer's disease may be redefined based on biochemical events rather than phenotype.

Alzheimer pathology disorganizes cortico-cortical circuitry: direct evidence from a transgenic animal model

It has been proposed that Alzheimer disease (AD) is associated with a "disconnection syndrome" due to the gradual loss of morphological and functional integrity of cortico-cortical pathways. This hypothesis derives from indirect neuropathological observations, but definitive evidence that AD primarily targets cortico-cortical networks is still lacking. By means of neuroanatomical anterograde tracing methods, we have investigated, in a murine transgenic model of AD, the impact of the amyloid burden on axonal terminals in different neural systems. Axonal tracings revealed, in accordance with the "disconnection syndrome" hypothesis, that cortico-cortical fibers are significantly disorganized. Terminal fields in local and distant cortical areas contained numerous swollen dystrophic neurites often grouped in grape-like clusters at the plaque periphery. In contrary to fibers of cortical origin, those originating from subcortical brain structures only showed limited signs of degeneration upon reaching their cortical targets. These observations suggest a selective disruption of cortico-cortical connections induced by AD brain pathology.

CA1 hippocampal neuronal loss in familial Alzheimer's disease presenilin-1 E280A mutation is related to epilepsy

PURPOSE

Alzheimer disease (AD) and epilepsy are brain disorders frequently associated with neuronal cell loss in mesial temporal lobe structures, but presenting different patterns of damage. Recently it was proposed that a causal relation may exist between AD pathology and the appearance of epilepsy in some cases with AD. This study aimed to determine the neuronal loss in CA1 hippocampal region from patients bearing the presenilin-1 [E280A] mutation (PS1[E280A]) associated with seizures.

METHODS

Coronal sections from the hippocampal formation (anterior one third) from controls (n = 5) and familial AD (FAD; n = 8) patients were stained by using thionin and thioflavine-S staining to evaluate the number of neurons in the CA1 field, beta-plaques, and neurofibrillary tangles, respectively.

RESULTS

Two distinct patterns of neuronal loss in the CA1 field of FAD patients were found: (a) diffuse-patchy neuronal loss (three FAD nonepilepsy patients) characterized by both a general decrease of neurons and the presence of multiple, small regions devoid of neurons; and (b) sclerotic-like neuronal loss (five FAD epilepsy patients) similar to that found typically in the CA1 field of epilepsy patients with hippocampal sclerosis.

CONCLUSIONS

This investigation shows for the first time CA1 neuronal depopulation in a subpopulation of patients (five of eight) bearing the PS1[E280A] mutation with epileptic seizures, indicating a relation between hippocampal neuronal loss and epileptic seizures in FAD patients.

Protein misfolding in neurodegenerative diseases

A common pathogenic mechanism shared by diverse neurodegenerative disorders, like Alzheimer's disease, Parkinson's disease, Huntington's disease and transmissible spongiform encephalopathies, may be altered protein homeostasis leading to protein misfolding and aggregation of a wide variety of different proteins in the form of insoluble fibrils. Mutations in the genes encoding protein constituents of these aggregates have been linked to the corresponding diseases, thus a reasonable scenario of pathogenesis was based on misfolding of a neurone-specific protein that forms insoluble fibrils that subsequently kill neuronal cells. However, during the past 5 years accumulating evidence has revealed the neurotoxic role of prefibrillar intermediate forms (soluble oligomers and protofibrils) produced during fibril formation. Many think these may be the predominant neurotoxic species, whereas microscopically visible fibrillar aggregates may not be toxic. Large protein aggregates may rather be simply inactive, or even represent a protective state that sequesters and inactivates toxic oligomers and protofibrils. Further understanding of the biochemical mechanisms involved in protein misfolding and fibrillization may optimize the planning of common therapeutic approaches for neurodegenerative diseases, directed towards reversal of protein misfolding, blockade of protein oligomerization and interference with the action of toxic proteins.

