Soluble pool of Abeta amyloid as a determinant of severity of neurodegeneration in Alzheimer's disease

Immunization reverses memory deficits without reducing brain Abeta burden in Alzheimer's disease model

We have previously shown that chronic treatment with the monoclonal antibody m266, which is specific for amyloid beta-peptide (Abeta), increases plasma concentrations of Abeta and reduces Abeta burden in the PDAPP transgenic mouse model of Alzheimer's disease (AD). We now report that administration of m266 to PDAPP mice can rapidly reverse memory deficits in both an object recognition task and a holeboard learning and memory task, but without altering brain Abeta burden. We also found that an Abeta/antibody complex was present in both the plasma and the cerebrospinal fluid of m266-treated mice. Our data indicate that passive immunization with this anti-Abeta monoclonal antibody can very rapidly reverse memory impairment in certain learning and memory tasks in the PDAPP mouse model of AD, owing perhaps to enhanced peripheral clearance and (or) sequestration of a soluble brain Abeta species.

Highly conserved and disease-specific patterns of carboxyterminally truncated Abeta peptides 1-37/38/39 in addition to 1-40/42 in Alzheimer's disease and in patients with chronic neuroinflammation

Human lumbar CSF patterns of Abeta peptides were analysed by urea-based beta-amyloid sodium dodecyl sulphate polyacrylamide gel electrophoresis with western immunoblot (Abeta-SDS-PAGE/immunoblot). A highly conserved pattern of carboxyterminally truncated Abeta1-37/38/39 was found in addition to Abeta1-40 and Abeta1-42. Remarkably, Abeta1-38 was present at a higher concentration than Abeta1-42, being the second prominent Abeta peptide species in CSF. Patients with Alzheimer's disease (AD, n = 12) and patients with chronic inflammatory CNS disease (CID, n = 10) were differentiated by unique CSF Abeta peptide patterns from patients with other neuropsychiatric diseases (OND, n = 37). This became evident only when we investigated the amount of Abeta peptides relative to their total Abeta peptide concentration (Abeta1-x%, fractional Abeta peptide pattern), which may reflect disease-specific gamma-secretase activities. Remarkably, patients with AD and CID shared elevated Abeta1-38% values, whereas otherwise the patterns were distinct, allowing separation of AD from CID or OND patients without overlap. The presence of one or two ApoE epsilon4 alleles resulted in an overall reduction of CSF Abeta peptides, which was pronounced for Abeta1-42. The severity of dementia was significantly correlated to the fractional Abeta peptide pattern but not to the absolute Abeta peptide concentrations.

Localization of beta-secretase memapsin 2 in the brain of Alzheimer's patients and normal aged controls

Chronic accumulation of beta-amyloid in the brain has been shown to result in complex molecular and cellular changes that accompany neurodegeneration in Alzheimer's disease (AD). In this study, we examined the expression of a newly identified beta-secretase, memapsin 2 (M2) or beta-site APP cleaving enzyme in deparaffinized sections from 10 AD patients and 10 aged matched controls and in frozen samples of parietal cortex from 11 AD and 8 controls. M2 is mainly expressed in neurons, with high levels in CA4 to CA2 regions and transentorhinal cortex and low or intermediate levels in CA1, subiculum, and granule cells of the dentate gyrus. The majority of AD brains showed an increase of M2 expression in the CA1, but a decrease in the transentorhinal cortex. A subset of controls and AD patients had high M2 expression in parietal neocortex. Double-staining revealed that senile plaques are not directly associated with the soma of M2-expressing neurons. Neurofibrillary tangles were associated with lower M2 expression in AD. These data indicate that beta-secretase M2 may not be straightforwardly involved in amyloid plaque formation in AD brain.

Naturally secreted oligomers of amyloid beta protein potently inhibit hippocampal long-term potentiation in vivo

Although extensive data support a central pathogenic role for amyloid beta protein (Abeta) in Alzheimer's disease, the amyloid hypothesis remains controversial, in part because a specific neurotoxic species of Abeta and the nature of its effects on synaptic function have not been defined in vivo. Here we report that natural oligomers of human Abeta are formed soon after generation of the peptide within specific intracellular vesicles and are subsequently secreted from the cell. Cerebral microinjection of cell medium containing these oligomers and abundant Abeta monomers but no amyloid fibrils markedly inhibited hippocampal long-term potentiation (LTP) in rats in vivo. Immunodepletion from the medium of all Abeta species completely abrogated this effect. Pretreatment of the medium with insulin-degrading enzyme, which degrades Abeta monomers but not oligomers, did not prevent the inhibition of LTP. Therefore, Abeta oligomers, in the absence of monomers and amyloid fibrils, disrupted synaptic plasticity in vivo at concentrations found in human brain and cerebrospinal fluid. Finally, treatment of cells with gamma-secretase inhibitors prevented oligomer formation at doses that allowed appreciable monomer production, and such medium no longer disrupted LTP, indicating that synaptotoxic Abeta oligomers can be targeted therapeutically.

