Soluble amyloid beta-protein in the cerebrospinal fluid from patients with Alzheimer's disease, vascular dementia and controls

The glymphatic system and cerebral small vessel disease

OBJECTIVES

Cerebral small vessel disease is a group of pathologies in which alterations of the brain's blood vessels contribute to stroke and neurocognitive changes. Recently, a neurotoxic waste clearance system composed of perivascular spaces abutting the brain's blood vessels, termed the glymphatic system, has been identified as a key player in brain homeostasis. Given that small vessel disease and the glymphatic system share anatomical structures, this review aims to reexamine small vessel disease in the context of the glymphatic system and highlight novel aspects of small vessel disease physiology.

MATERIALS AND METHODS

This review was conducted with an emphasis on studies that examined aspects of small vessel disease and on works characterizing the glymphatic system. We searched PubMed for relevant articles using the following keywords: glymphatics, cerebral small vessel disease, arterial pulsatility, hypertension, blood-brain barrier, endothelial dysfunction, stroke, diabetes.

RESULTS

Cerebral small vessel disease and glymphatic dysfunction are anatomically connected and significant risk factors are shared between the two. These include hypertension, type 2 diabetes, advanced age, poor sleep, obesity, and neuroinflammation. There is clear evidence that CSVD hinders the effective functioning of glymphatic system.

CONCLUSION

These shared risk factors, as well as the model of cerebral amyloid angiopathy pathogenesis, hint at the possibility that glymphatic dysfunction could independently contribute to the pathogenesis of cerebral small vessel disease. However, the current evidence supports a model of cascading dysfunction, wherein concurrent small vessel and glymphatic injury hinder glymphatic-mediated recovery and promote the progression of subclinical to clinical disease.

Temporal and spatially controlled APP transgene expression using Cre-dependent alleles

Although a large number of mouse models have been made to study Alzheimer's disease, only a handful allow experimental control over the location or timing of the protein being used to drive pathology. Other fields have used the Cre and the tamoxifen-inducible CreER driver lines to achieve precise spatial and temporal control over gene deletion and transgene expression, yet these tools have not been widely used in studies of neurodegeneration. Here, we describe two strategies for harnessing the wide range of Cre and CreER driver lines to control expression of disease-associated amyloid precursor protein (APP) in modeling Alzheimer's amyloid pathology. We show that CreER-based spatial and temporal control over APP expression can be achieved with existing lines by combining a Cre driver with a tetracycline-transactivator (tTA)-dependent APP responder using a Cre-to-tTA converter line. We then describe a new mouse line that places APP expression under direct control of Cre recombinase using an intervening lox-stop-lox cassette. Mating this allele with a CreER driver allows both spatial and temporal control over APP expression, and with it, amyloid onset. This article has an associated First Person interview with the first author of the paper.

A comparative study of the effects of Aducanumab and scanning ultrasound on amyloid plaques and behavior in the APP23 mouse model of Alzheimer disease

BACKGROUND

Aducanumab is an anti-amyloid-β (Aβ) antibody that achieved reduced amyloid pathology in Alzheimer's disease (AD) trials; however, it is controversial whether it also improved cognition, which has been suggested would require a sufficiently high cumulative dose of the antibody in the brain. Therapeutic ultrasound, in contrast, has only begun to be investigated in human AD clinical trials. We have previously shown that scanning ultrasound in combination with intravenously injected microbubbles (SUS), which temporarily and safely opens the blood-brain barrier (BBB), removes amyloid and restores cognition in APP23 mice. However, there has been no direct testing of how the effects of SUS compare to immunotherapy or whether a combination therapy is more effective.

METHODS

In a study comprising four treatment arms, we tested the efficacy of an Aducanumab analog, Adu, both in comparison to SUS, and as a combination therapy, in APP23 mice (aged 13-22 months), using sham as a control. The active place avoidance (APA) test was used to test spatial memory, and histology and ELISA were used to measure amyloid. Brain antibody levels were also determined.

RESULTS

We found that both Adu and SUS reduced the total plaque area in the hippocampus with no additive effect observed with the combination treatment (SUS + Adu). Whereas in the cortex where there was a trend towards reducing the total plaque area from either Adu or SUS, only the combination treatment yielded a statistically significant decrease in total plaque area compared to sham. Only the SUS and SUS + Adu groups included animals that had their plaque load reduced to below 1% from above 10%. There was a robust improvement in spatial memory for the SUS + Adu group only, and in this group the level of Adu, when measured 3 days post-treatment, was 5-fold higher compared to those mice that received Adu on its own. Together, these findings suggest that SUS should be considered as a treatment option for AD. Alternatively, a combination trial using Aducanumab together with ultrasound to increase brain levels of the antibody may be warranted.