Dimeric amyloid beta protein rapidly accumulates in lipid rafts followed by apolipoprotein E and phosphorylated tau accumulation in the Tg2576 mouse model of Alzheimer's disease

To investigate lipid rafts as a site where amyloid beta protein (Abeta) oligomers might accumulate and cause toxicity in Alzheimer's disease (AD), we analyzed Abeta in the Tg2576 transgenic mouse model of AD. Abeta was highly concentrated in lipid rafts, which comprise a small fraction of brain volume but contain 27% of brain Abeta42 and 24% of Abeta40 in young mice. In the Tg2576 model, memory impairment begins at 6 months before amyloid plaques are visible. Here we show that Abeta dimers appear in lipid rafts at 6 months and that raft Abeta, which is primarily dimeric, rapidly accumulates reaching levels >500x those in young mice by 24-28 months. A similar large accumulation of dimeric Abeta was observed in lipid rafts from AD brain. In contrast to extracellular amyloid fibrils, which are SDS-insoluble, virtually all Abeta in lipid rafts is SDS soluble. Coupled with recent studies showing that synthetic and naturally occurring Abeta oligomers can inhibit hippocampal long-term potentiation, the in vivo age-dependent accumulation of SDS-soluble Abeta dimers in lipid rafts at the time when memory impairment begins in Tg2576 mice provides strong evidence linking Abeta oligomers to memory impairment. After dimeric Abeta began to accumulate in lipid rafts of the Tg2576 brain, apolipoprotein E (ApoE) and then phosphorylated tau accumulated. A similar increase in ApoE and a large increase in phosphorylated tau was observed in lipid rafts from AD brain. These findings suggest that lipid rafts may be an important site for interaction between dimeric Abeta, ApoE, and tau.

Memantine inhibits and reverses the Alzheimer type abnormal hyperphosphorylation of tau and associated neurodegeneration

Memantine, an N-methyl-D-aspartate (NMDA) receptor antagonist, reduces the clinical deterioration in moderate-to-severe Alzheimer disease (AD) for which other treatments are not available. The activity of protein phosphatase (PP)-2A is compromised in AD brain and is believed to be a cause of the abnormal hyperphosphorylation of tau and the consequent neurofibrillary degeneration. Here we show that memantine inhibits and reverses the PP-2A inhibition-induced abnormal hyperphosphorylation and accumulation of tau in organotypic culture of rat hippocampal slices. Such restorative effects of memantine were not detected either with 5,7-dichlorokynurenic acid or with D(-)-2-amino-5-phosphopentanoic acid, NMDA receptor antagonists active at the glycine binding site and at the glutamate binding site, respectively. These findings show (1) that memantine inhibits and reverses the PP-2A inhibition-induced abnormal hyperphosphorylation of tau/neurofibrillary degeneration and (2) that this drug might be useful for the treatment of AD and related tauopathies.

Increased MAP kinase activity in Alzheimer's and Down syndrome but not in schizophrenia human brain

Abnormal phosphorylation of tau is a feature of Alzheimer's disease (AD), which develops prematurely in Down syndrome (DS) patients. Cognitive impairment is also recognized as a clinical characteristic of schizophrenia, which does not appear to be associated with tau-aggregate formation. Several kinases can phosphorylate tau in cell-free assays. Here we show increased activity of mitogen-activated protein kinases (MAPKs) (including ERK1/2, SAPKs and p38) in post mortem AD and DS brains, which could not be accounted for by expression changes. In contrast, glycogen synthase kinase-3 activity (GSK-3 alpha beta) was reduced significantly. Examination of tau in AD and DS using antibodies selective for MAPK phosphorylation sites showed increased immunoreactivity. In addition, phosphorylation of S(199), reportedly a selective substrate for cyclin-dependent kinase-5 (cdk5) or GSK-3 alpha beta was only observed in AD samples, which showed a concomitant increase in the expression of p25, the enhancing cofactor for cdk5 activity. However, in schizophrenia brain, MAPK-phosphorylated tau was unchanged compared to matched controls, despite similar expression levels to those in AD. The activities of the MAPKs and GSK-3 alpha beta were also unchanged. These data demonstrate that in AD and DS, enhanced MAPK activity, which has an established role in regulating neuronal plasticity and survival, can account for irregular tau phosphorylation, and that the molecular processes involved in these neurodegenerative disorders are distinct from those in schizophrenia. These data also question the significance of GSK-3 alpha beta, as much previous work carried out in vitro has placed this kinase as a favoured candidate for involvement in the pathological phosphorylation of tau.