Hippocampal A beta 42 levels correlate with spatial memory deficit in APP and PS1 double transgenic mice

We investigated the role of hippocampal amyloid pathology in spatial learning impairment of a new mouse line carrying mutated human amyloid precursor protein (APP) and presenilin-1 (PS1) transgenes. The APP + PS1 mice were tested in spatial navigation in the water maze and in position discrimination in the T-maze at ages of 3-4 and 11-12 months, before and after the appearance of first amyloid plaques. The APP + PS1 mice were impaired in water maze acquisition and retention only at the age of 11-12 months, but performed equally to controls in the T-maze task at both ages. In the impaired older age group, the levels of total Abeta1-42 in the hippocampus of APP + PS1 mice correlated negatively with the retention score. Here we show for the first time that the age-dependent impairment in memory retention in the traditional water maze of APP + PS1 mice correlates with the amount of total Abeta in hippocampus even at a stage when the amyloid deposits cover less than 1% of the hippocampal volume.

Structure and location of amyloid beta peptide chains and arrays in Alzheimer's disease: new findings require reevaluation of the amyloid hypothesis and of tests of the hypothesis

New in situ high resolution electronmicroscopic examination of amyloid fibrils in situ indicate that in Alzheimer's disease these fibrils are not simply long chains of self aggregated amyloid beta peptide. The amyloid beta is not only associated with P protein and glycans, as was well known from previous immunohistologic studies, but is arranged in the form of short chains at right angles to a P protein backbone with the glycans wrapped around that backbone. These findings suggest that the hypothesis causally relating simple, fibrillar amyloid beta to Alzheimer's disease must be reevaluated since such simple fibrils may be absent, or not the major form of the amyloid beta in the brain. Other data shows that shorter multimers, so-called protofibrils, or dimers of amyloid beta or molecules cleaved from it can be highly toxic. Some of these may be in the soluble amyloid beta fraction. Shorter multimers or dimers of amyloid beta, either extra or intracellular, may be the real links between amyloid beta production and Alzheimer's disease. Toxicity studies employing fibrillar amyloid beta may not be relevant, even if they produce lesions, because they do not employ amyloid beta in the form in which it actually exists in the Alzheimer brain. Studies of treatments designed to remove fibrils or to prevent their formation may be ineffective or suboptimal in effectiveness because they do not reduce the relevant amyloid burden and/or fail to alter the arrangement of shorter multimers of amyloid beta around its P-protein and glycan core.

'...and C is for Clioquinol' - the AbetaCs of Alzheimer's disease

Alzheimer's disease (AD) is a devastating age-related neurodegenerative disorder that has been intensively studied over the last several years. In vitro and in vivo studies have led to an understanding of some of the physico-chemical properties of amyloid, a well-characterized hallmark of AD. Clioquinol is a drug that acts on amyloid by perturbing amyloid's metallo-chemistry, and Clioquinol treatment has been shown to be beneficial in a mouse model of AD. This short review examines the recent studies relating to Clioquinol and AD, and anticipates the imminent results of a Phase II trial of Clioquinol in AD, due in March 2002.

Association of CSF apolipoprotein E, Abeta42 and cognition in Alzheimer's disease

A significant association between CSF Abeta42 and cognition in patients with Alzheimer's disease (AD) homozygous for the epsilon3 allele of the apolipoprotein E (apoE) has been described. In this study we extended our observations on apoE, as another plaque component, and investigated the association between CSF apoE concentrations and cognitive performance after stratification for the apoE genotype in 62 patients with AD, 19 other forms of dementia and 18 controls. CSF Abeta42 and apoE concentrations were significantly and positively associated with Mini Mental State Examination (MMSE) score in AD (Abeta42: r = 0.332; P = 0.026; apoE: r = 0.386; P = 0.006). For Abeta42 this association was exclusively present in epsilon3 homozygotes (r = 0.44; P = 0.014), whereas apoE was correlated with MMSE in epsilon4 hetero- or homozygotes subjects (epsilon4/epsilonX: r = 0.638; P = 0.004: epsilon4/epsilon4; r = 0.812; P = 0.05). No association was observed between CSF concentrations of Abeta42 and apoE. The significant relationship between MMSE and CSF Abeta42 in epsilon3 homozygotes and apoE in epsilon4 hetero- and homozygotes respectively may suggest that both proteins may be associated independently from each other with cognitive decline.

Species-barrier-independent prion replication in apparently resistant species

Prion diseases of humans and animals are associated with the accumulation of an abnormal isoform (PrP(Sc)) of the host-encoded prion protein (PrP(C)). Transmission of these diseases between mammalian species is usually limited by a 'species barrier', which can be mediated by differences in primary sequence of the prion protein between donor and host species. Studies on species barriers usually rely on the development of clinical disease in inoculated animals as an assessment of susceptibility in a particular host. Recent studies by a number of groups have demonstrated that the absence of clinical symptoms does not necessarily exclude transmission of prion disease across a species barrier. Such results indicate that subclinical or carrier states exist in these diseases, which has public health implications regarding human exposure to BSE prions and iatrogenic transmission from apparently healthy humans. Here the issue of subclinical prion diseases is reviewed and implications are discussed.