The inhibition of cellular toxicity of amyloid-β by dissociated transthyretin

The protective effect of transthyretin (TTR) on cellular toxicity of β-amyloid (Aβ) has been previously reported. TTR is a tetrameric carrier of thyroxine in blood and cerebrospinal fluid, the pathogenic aggregation of which causes systemic amyloidosis. However, studies have documented a protective effect of TTR against cellular toxicity of pathogenic Aβ, a protein associated with Alzheimer's disease. TTR binds Aβ, alters its aggregation, and inhibits its toxicity both in vitro and in vivo In this study, we investigate whether the amyloidogenic ability of TTR and its antiamyloid inhibitory effect are associated. Using protein aggregation and cytotoxicity assays, we found that the dissociation of the TTR tetramer, required for its amyloid pathogenesis, is also necessary to prevent cellular toxicity from Aβ oligomers. These findings suggest that the Aβ-binding site of TTR may be hidden in its tetrameric form. Aided by computational docking and peptide screening, we identified a TTR segment that is capable of altering Aβ aggregation and toxicity, mimicking TTR cellular protection. EM, immune detection analysis, and assessment of aggregation and cytotoxicity revealed that the TTR segment inhibits Aβ oligomer formation and also promotes the formation of nontoxic, nonamyloid amorphous aggregates, which are more sensitive to protease digestion. Finally, this segment also inhibits seeding of Aβ catalyzed by Aβ fibrils extracted from the brain of an Alzheimer's patient. Together, these findings suggest that mimicking the inhibitory effect of TTR with peptide-based therapeutics represents an additional avenue to explore for the treatment of Alzheimer's disease.

Diversity in Aβ deposit morphology and secondary proteome insolubility across models of Alzheimer-type amyloidosis

A hallmark pathology of Alzheimer's disease (AD) is the formation of amyloid β (Aβ) deposits that exhibit diverse localization and morphologies, ranging from diffuse to cored-neuritic deposits in brain parenchyma, with cerebral vascular deposition in leptomeningeal and parenchymal compartments. Most AD brains exhibit the full spectrum of pathologic Aβ morphologies. In the course of studies to model AD amyloidosis, we have generated multiple transgenic mouse models that vary in the nature of the transgene constructs that are expressed; including the species origin of Aβ peptides, the levels and length of Aβ that is deposited, and whether mutant presenilin 1 (PS1) is co-expressed. These models recapitulate features of human AD amyloidosis, but interestingly some models can produce pathology in which one type of Aβ morphology dominates. In prior studies of mice that primarily develop cored-neuritic deposits, we determined that Aβ deposition is associated with changes in cytosolic protein solubility in which a subset of proteins become detergent-insoluble, indicative of secondary proteome instability. Here, we survey changes in cytosolic protein solubility across seven different transgenic mouse models that exhibit a range of Aβ deposit morphologies. We find a surprisingly diverse range of changes in proteome solubility across these models. Mice that deposit human Aβ40 and Aβ42 in cored-neuritic plaques had the most robust changes in proteome solubility. Insoluble cytosolic proteins were also detected in the brains of mice that develop diffuse Aβ42 deposits but to a lesser extent. Notably, mice with cored deposits containing only Aβ42 had relatively few proteins that became detergent-insoluble. Our data provide new insight into the diversity of biological effects that can be attributed to different types of Aβ pathology and support the view that fibrillar cored-neuritic plaque pathology is the more disruptive Aβ pathology in the Alzheimer's cascade.

Critical Steps to be Taken into Consideration Before Quantification of β-Amyloid and Tau Isoforms in Blood can be Implemented in a Clinical Environment

This review aims to document difficulties, limitations, and pitfalls when considering protein analysis in blood samples. It proposes an improved workflow for design, development, and validation of (immuno)assays for blood proteins, without providing reflections on a potential hypothesis of the origin of protein mismetabolism and deposition. There is a special focus on assay development for quantification of β-amyloid (Aβ) and tau in blood for diagnostic use or for integration in clinical trials in the field of Alzheimer's disease (AD).