Caspase-cleavage of tau is an early event in Alzheimer disease tangle pathology

Neurofibrillary tangles (NFTs) are composed of abnormal aggregates of the cytoskeletal protein tau. Together with amyloid beta (Abeta) plaques and neuronal and synaptic loss, NFTs constitute the primary pathological hallmarks of Alzheimer disease (AD). Recent evidence also suggests that caspases are activated early in the progression of AD and may play a role in neuronal loss and NFT pathology. Here we demonstrate that tau is cleaved at D421 (DeltaTau) by executioner caspases. Following caspase-cleavage, DeltaTau facilitates nucleation-dependent filament formation and readily adopts a conformational change recognized by the early pathological tau marker MC1. DeltaTau can be phosphorylated by glycogen synthase kinase-3beta and subsequently recognized by the NFT antibody PHF-1. In transgenic mice and AD brains, DeltaTau associates with both early and late markers of NFTs and is correlated with cognitive decline. Additionally, DeltaTau colocalizes with Abeta(1-42) and is induced by Abeta(1-42) in vitro. Collectively, our data imply that Abeta accumulation triggers caspase activation, leading to caspase-cleavage of tau, and that this is an early event that may precede hyperphosphorylation in the evolution of AD tangle pathology. These results suggest that therapeutics aimed at inhibiting tau caspase-cleavage may prove beneficial not only in preventing NFT formation, but also in slowing cognitive decline.

Molecular Genetics of Late-Onset Alzheimer's Disease

Late-onset Alzheimer's disease (AD) is a complex and multifactorial disease with the possible involvement of several genes. Apolipoprotein E (APOE), especially the APOE*4 allele, has been established as a strong susceptibility marker that accounts for nearly 30% of the risk in late-onset AD. However, as the APOE*4 allele is neither necessary nor sufficient for the development of AD, it emphasizes the involvement of other genetic and/or environmental factors which, alone or in conjunction with APOE*4, can modify the risk of AD. Recently, genome-wide linkage or linkage disequilibrium studies on late-onset AD have provided informative data for the existence of multiple putative genes for AD on several chromosomes, with the strongest evidence on chromosomes 12, 10, 9 and 6. This paper attempts to review the current progress on the identification of additional genetic loci for late-onset AD.

Transgenic animal models of Alzheimer's disease and related disorders: histopathology, behavior and therapy

Alzheimer's disease (AD) is a devastating neurodegenerative disease that affects more than 15 million people worldwide. Within the next generation, these numbers will more than double. To assist in the elucidation of pathogenic mechanisms of AD and related disorders, such as frontotemporal dementia (FTDP-17), genetically modified mice, flies, fish and worms were developed, which reproduce aspects of the human histopathology, such as beta-amyloid-containing plaques and tau-containing neurofibrillary tangles (NFT). In mice, the tau pathology caused selective behavioral impairment, depending on the distribution of the tau aggregates in the brain. Beta-amyloid induced an increase in the numbers of NFT, whereas the opposite was not observed in mice. In beta-amyloid-producing transgenic mice, memory impairment was associated with increased levels of beta-amyloid. Active and passive beta-amyloid-directed immunization caused the removal of beta-amyloid plaques and restored memory functions. These findings have since been translated to human therapy. This review aims to discuss the suitability and limitations of the various animal models and their contribution to an understanding of the pathophysiology of AD and related disorders.

A disintegrin-metalloproteinase prevents amyloid plaque formation and hippocampal defects in an Alzheimer disease mouse model

Alzheimer disease (AD) is characterized by excessive deposition of amyloid beta-peptides (A beta peptides) in the brain. In the nonamyloidogenic pathway, the amyloid precursor protein (APP) is cleaved by the alpha-secretase within the A beta peptide sequence. Proteinases of the ADAM family (adisintegrin and metalloproteinase) are the main candidates as physiologically relevant alpha-secretases, but early lethality of knockout animals prevented a detailed analysis in neuronal cells. To overcome this restriction, we have generated transgenic mice that overexpress either ADAM10 or a catalytically inactive ADAM10 mutant. In this report we show that a moderate neuronal overexpression of ADAM10 in mice transgenic for human APP([V717I]) increased the secretion of the neurotrophic soluble alpha-secretase-released N-terminal APP domain (APPs alpha), reduced the formation of A beta peptides, and prevented their deposition in plaques. Functionally, impaired long-term potentiation and cognitive deficits were alleviated. Expression of mutant catalytically inactive ADAM10 led to an enhancement of the number and size of amyloid plaques in the brains of double-transgenic mice. The results provide the first in vivo evidence for a proteinase of the ADAM family as an alpha-secretase of APP, reveal activation of ADAM10 as a promising therapeutic target, and support the hypothesis that a decrease in alpha-secretase activity contributes to the development of AD.