Deposition of transthyretin in early stages of familial amyloidotic polyneuropathy: evidence for toxicity of nonfibrillar aggregates

Familial amyloidotic polyneuropathy (FAP) is a neurodegenerative disorder characterized by extracellular deposition of transthyretin (TTR) amyloid fibrils, particularly in the peripheral nervous system. No systematic immunohistochemical data exists relating TTR deposition with FAP progression. We assessed nerves from FAP patients in different stages of disease progression (FAP 0 to FAP 3) for TTR deposition by immunohistochemistry, and for the presence of amyloid fibrils by Congo Red staining. The nature of the deposited material was further studied by electron microscopy. We observed that early in FAP (FAP 0), TTR is already deposited in an aggregated nonfibrillar form, negative by Congo Red staining. This suggested that in vivo, preamyloidogenic forms of TTR exist in the nerve, in a stage before fibril formation. Cytotoxicity of nonfibrillar TTR was assessed in nerves of different FAP stages by immunohistochemistry for macrophage colony-stimulating factor. FAP 0 patients already presented increased axonal expression of macrophage colony-stimulating factor that was maintained in all other stages, in sites related to TTR deposition. Toxicity of synthetic TTR fibrils formed in vitro at physiological pH was studied on a Schwannoma cell line by caspase-3 activation assays and showed that early aggregates but not mature fibrils are toxic to cells. Taken together, these results show that nonfibrillar cytotoxic deposits occur in early stages of FAP.

Vaccination with soluble Abeta oligomers generates toxicity-neutralizing antibodies

In recent studies of transgenic models of Alzheimer's disease (AD), it has been reported that antibodies to aged beta amyloid peptide 1-42 (Abeta(1-42)) solutions (mixtures of Abeta monomers, oligomers and amyloid fibrils) cause conspicuous reduction of amyloid plaques and neurological improvement. In some cases, however, neurological improvement has been independent of obvious plaque reduction, and it has been suggested that immunization might neutralize soluble, non-fibrillar forms of Abeta. It is now known that Abeta toxicity resides not only in fibrils, but also in soluble protofibrils and oligomers. The current study has investigated the immune response to low doses of Abeta(1-42) oligomers and the characteristics of the antibodies they induce. Rabbits that were injected with Abeta(1-42) solutions containing only monomers and oligomers produced antibodies that preferentially bound to assembled forms of Abeta in immunoblots and in physiological solutions. The antibodies have proven useful for assays that can detect inhibitors of oligomer formation, for immunofluorescence localization of cell-attached oligomers to receptor-like puncta, and for immunoblots that show the presence of SDS-stable oligomers in Alzheimer's brain tissue. The antibodies, moreover, were found to neutralize the toxicity of soluble oligomers in cell culture. Results support the hypothesis that immunizations of transgenic mice derive therapeutic benefit from the immuno-neutralization of soluble Abeta-derived toxins. Analogous immuno-neutralization of oligomers in humans may be a key in AD vaccines.

Treatment of Alzheimer's disease with clioquinol

As heavy metal ions may be implicated in the formation of senile plaques in Alzheimer-afflicted brains, treatment with clioquinol was tested in 20 patients with Alzheimer's disease. Clioquinol is a chelator that crosses the blood-brain barrier and has greater affinity for zinc and copper ions than for calcium and magnesium ions. Treatment was given for 21 days at doses of 20 mg/day to 10 patients and 80 mg/day to another 10 patients. The study was blind to the dosages but included no controls. Cerebrospinal fluid (CSF) investigations revealed a significant increase at day 7 and a decrease at day 21 in Tau protein and growth-associated protein (GAP43). These proteins are increased in Alzheimer's disease and considered as rather stable markers. The initial increase may indicate a temporary cytotoxicity to the brain and/or an increased release into the CSF from stores in the tissue, possibly from senile plaques where the proteins are accumulated. The levels of CSF-Tau protein correlated positively and significantly with the serum levels of copper and also with the serum copper/zinc ratio. Clinical ratings showed slight improvement after 3 weeks treatment with clioquinol in this open study.

Beta amyloid peptide (Abeta42) is internalized via the G-protein-coupled receptor FPRL1 and forms fibrillar aggregates in macrophages

The 42 amino acid form of beta amyloid (Abeta42) plays a pivotal role in neurotoxicity and the activation of mononuclear phagocytes in Alzheimer's disease (AD). Our recent study revealed that FPRL1, a G-protein-coupled receptor, mediates the chemotactic and activating effect of Abeta42 on mononuclear phagocytes (monocytes and microglia), suggesting that FPRL1 may be involved in the proinflammatory responses in AD. We investigated the role of FPRL1 in cellular uptake and the subsequent fibrillar formation of Abeta42 by using fluorescence confocal microscopy. We found that upon incubation with macrophages or HEK293 cells genetically engineered to express FPRL1, Abeta42 associated with FPRL1 and the Abeta42/FPRL1 complexes were rapidly internalized into the cytoplasmic compartment. The maximal internalization of Abeta42/FPRL1 complexes occurred by 30 min after incubation. Removal of free Abeta42 from culture supernatants at 30 min resulted in a progressive recycling of FPRL1 to the cell surface and degradation of the internalized Abeta42. However, persistent exposure of the cells to Abeta42 over 24 h resulted in retention of Abeta42/FPRL1 complexes in the cytoplasmic compartment and the formation of Congo red positive fibrils in macrophages but not in HEK 293 cell transfected with FPRL1. These results suggest that besides mediating the proinflammatory activity of Abeta42, FPRL1 is also involved in the internalization of Abeta42, which culminates in the formation of fibrils only in macrophages.