Aptamers Selected for Recognizing Amyloid β-Protein-A Case for Cautious Optimism

Aptamers are versatile oligonucleotide ligands used for molecular recognition of diverse targets. However, application of aptamers to the field of amyloid β-protein (Aβ) has been limited so far. Aβ is an intrinsically disordered protein that exists in a dynamic conformational equilibrium, presenting time-dependent ensembles of short-lived, metastable structures and assemblies that have been generally difficult to isolate and characterize. Moreover, despite understanding of potential physiological roles of Aβ, this peptide has been linked to the pathogenesis of Alzheimer disease, and its pathogenic roles remain controversial. Accumulated scientific evidence thus far highlights undesirable or nonspecific interactions between selected aptamers and different Aβ assemblies likely due to the metastable nature of Aβ or inherent affinity of RNA oligonucleotides to β-sheet-rich fibrillar structures of amyloidogenic proteins. Accordingly, lessons drawn from Aβ–aptamer studies emphasize that purity and uniformity of the protein target and rigorous characterization of aptamers’ specificity are important for realizing and garnering the full potential of aptamers selected for recognizing Aβ or other intrinsically disordered proteins. This review summarizes studies of aptamers selected for recognizing different Aβ assemblies and highlights controversies, difficulties, and limitations of such studies.

Role of Syndecans in Lipid Metabolism and Human Diseases

Syndecans are transmembrane heparan sulfate proteoglycans involved in the regulation of cell growth, differentiation, adhesion, neuronal development, and lipid metabolism. Syndecans are expressed in a tissue-specific manner to facilitate diverse cellular processes. As receptors and co-receptors, syndecans provide promising therapeutic targets that bind to a variety of physiologically important ligands. Negatively charged glycosaminoglycan chains of syndecans, located in the extracellular compartment, are critical for such binding. Functions of syndecans are as diverse as their ligands. For example, hepatic syndecan-1 mediates clearance of triglyceride-rich lipoproteins. Syndecan-2 promotes localization of Alzheimer's amyloid Aβ peptide to the cell surface, which is proposed to contribute to amyloid plaque formation. Syndecan-3 helps co-localize the appetite-regulating melanocortin-4 receptor with its agonist, leading to an increased appetite. Finally, syndecan-4 initiates the capture of modified low-density lipoproteins by macrophages and thereby promotes the atheroma formation. We hypothesize that syndecan modifications such as desulfation of glycosaminoglycan chains may contribute to a wide range of diseases, from atherosclerosis to type 2 diabetes. At the same time, desulfated syndecans may have beneficial effects, as they can inhibit amyloid plaque formation or decrease the appetite. Despite considerable progress in understanding diverse functions of syndecans, the complex physiological roles of this intriguing family of proteoglycans are far from clear. Additional studies of syndecans may potentially help develop novel therapeutic approaches and diagnostic tools to alleviate complex human diseases such as cardiovascular and Alzheimer's diseases.

RNA aptamer probes as optical imaging agents for the detection of amyloid plaques

Optical imaging using multiphoton microscopy and whole body near infrared imaging has become a routine part of biomedical research. However, optical imaging methods rely on the availability of either small molecule reporters or genetically encoded fluorescent proteins, which are challenging and time consuming to develop. While directly labeled antibodies can also be used as imaging agents, antibodies are species specific, can typically not be tagged with multiple fluorescent reporters without interfering with target binding, and are bioactive, almost always eliciting a biological response and thereby influencing the process that is being studied. We examined the possibility of developing highly specific and sensitive optical imaging agents using aptamer technology. We developed a fluorescently tagged anti-Aβ RNA aptamer, β55, which binds amyloid plaques in both ex vivo human Alzheimer’s disease brain tissue and in vivo APP/PS1 transgenic mice. Diffuse β55 positive halos, attributed to oligomeric Aβ, were observed surrounding the methoxy-XO4 positive plaque cores. Dot blots of synthetic Aβ aggregates provide further evidence that β55 binds both fibrillar and non-fibrillar Aβ. The high binding affinity, the ease of probe development, and the ability to incorporate multiple and multimodal imaging reporters suggest that RNA aptamers may have complementary and perhaps advantageous properties compared to conventional optical imaging probes and reporters.