Role of hydrogen peroxide in the aetiology of Alzheimer's disease: implications for treatment

Hydrogen peroxide (H(2)O(2)) is a stable, uncharged and freely diffusable reactive oxygen species (ROS) and second messenger. The generation of H(2)O(2) in the brain is relatively high because of the high oxygen consumption in the tissue. Alzheimer's disease is a neurodegenerative disorder characterised by the appearance of amyloid-beta (Abeta)-containing plaques and hyperphosphorylated tau-containing neurofibrillary tangles. The pathology of Alzheimer's disease is also associated with oxidative stress and H(2)O(2) is implicated in this and the neurotoxicity of the Abeta peptide. The ability for Abeta to generate H(2)O(2), and interactions of H(2)O(2) with iron and copper to generate highly toxic ROS, may provide a mechanism for the oxidative stress associated with Alzheimer's disease. The role of heavy metals in Alzheimer's disease pathology and the toxicity of the H(2)O(2) molecule may be closely linked. Drugs that prevent oxidative stress include antioxidants, modifiers of the enzymes involved in ROS generation and metabolism, metal chelating agents and agents that can remove the stimulus for ROS generation. In Alzheimer's disease the H(2)O(2) molecule must be considered a therapeutic target for treatment of the oxidative stress associated with the disease. The actions of H(2)O(2) include modifications of proteins, lipids and DNA, all of which are effects seen in the Alzheimer's disease brain and may contribute to the loss of synaptic function characteristic of the disease. The effectiveness of drugs to target this component of the disease pathology remains to be determined; however, metal chelators may provide an effective route and have the added bonus in the case of clioquinol of potentially reducing the Abeta load. Future research and development of agents that specifically target the H(2)O(2) molecule or enzymes involved in its metabolism may provide the future route to Alzheimer's disease therapy.

Accelerated extinction of conditioned taste aversion in P301L tau transgenic mice

Neurofibrillary tangles, insoluble protein deposits composed of filamentous tau aggregates, are neuropathological hallmarks of Alzheimer's disease and familial frontotemporal dementia (FTDP-17). Transgenic mice expressing the FTDP-17 mutation P301L of tau recapitulate key features of the human pathology, that is, tau proteins aggregate and neurofibrillary tangles begin to appear in the amygdala at 6 months of age. To detect early signs of tau aggregate-associated changes, we investigated behavioral alterations and cognitive deficits in such mice using an amygdala-specific test battery for anxiety-related and cognitive behavior. P301L mice had anxiety levels not different from wild-types, but their exploratory behavior was significantly increased. Acquisition of a fear response to tone and context as well as taste aversion was comparable to wild-types. However, extinction of a conditioned taste aversion was significantly accelerated. We conclude that already aggregation of tau proteins not yet accompanied by massive formation of neurofibrillary tangles causes selective behavioral deficits.

Role for glyoxalase I in Alzheimer's disease

P301L mutant tau transgenic mice develop neurofibrillary tangles, a histopathologic hallmark of Alzheimer's disease and frontotemporal dementia (FTDP-17). To identify differentially expressed genes and to gain insight into pathogenic mechanisms, we performed a stringent analysis of the microarray dataset obtained with RNA from whole brains of P301L mutant mice and identified a single up-regulated gene, glyoxalase I. This enzyme plays a critical role in the detoxification of dicarbonyl compounds and thereby reduces the formation of advanced glycation end products. In situ hybridization analysis revealed expression of glyoxalase I in all brain areas analyzed, both in transgenic and control mice. However, levels of glyoxalase I protein were significantly elevated in P301L brains, as shown by Western blot analysis and immunohistochemistry. Moreover, a glyoxalase I-specific antiserum revealed many intensely stained flame-shaped neurons in Alzheimer's disease brain compared with brains from nondemented controls. In addition, we examined a single nucleotide polymorphism predicting a nonconservative amino acid substitution at position 111 (E111A) in ethnically independent populations. We identified significant and consistent deviations from Hardy-Weinberg equilibrium, which points to the presence of selection forces. The E111A single nucleotide polymorphism was not associated with the risk for Alzheimer's disease in the overall population. Together, our data demonstrate the potential of transcriptomics applied to animal models of human diseases. They suggest a previously unidentified role for glyoxalase I in neurodegenerative disease.