The C-terminal fragment of the Alzheimer's disease amyloid protein precursor is degraded by a proteasome-dependent mechanism distinct from gamma-secretase

The beta-amyloid protein (Abeta) is derived by proteolytic processing of the amyloid protein precursor (APP). Cleavage of APP by beta-secretase generates a C-terminal fragment (APP-CTFbeta), which is subsequently cleaved by gamma-secretase to produce Abeta. The aim of this study was to examine the cleavage of APP-CTFbeta by gamma-secretase in primary cortical neurons from transgenic mice engineered to express the human APP-CTFbeta sequence. Neurons were prepared from transgenic mouse cortex and proteins labelled by incubation with [35S]methionine and [35S]cysteine. Labelled APP-CTFbeta and Abeta were then immunoprecipitated with a monoclonal antibody (WO2) specific for the transgene sequences. Approximately 30% of the human APP-CTFbeta (hAPP-CTFbeta) was converted to human Abeta (hAbeta), which was rapidly secreted. The remaining 70% of the hAPP-CTFbeta was degraded by an alternative pathway. The cleavage of hAPP-CTFbeta to produce hAbeta was inhibited by specific gamma-secretase inhibitors. However, treatment with proteasome inhibitors caused an increase in both hAPP-CTFbeta and hAbeta levels, suggesting that the alternative pathway was proteasome-dependent. A preparation of recombinant 20S proteasome was found to cleave a recombinant cytoplasmic domain fragment of APP (APPcyt) directly. The study suggests that in primary cortical neurons, APP-CTFbeta is degraded by two distinct pathways, one involving gamma-secretase, which produces Abeta, and a second major pathway involving direct cleavage of APP-CTFbeta within the cytoplasmic domain by the proteasome. These results raise the possibility that defective proteasome function could lead to an increase in Abeta production in the AD brain.

New developments in animal models of Alzheimer's disease

Alzheimer's disease (AD) is characterized by deterioration in mental function 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 are assemblies of hyperphosphorylated forms of the microtubule-associated protein tau. Electron microscopy of NFTs reveals structures known as paired helical filaments (PHFs). In familial AD (FAD), mutations in three distinct genes drive A beta synthesis by favoring endoproteolytic secretase cleavages that liberate A beta from the Alzheimer beta-amyloid precursor protein (APP). This suggests that excess A beta initiates a pathogenic cascade in humans that culminates in all the pathologic and cellular hallmarks of AD. Building upon the knowledge of FAD mutations, incremental technical advances have now allowed reproduceable creation of APP transgenic mice that exhibit AD-like amyloid pathology and A beta burdens. These transgenic mouse lines also exhibit deficits in spatial reference and working memory, with immunization against A beta abrogating both AD-associated phenotypes. Besides establishing a proof of principle for A beta-directed therapies, these findings suggest a potential to identify individual elements in the pathogenic pathway that lead to cognitive dysfunction. Furthermore, transgenic APP mice with potent amyloid deposition will likely form a beach-head to capture the final elements of AD neuropathology--cell loss and NFTs composed of PHFs--that are missing from current transgenic models.

Transgenic mouse models of Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder, characterized by a progressive loss of cognitive function. Despite considerable progress, a complete description of the molecular pathology of this disease has yet to be elucidated. In this respect, the need for an animal model that develops some or all aspects of this uniquely human disease in a reproducible fashion is crucial for the development and testing of potential treatments. A valid animal model for AD should exhibit (1) progressive AD-like neuropathology and (2) cognitive deficits, and (3) should be verified in several laboratories. Transgenic models should be able to (4) discern pathogenic effects of familial forms (FAD) mutations from those of transgene overexpression. Models derived from microinjection of FAD mutant alleles should (5) encompass more than one Tg line. At present, however, no model that replicates all of these desirable features exists. In this review, we discuss transgenic mouse models with well-characterized AD-like neuropathology that show some form of cognitive impairment. We argue that conclusions drawn from a limited selection of cross-sectional experiments should be verified in longitudinally designed experiments. Future studies should attempt to establish a closer relationship between molecular pathology and the degree of cognitive impairment. While exact replication of AD in mice may not attainable (due to phylogenetic differences and fundamental differences in behavioral ecology), rigorous comparative analysis of cognitive behavior observed in various mouse models of AD should provide a framework for better understanding of molecular mechanisms underlying cognitive impairment observed in AD patients.

Protein oxidation in the brain in Alzheimer's disease

In this study we used immunohistochemistry and two-dimensional fingerprinting of oxidatively modified proteins (two-dimensional Oxyblot) together to investigate protein carbonyl formation in the Alzheimer's disease brain. Increased protein oxidation was detected in sections from the hippocampus and parahippocampal gyrus, superior and middle temporal gyri of six Alzheimer's disease and six age-matched control human subjects, but not in the cerebellum. In two brain regions severely affected by Alzheimer's disease pathology, prominent protein carbonyl immunoreactivity was localized in the cytoplasm of neurons without visual pathomorphological changes and degenerating neurons, suggesting that intracellular proteins might be significantly affected by oxidative modifications. Following two-dimensional electrophoresis the positions of some individual proteins were identified using specific antibodies, and immunoblot analysis for protein carbonyls was performed. These studies demonstrated the presence of protein carbonyl immunoreactivity in beta-tubulin, beta-actin and creatine kinase BB in Alzheimer's disease and control brain extracts. Protein carbonyls were undetectable in spots matching glial fibrillary acidic protein and tau isoforms. Specific protein carbonyl levels in beta-actin and creatine kinase BB were significantly higher in Alzheimer's disease than in control brain extract. beta-Tubulin did not demonstrate a significant increase in specific protein carbonyl content in Alzheimer's disease brains. We suggest that oxidative stress-induced injury may involve the selective modification of different intracellular proteins, including key enzymes and structural proteins, which precedes and may lead to the neurofibrillary degeneration of neurons in the Alzheimer's disease brain.

Early-onset amyloid deposition and cognitive deficits in transgenic mice expressing a double mutant form of amyloid precursor protein 695

We have created early-onset transgenic (Tg) models by exploiting the synergistic effects of familial Alzheimer's disease mutations on amyloid beta-peptide (Abeta) biogenesis. TgCRND8 mice encode a double mutant form of amyloid precursor protein 695 (KM670/671NL+V717F) under the control of the PrP gene promoter. Thioflavine S-positive Abeta amyloid deposits are present at 3 months, with dense-cored plaques and neuritic pathology evident from 5 months of age. TgCRND8 mice exhibit 3,200-4,600 pmol of Abeta42 per g brain at age 6 months, with an excess of Abeta42 over Abeta40. High level production of the pathogenic Abeta42 form of Abeta peptide was associated with an early impairment in TgCRND8 mice in acquisition and learning reversal in the reference memory version of the Morris water maze, present by 3 months of age. Notably, learning impairment in young mice was offset by immunization against Abeta42 (Janus, C., Pearson, J., McLaurin, J., Mathews, P. M., Jiang, Y., Schmidt, S. D., Chishti, M. A., Horne, P., Heslin, D., French, J., Mount, H. T. J., Nixon, R. A., Mercken, M., Bergeron, C., Fraser, P. E., St. George-Hyslop, P., and Westaway, D. (2000) Nature 408, 979-982). Amyloid deposition in TgCRND8 mice was enhanced by the expression of presenilin 1 transgenes including familial Alzheimer's disease mutations; for mice also expressing a M146L+L286V presenilin 1 transgene, amyloid deposits were apparent by 1 month of age. The Tg mice described here suggest a potential to investigate aspects of Alzheimer's disease pathogenesis, prophylaxis, and therapy within short time frames.

Use-dependent effects of amyloidogenic fragments of (beta)-amyloid precursor protein on synaptic plasticity in rat hippocampus in vivo

The Alzheimer's disease-related beta-amyloid precursor protein (beta-APP) is metabolized to a number of potentially amyloidogenic peptides that are believed to be pathogenic. Application of relatively low concentrations of the soluble forms of these peptides has previously been shown to block high-frequency stimulation-induced long-term potentiation (LTP) of glutamatergic transmission in the hippocampus. The present experiments examined how these peptides affect low-frequency stimulation-induced long-term depression (LTD) and the reversal of LTP (depotentiation). We discovered that beta-amyloid peptide (Abeta1-42) and the Abeta-containing C -terminus of beta-APP (CT) facilitate the induction of LTD in the CA1 area of the intact rat hippocampus. The LTD was frequency- and NMDA receptor-dependent. Thus, although low-frequency stimulation alone was ineffective, after intracerebroventricular injection of Abeta1-42, it induced an LTD that was blocked by d-(-)-2-amino-5-phosphonopentanoic acid. Furthermore, an NMDA receptor-dependent depotentiation was induced in a time-dependent manner, being evoked by injection of CT 10 min, but not 1 hr, after LTP induction. These use- and time-dependent effects of the amyloidogenic peptides on synaptic plasticity promote long-lasting reductions in synaptic strength and oppose activity-dependent strengthening of transmission in the hippocampus. This will result in a profound disruption of information processing dependent on hippocampal synaptic plasticity.

Metal chelator decreases Alzheimer beta-amyloid plaques

Transgenic mice developing beta-amyloid (Abeta) plaques are advancing experimental treatment strategies for Alzheimer's disease. The metal chelator, clioquinol, is reported by Cherny et al. (2001) to reduce Abeta plaques, presumably by chelation of Abeta-associated zinc and copper. This and other recent Abeta-modulating treatment approaches are discussed.