Active Immunization of Mice with an Aβ-Hsp70 Vaccine

Heat-shock proteins are highly immunogenic. Complexed with an antigen, they act as adjuvants, inducing a humoral and cellular immune response against both the antigen and the chaperone. In this study, we produced an Hsp70-supported vaccine to induce the generation of antibodies against amyloid-beta (Abeta) peptides, the major constituent of beta-amyloid plaques in Alzheimer's disease. The vaccine consisted of synthetic human Abeta42 covalently cross-linked with DnaK, an Hsp70 homolog of Escherichia coli. Active immunization of mice with this vaccine resulted in the generation of antibodies against Abeta, that were detectable in sera after the first booster immunization. Antibody titers varied markedly with the genetic background of the mice. Prophylactic short-term immunization of transgenic mice (APP tg2576) before the onset of plaques, however, did not prevent amyloid plaque deposition. There were no differences in the plaque load and in the level of Triton X-100-soluble Abeta peptides in the brains of immunized and control-treated transgenic mice. Unexpectedly, the level of formic-acid soluble Abeta peptides tended to be higher in immunized mice. The reason for the increase may be an enhanced deposition of Abeta in the small cerebral blood vessels. These data emphasize the need for anti-Abeta antibodies that remove Abeta peptides from the central nervous system without negative side effects.

Characterisation of cytoskeletal abnormalities in mice transgenic for wild-type human tau and familial Alzheimer's disease mutants of APP and presenilin-1

To study the role of Abeta amyloid deposits in the generation of cytoskeletal lesions, we have generated a transgenic mouse line coexpressing in the same neurons a wild-type human tau isoform (0N3R), a mutant form of APP (751SL) and a mutant form of PS1 (M146L). These mice developed early cerebral extracellular deposits of Abeta, starting at 2.5 months. A somatodendritic neuronal accumulation of transgenic tau protein was observed in tau only and in tau/PS1/APP transgenic mice, including in neurons adjacent to Abeta deposits. The phosphorylation status of this somatodendritic tau was similar in the two transgenic lines. The Abeta deposits were surrounded by a neuritic reaction composed of axonal dystrophic processes, immunoreactive for many phosphotau epitopes and for the human tau transgenic protein. Ultrastructural observation showed in these dystrophic neurites a disorganisation of the microtubule and the neurofilament network but animals that were observed up to 18 months of age did not develop neurofibrillary tangles. These results indicate that overexpression of mutant PS1, mutant APP and of wild-type human tau were not sufficient per se to drive the formation of neurofibrillary tangles in a transgenic model. The Abeta deposits, however, were associated to marked changes in cytoskeletal organisation and in tau phosphorylation in adjacent dystrophic neurites.

In vitro cultured neurons for molecular studies correlating apoptosis with events related to Alzheimer disease

This short review analyses the possible molecular events linking a general program of death such as apoptosis to highly specific intracellular pathways involving the function and degradation of two proteins--tau and amyloid precursor protein--which in their aggregated state constitute the hallmark of Alzheimer disease. By surveying the recent studies carried out in 'in vitro' neuronal cultures--with special emphasis to cerebellar granule neurons--the apparent correlation between onset of apoptosis, tau cleavage with formation of potential toxic fragments, and activation of an amyloidogenic route are discussed. Within this framework, proteasomes seem to play a crucial role upstream of the proteolytic cascade involving calpain(s) and caspase(s) by contributing to tau and amyloid precursor protein-altered breakdown and consequent tendency to aggregation of their degradation fragments. Thus, apoptotic death due to altered supply of anti apoptotic agents, neurotrophic factors, deafferentiation or other causes, may constitute a major trigger of the onset of Alzheimer disease.