Treatment with a copper-zinc chelator markedly and rapidly inhibits beta-amyloid accumulation in Alzheimer's disease transgenic mice

Inhibition of neocortical beta-amyloid (Abeta) accumulation may be essential in an effective therapeutic intervention for Alzheimer's disease (AD). Cu and Zn are enriched in Abeta deposits in AD, which are solubilized by Cu/Zn-selective chelators in vitro. Here we report a 49% decrease in brain Abeta deposition (-375 microg/g wet weight, p = 0.0001) in a blinded study of APP2576 transgenic mice treated orally for 9 weeks with clioquinol, an antibiotic and bioavailable Cu/Zn chelator. This was accompanied by a modest increase in soluble Abeta (1.45% of total cerebral Abeta); APP, synaptophysin, and GFAP levels were unaffected. General health and body weight parameters were significantly more stable in the treated animals. These results support targeting the interactions of Cu and Zn with Abeta as a novel therapy for the prevention and treatment of AD.

How well does the CSF inform upon pathology in the brain in Creutzfeldt-Jakob and Alzheimer's diseases?

Analysis of lumbar cerebrospinal fluid (CSF) plays a major role in the investigation of central nervous system disease, but how well do the changes in the CSF reflect pathology within the brain and spinal cord parenchyma? Both Creutzfeldt-Jakob (CJD) and Alzheimer's disease (AD) are characterized by the deposition of insoluble beta-pleated sheet peptides [prion protein (PrP) and beta-amyloid (Abeta), respectively] in the extracellular spaces of grey matter in the brain, but there is discordance in both diseases between the peptide levels in the brain and in the CSF. Experimental studies using tracers have shown that interstitial fluid (ISF) drains through very narrow intercellular spaces within grey matter into bulk flow perivascular channels that surround penetrating arteries. ISF then flows to the surface of the brain and joins CSF to drain to cervical lymph nodes. Such drainage of ISF and CSF to regional lymph nodes in the rat plays a significant role in B-cell and T-cell immune reactions within the brain. In man, the pia mater separates the periarterial ISF drainage pathways from the CSF in the subarachnoid space. The almost complete lack of insoluble protease-resistant PrP entering the CSF from the brain in patients with CJD, reported by Wong et al. in this issue of the Journal of Pathology, illustrates the limitations of ISF drainage pathways for the elimination of insoluble peptides from brain tissue. Insoluble Abeta accumulates in the extracellular spaces as plaques in AD and in periarterial ISF drainage pathways as cerebral amyloid angiopathy. Soluble Abeta appears to become entrapped by the insoluble Abeta in the ISF drainage pathways; thus, as the level of soluble Abeta in the brain rises in AD, the level in the CSF falls. Thus, the changes in the CSF do not accurately reflect the accumulation of the abnormal peptides in the brain parenchyma in either CJD or AD. In both diseases, facilitation of ISF drainage and elimination of PrP and Abeta peptides from the extracellular spaces of the brain may lead to practical therapeutic strategies for these devastating disorders.

Sublethal concentrations of prion peptide PrP106-126 or the amyloid beta peptide of Alzheimer's disease activates expression of proapoptotic markers in primary cortical neurons

Neurodegenerative disorders such as prion diseases and Alzheimer's disease (AD) are characterized by neuronal dysfunction and accumulation of amyloidogenic protein. In vitro studies have demonstrated that these amyloidogenic proteins can induce cellular oxidative stress and therefore may contribute to the neuronal dysfunction observed in these illnesses. Although the neurotoxic pathways are not fully elucidated, recent studies in AD have demonstrated up-regulation of caspases in neurons treated with amyloid beta (Abeta) peptide, suggesting involvement of apoptotic processes. To examine the role of proapoptotic pathways in prion diseases we treated primary mouse cortical neurons with the toxic prion protein peptide PrP106-126 and measured caspase activation and annexin V binding. We found that PrP106-126 induced a rapid and marked elevation in caspase 3, 6, and 8-like activity in neuronal cultures. Increased annexin V binding was observed predominantly on cortical cell neurites in peptide-treated cultures. Interestingly, these effects were induced by sublethal (5-50 microM) or lethal (100-200 microM) concentrations of PrP106-126. Sublethal concentrations of PrP106-126 maintained elevated caspase activation for at least 10 days with no loss of cell viability. Abeta1-40 also up-regulated caspase 3 activity and annexin V binding at both sublethal (5 microM) and lethal (25 microM) concentrations. There were no changes to proapoptotic marker expression in cultures treated with scrambled PrP106-126 (200 microM) or Abeta1-28 (25 microM) peptides. These studies demonstrate that amyloidogenic peptides can induce prolonged activation of proapoptotic marker expression in cultured neurons even at sublethal concentrations. These effects could contribute to chronic neuronal dysfunction and increase susceptibility to additional metabolic insults in neurodegenerative disorders. If so, targeting of therapeutic strategies against neuronal caspase activation early in the disease course could be beneficial in AD and prion diseases.

Metal chelation as a potential therapy for Alzheimer's disease

Alzheimer's disease is a rapidly worsening public health problem. The current lack of effective treatments for Alzheimer's disease makes it imperative to find new pharmacotherapies. At present, the treatment of symptoms includes use of acetylcholinesterase inhibitors, which enhance acetylcholine levels and improve cognitive functioning. Current reports provide evidence that the pathogenesis of Alzheimer's disease is linked to the characteristic neocortical amyloid-beta deposition, which may be mediated by abnormal metal interaction with A beta as well as metal-mediated oxidative stress. In light of these observations, we have considered the development of drugs that target abnormal metal accumulation and its adverse consequences, as well as prevention or reversal of amyloid-beta plaque formation. This paper reviews recent observations on the possible etiologic role of A beta deposition, its redox activity, and its interaction with transition metals that are enriched in the neocortex. We discuss the effects of metal chelators on these processes, list existing drugs with chelating properties, and explore the promise of this approach as a basis for medicinal chemistry in the development of novel Alzheimer's disease therapeutics.

The solubility of alpha-synuclein in multiple system atrophy differs from that of dementia with Lewy bodies and Parkinson's disease

Intracellular inclusions containing alpha-synuclein (alpha SN) are pathognomonic features of several neurodegenerative disorders. Inclusions occur in oligodendrocytes in multiple system atrophy (MSA) and in neurons in dementia with Lewy bodies (DLB) and Parkinson's disease (PD). In order to identify disease-associated changes of alpha SN, this study compared the levels, solubility and molecular weight species of alpha SN in brain homogenates from MSA, DLB, PD and normal aged controls. In DLB and PD, substantial amounts of detergent-soluble and detergent-insoluble alpha SN were detected compared with controls in grey matter homogenate. Compared with controls, MSA cases had significantly higher levels of alpha SN in the detergent-soluble fraction of brain samples from pons and white matter but detergent-insoluble alpha SN was not detected. There was an inverse correlation between buffered saline-soluble and detergent-soluble levels of alpha SN in individual MSA cases suggesting a transition towards insolubility in disease. The differences in solubility of alpha SN between grey and white matter in disease may result from different processing of alpha SN in neurons compared with oligodendrocytes. Highly insoluble alpha SN is not involved in the pathogenesis of MSA. It is therefore possible that buffered saline-soluble or detergent-soluble forms of alpha SN are involved in the pathogenesis of other alpha SN-related diseases.

A beta peptide immunization reduces behavioural impairment and plaques in a model of Alzheimer's disease

Much evidence indicates that abnormal processing and extracellular deposition of amyloid-beta peptide (A beta), a proteolytic derivative of the beta-amyloid precursor protein (betaAPP), is central to the pathogenesis of Alzheimer's disease (reviewed in ref. 1). In the PDAPP transgenic mouse model of Alzheimer's disease, immunization with A beta causes a marked reduction in burden of the brain amyloid. Evidence that A beta immunization also reduces cognitive dysfunction in murine models of Alzheimer's disease would support the hypothesis that abnormal A beta processing is essential to the pathogenesis of Alzheimer's disease, and would encourage the development of other strategies directed at the 'amyloid cascade'. Here we show that A beta immunization reduces both deposition of cerebral fibrillar A beta and cognitive dysfunction in the TgCRND8 murine model of Alzheimer's disease without, however, altering total levels of A beta in the brain. This implies that either a approximately 50% reduction in dense-cored A beta plaques is sufficient to affect cognition, or that vaccination may modulate the activity/abundance of a small subpopulation of especially toxic A beta species.

Astroglial expression of human alpha(1)-antichymotrypsin enhances alzheimer-like pathology in amyloid protein precursor transgenic mice

Proteases and their inhibitors play key roles in physiological and pathological processes. Cerebral amyloid plaques are a pathological hallmark of Alzheimer's disease (AD). They contain amyloid-ss (Ass) peptides in tight association with the serine protease inhibitor alpha(1)-antichymotrypsin.(1,2) However, it is unknown whether the increased expression of alpha(1)-antichymotrypsin found in AD brains counteracts or contributes to the disease. We used regulatory sequences of the glial fibrillary acidic protein gene(3) to express human alpha(1)-antichymotrypsin (hACT) in astrocytes of transgenic mice. These mice were crossed with transgenic mice that produce human amyloid protein precursors (hAPP) and Ass in neurons.(4,5) No amyloid plaques were found in transgenic mice expressing hACT alone, whereas hAPP transgenic mice and hAPP/hACT doubly transgenic mice developed typical AD-like amyloid plaques in the hippocampus and neocortex around 6 to 8 months of age. Co-expression of hAPP and hACT significantly increased the plaque burden at 7 to 8, 14, and 20 months. Both hAPP and hAPP/hACT mice showed significant decreases in synaptophysin-immunoreactive presynaptic terminals in the dentate gyrus, compared with nontransgenic littermates. Our results demonstrate that hACT acts as an amyloidogenic co-factor in vivo and suggest that the role of hACT in AD is pathogenic.