PAR-1 Kinase Plays an Initiator Role in a Temporally Ordered Phosphorylation Process that Confers Tau Toxicity in Drosophila

Multisite hyperphosphorylation of tau has been implicated in the pathogenesis of neurodegenerative diseases including Alzheimer's disease (AD). However, the phosphorylation events critical for tau toxicity and mechanisms regulating these events are largely unknown. Here we show that Drosophila PAR-1 kinase initiates tau toxicity by triggering a temporally ordered phosphorylation process. PAR-1 directly phosphorylates tau at S262 and S356. This phosphorylation event is a prerequisite for the action of downstream kinases, including glycogen synthase kinase 3 (GSK-3) and cyclin-dependent kinase-5 (Cdk5), to phosphorylate several other sites and generate disease-associated phospho-epitopes. The initiator role of PAR-1 is further underscored by the fact that mutating PAR-1 phosphorylation sites causes a much greater reduction of overall tau phosphorylation and toxicity than mutating S202, one of the downstream sites whose phosphorylation depends on prior PAR-1 action. These findings begin to differentiate the effects of various phosphorylation events on tau toxicity and provide potential therapeutic targets.

Transient cerebral ischemia induces aberrant neuronal cell cycle re-entry and Alzheimer's disease-like tauopathy in female rats

Aberrant mitosis occurs in many tauopathy-related neurodegenerative diseases and is believed to precede the formation of neurofibrillary tangles. In this study, we report for the first time that transient cerebral ischemia induces aberrant mitotic proteins and hyperphosphorylation of tau protein with neurofibrillary tangle-like conformational epitopes in adult female rat cortex. Following transient cerebral ischemia in rats, initiation of apoptosis precedes and is potentially integrated with subsequent aberrant mitosis and tau hyperphosphorylation. Furthermore, inhibition of mitosis-related cyclin-dependent kinases (Cdks) by roscovitine significantly reduced the hyperphosphorylation of tau. Administration of the female sex steroid and potent neuroprotective agent, 17beta-estradiol, reduced ischemia-reperfusion-induced cerebral damage and the subsequent aberrant mitosis and tauopathies. These results provide a neuropathological basis for the higher prevalence of dementia in stroke patients and support the hypothesis that apoptosis and aberrant mitosis are integrated pathological events in neurons that may play a critical role in the development of Alzheimer's disease and other tauopathy-related neuropathology.

Altered mitogen-activated protein kinase signaling, tau hyperphosphorylation and mild spatial learning dysfunction in transgenic rats expressing the β-amyloid peptide intracellularly in hippocampal and cortical neurons

The pathological significance of intracellular Abeta accumulation in vivo is not yet fully understood. To address this, we have studied transgenic rats expressing Alzheimer's-related transgenes that accumulate Abeta intraneuronally in the cerebral and hippocampal cortices but do not develop extracellular amyloid plaques. In these rats, the presence of intraneuronal Abeta is sufficient to provoke up-regulation of the phosphorylated form of extracellular-regulated kinase (ERK) 2 and its enzymatic activity in the hippocampus while no changes were observed in the activity or phosphorylation status of other putative tau kinases such as p38, glycogen synthase kinase 3, and cycline-dependent kinase 5. The increase in active phospho-ERK2 was accompanied by increased levels of tau phosphorylation at S396 and S404 ERK2 sites and a decrease in the phosphorylation of the CREB kinase p90RSK. In a water maze paradigm, male transgenic rats displayed a mild spatial learning deficit relative to control littermates. Our results suggest that in the absence of plaques, intraneuronal accumulation of Abeta peptide correlates with the initial steps in the tau-phosphorylation cascade, alterations in ERK2 signaling and impairment of higher CNS functions in male rats.

The role of tau (MAPT) in frontotemporal dementia and related tauopathies

Tau is a multifunctional protein that was originally identified as a microtubule-associated protein. In patients diagnosed with frontotemporal dementia and parkinsonism linked to chromosome 17, mutations in the gene encoding tau (MAPT) have been identified that disrupt the normal binding of tau to tubulin resulting in pathological deposits of hyperphosphorylated tau. Abnormal filamentous tau deposits have been reported as a pathological characteristic in several other neurodegenerative diseases, including frontotemporal dementia, Pick Disease, Alzheimer disease, argyrophilic grain disease, progressive supranuclear palsy, and corticobasal degeneration. In the last five years, extensive research has identified 34 different pathogenic MAPT mutations in 101 families worldwide. In vitro, cell-free and transfected cell studies have provided valuable information on tau dysfunction and transgenic mice carrying human MAPT mutations are being generated to study the influence of MAPT mutations in vivo. This mutation update describes the considerable differences in clinical and pathological presentation of patients with MAPT mutations and summarizes the effect of the different mutations on tau functioning. In addition, the role of tau as a genetic susceptibility factor is discussed, together with the genetic evidence for additional causal genes for tau-positive as well as tau-negative dementia.