Neuronal oxidative stress precedes amyloid-beta deposition in Down syndrome

The predictable chronological sequence of pathological events in Down syndrome (DS) provides the opportunity to rigorously investigate the relationship between oxidative stress and amyloid-beta (Abeta) deposition. In this study, we report a marked accumulation of oxidized nucleic acid, 8-hydroxyguanosine (8OHG), and oxidized protein, nitrotyrosine, in the cytoplasm of cerebral neurons in DS with the levels of nucleic acid and protein oxidation paralleling each other. Relative density measurements of neuronal 8OHG immunoreactivity showed that there was a significant increase (p < 0.02) in DS (n = 22, ages 0.3-65 yr) compared with age-matched controls (n = 10, ages 0.3-64 yr). As a function of age, 8OHG immunoreactivity increased significantly in the teens and twenties (p < 0.04), while Abeta burden only increased after age 30 (p < 0.0001). In 9 cases of DS bearing Abeta deposition, the extent of deposits of Abeta ending at amino acid 42 (Abeta42) was actually associated with a decrease in relative 8OHG (r = -0.79, p < 0.015) while Abeta40 was not. These findings suggest that in brains of patients with DS, increased levels of oxidative damage occur prior to the onset of Abeta deposition.

Evidence that the beta-amyloid plaques of Alzheimer's disease represent the redox-silencing and entombment of abeta by zinc

Abeta binds Zn(2+), Cu(2+), and Fe(3+) in vitro, and these metals are markedly elevated in the neocortex and especially enriched in amyloid plaque deposits of individuals with Alzheimer's disease (AD). Zn(2+) precipitates Abeta in vitro, and Cu(2+) interaction with Abeta promotes its neurotoxicity, correlating with metal reduction and the cell-free generation of H(2)O(2) (Abeta1-42 > Abeta1-40 > ratAbeta1-40). Because Zn(2+) is redox-inert, we studied the possibility that it may play an inhibitory role in H(2)O(2)-mediated Abeta toxicity. In competition to the cytotoxic potentiation caused by coincubation with Cu(2+), Zn(2+) rescued primary cortical and human embryonic kidney 293 cells that were exposed to Abeta1-42, correlating with the effect of Zn(2+) in suppressing Cu(2+)-dependent H(2)O(2) formation from Abeta1-42. Since plaques contain exceptionally high concentrations of Zn(2+), we examined the relationship between oxidation (8-OH guanosine) levels in AD-affected tissue and histological amyloid burden and found a significant negative correlation. These data suggest a protective role for Zn(2+) in AD, where plaques form as the result of a more robust Zn(2+) antioxidant response to the underlying oxidative attack.

High-level neuronal expression of abeta 1-42 in wild-type human amyloid protein precursor transgenic mice: synaptotoxicity without plaque formation

Amyloid plaques are a neuropathological hallmark of Alzheimer's disease (AD), but their relationship to neurodegeneration and dementia remains controversial. In contrast, there is a good correlation in AD between cognitive decline and loss of synaptophysin-immunoreactive (SYN-IR) presynaptic terminals in specific brain regions. We used expression-matched transgenic mouse lines to compare the effects of different human amyloid protein precursors (hAPP) and their products on plaque formation and SYN-IR presynaptic terminals. Four distinct minigenes were generated encoding wild-type hAPP or hAPP carrying mutations that alter the production of amyloidogenic Abeta peptides. The platelet-derived growth factor beta chain promoter was used to express these constructs in neurons. hAPP mutations associated with familial AD (FAD) increased cerebral Abeta(1-42) levels, whereas an experimental mutation of the beta-secretase cleavage site (671(M-->I)) eliminated production of human Abeta. High levels of Abeta(1-42) resulted in age-dependent formation of amyloid plaques in FAD-mutant hAPP mice but not in expression-matched wild-type hAPP mice. Yet, significant decreases in the density of SYN-IR presynaptic terminals were found in both groups of mice. Across mice from different transgenic lines, the density of SYN-IR presynaptic terminals correlated inversely with Abeta levels but not with hAPP levels or plaque load. We conclude that Abeta is synaptotoxic even in the absence of plaques and that high levels of Abeta(1-42) are insufficient to induce plaque formation in mice expressing wild-type hAPP. Our results support the emerging view that plaque-independent Abeta toxicity plays an important role in the development of synaptic deficits in AD and related conditions.

Chelation and intercalation: complementary properties in a compound for the treatment of Alzheimer's disease

Selective application of metal chelators to homogenates of human Alzheimer's disease (AD) brain has led us to propose that the architecture of aggregated beta-amyloid peptide, whether in the form of plaques or soluble oligomers, is determined at least in part by high-affinity binding of transition metals, especially copper and zinc. Of the two metals, copper is implicated in reactive oxygen species generating reactions, while zinc appears to be associated with conformational and antioxidant activity. We tested the copper chelators trientine, penicillamine, and bathophenanthroline for their ability to mobilize brain Abeta as measured against our benchmark compound bathocuproine (BC). All of these agents were effective in solubilizing brain Abeta, although BC was the most consistent across the range of AD brain tissue samples tested. Similarly, all of the copper chelators depleted copper in the high-speed supernatants. BC alone had no significant effect upon zinc levels in the soluble fraction. BC extraction of brain tissue from C100 transgenic mice (which express human Abeta but do not develop amyloid) revealed SDS-resistant dimers as Abeta was mobilized from the sedimentable to the soluble fraction. NMR analysis showed that, in addition to its copper chelating properties, BC interacts with Abeta to form a complex independent of the presence of copper. Such hybrid copper chelating and "chain breaking" properties may form the basis of a rational design for a therapy for Alzheimer's disease.