Mouse models of Alzheimer's disease: the long and filamentous road

Alzheimer's disease (AD) is characterized by memory impairment leading to dementia, deposition of amyloid plaques and neurofibrillary tangles (NFTs), and neuronal loss. The major component of plaques is the amyloid beta peptide, A beta, whereas NFTs contain hyperphosphorylated forms of the microtubule-associated protein tau (tau). Familial AD (FAD) mutations either elevate A beta synthesis by favoring 'secretase' of the Alzheimer beta-amyloid precursor protein (APP) or enhance the fibrillogenic properties of this peptide. Mutations in the tau gene cause a different disease denoted FTPD-17, but suggest that the aberrant forms of tau seen in AD are unlikely to be benign. These findings imply a complex pathogenic cascade in AD and important goals of transgenic modeling are to capture and stratify this pathogenic process. Several laboratories have created APP transgenic (Tg) mice that exhibit AD-like amyloid pathology and A beta burdens. These Tg lines also exhibit deficits in spatial reference and/or working memory, with immunization against A beta attenuating both AD-associated phenotypes. Tangle-like pathologies are observed in mice expressing FTPD-17 mutant forms of tau, but florid tau pathologies based upon the wild type (wt) tau isoforms present in AD have proven more elusive. Creation of animal models with robust amyloid and tau pathologies, yet free of irrelevant confounding pathologies, remains a major objective in this field.

Amyloid beta-protein (Abeta)1-40 protects neurons from damage induced by Abeta1-42 in culture and in rat brain

Previously, we found that amyloid beta-protein (Abeta)1-42 exhibits neurotoxicity, while Abeta1-40 serves as an antioxidant molecule by quenching metal ions and inhibiting metal-mediated oxygen radical generation. Here, we show another neuroprotective action of nonamyloidogenic Abeta1-40 against Abeta1-42-induced neurotoxicity in culture and in vivo. Neuronal death was induced by Abeta1-42 at concentrations higher than 2 microm, which was prevented by concurrent treatment with Abeta1-40 in a dose-dependent manner. However, metal chelators did not prevent Abeta1-42-induced neuronal death. Circular dichroism spectroscopy showed that Abeta1-40 inhibited the beta-sheet transformation of Abeta1-42. Thioflavin-T assay and electron microscopy analysis revealed that Abeta1-40 inhibited the fibril formation of Abeta1-42. In contrast, Abeta1-16, Abeta25-35, and Abeta40-1 did not inhibit the fibril formation of Abeta1-42 nor prevent Abeta1-42-induced neuronal death. Abeta1-42 injection into the rat entorhinal cortex (EC) caused the hyperphosphorylation of tau on both sides of EC and hippocampus and increased the number of glial fibrillary acidic protein (GFAP)-positive astrocytes in the ipsilateral EC, which were prevented by the concurrent injection of Abeta1-40. These results indicate that Abeta1-40 protects neurons from Abeta1-42-induced neuronal damage in vitro and in vivo, not by sequestrating metals, but by inhibiting the beta-sheet transformation and fibril formation of Abeta1-42. Our data suggest a mechanism by which elevated Abeta1-42/Abeta1-40 ratio accelerates the development of Alzheimer's disease (AD) in familial AD.

In vitro models of age-related neurodegenerative disorders

Aging is the major risk factor for numerous brain diseases. This is especially true for Alzheimer's disease (AD), a peculiar neurodegenerative disorder in that it results from the synergy of two simultaneous but distinct degenerating processes: A beta and tau pathologies. For AD, and for most neurodegenerative disorders, aggregation of full length or truncated proteins, in neurons or glial cells, or in the parenchyma, is central, but still a mystery. In addition, the late onset of these pathologies links them to ageing processes. Cause or consequence? Experimental models, that allow us to dissect these pathophysiological defects, are presented.

Detangling Alzheimer's Disease

Alzheimer's disease afflicts 4.5 million people in the United States, and the number is expected to rise to 16 million by the year 2050, as the population ages. Researchers are scrambling to find genetic risk factors, decipher disease mechanisms, and develop reliable diagnostic tests that detect the illness at its earliest, potentially most treatable stage. Using these findings, they hope to devise new therapeutic approaches. Current clinical trials are assessing novel techniques that stall or reverse Alzheimer-like neuropathology in mice.