Flexible antibodies with nonprotein hinges

There is a significant need for antibodies that can bind targets with greater affinity. Here we describe a novel strategy employing chemical semisynthesis to produce symmetroadhesins: antibody-like molecules having nonprotein hinge regions that are more flexible and extendible and are capable of two-handed binding. Native chemical ligation was carried out under mild, non-denaturing conditions to join a ligand binding domain (Aβ peptide) to an IgG1 Fc dimer via discrete oxyethylene oligomers of various lengths. Two-handed Aβ-Fc fusion proteins were obtained in quantitative yield and shown by surface plasmon resonance to bind an anti-Aβ antibody with a K(D) at least two orders of magnitude greater than the cognate Aβ peptide. MALDI-TOF MS analysis confirmed the protein/nonprotein/protein structure of the two-handed molecules, demonstrating its power to characterize complex protein-nonprotein hybrids by virtue of desorption/ionization mediated by peptide sequences contained therein. We anticipate many applications for symmetroadhesins that combine the target specificity of antibodies with the novel physical, chemical and biological properties of nonprotein hinges.

Abeta oligomers cause localized Ca(2+) elevation, missorting of endogenous Tau into dendrites, Tau phosphorylation, and destruction of microtubules and spines

Aggregation of amyloid-beta (Abeta) and Tau protein are hallmarks of Alzheimer's disease (AD), and according to the Abeta-cascade hypothesis, Abeta is considered toxic for neurons and Tau a downstream target of Abeta. We have investigated differentiated primary hippocampal neurons for early localized changes following exposure to Abeta oligomers. Initial events become evident by missorting of endogenous Tau into the somatodendritic compartment, in contrast to axonal sorting in normal neurons. In missorted dendritic regions there is a depletion of spines and local increase in Ca(2+), and breakdown of microtubules. Tau in these regions shows elevated phosphorylation at certain sites diagnostic of AD-Tau (e.g., epitope of antibody 12E8, whose phosphorylation causes detachment of Tau from microtubules, and AT8 epitope), and local elevation of certain kinase activities (e.g., MARK/par-1, BRSK/SADK, p70S6K, cdk5, but not GSK3beta, JNK, MAPK). These local effects occur without global changes in Tau, tubulin, or kinase levels. Somatodendritic missorting occurs not only with Tau, but also with other axonal proteins such as neurofilaments, and correlates with pronounced depletion of microtubules and mitochondria. The Abeta-induced effects on microtubule and mitochondria depletion, Tau missorting, and loss of spines are prevented by taxol, indicating that Abeta-induced microtubule destabilization and corresponding traffic defects are key factors in incipient degeneration. By contrast, the rise in Ca(2+) levels, kinase activities, and Tau phosphorylation cannot be prevented by taxol. Incipient and local changes similar to those of Abeta oligomers can be evoked by cell stressors (e.g., H(2)O(2), glutamate, serum deprivation), suggesting some common mechanism of signaling.

RNA aptamers generated against oligomeric Abeta40 recognize common amyloid aptatopes with low specificity but high sensitivity

Aptamers are useful molecular recognition tools in research, diagnostics, and therapy. Despite promising results in other fields, aptamer use has remained scarce in amyloid research, including Alzheimer's disease (AD). AD is a progressive neurodegenerative disease believed to be caused by neurotoxic amyloid β-protein (Aβ) oligomers. Aβ oligomers therefore are an attractive target for development of diagnostic and therapeutic reagents. We used covalently-stabilized oligomers of the 40-residue form of Aβ (Aβ40) for aptamer selection. Despite gradually increasing the stringency of selection conditions, the selected aptamers did not recognize Aβ40 oligomers but reacted with fibrils of Aβ40, Aβ42, and several other amyloidogenic proteins. Aptamer reactivity with amyloid fibrils showed some degree of protein-sequence dependency. Significant fibril binding also was found for the naïve library and could not be eliminated by counter-selection using Aβ40 fibrils, suggesting that aptamer binding to amyloid fibrils was RNA-sequence-independent. Aptamer binding depended on fibrillogenesis and showed a lag phase. Interestingly, aptamers detected fibril formation with ≥15-fold higher sensitivity than thioflavin T (ThT), revealing substantial β-sheet and fibril formation undetected by ThT. The data suggest that under physiologic conditions, aptamers for oligomeric forms of amyloidogenic proteins cannot be selected due to high, non-specific affinity of oligonucleotides for amyloid fibrils. Nevertheless, the high sensitivity, whereby aptamers detect β-sheet formation, suggests that they can serve as superior amyloid recognition tools.

Synergistic effect of apolipoprotein E epsilon4 and butyrylcholinesterase K-variant on progression from mild cognitive impairment to Alzheimer's disease

OBJECTIVE

To evaluate the synergistic effects of the apolipoprotein E (APOE) epsilon4 and butyrylcholinesterase K-variant (BCHE-K) alleles on progression to Alzheimer's disease (AD) in individuals with mild cognitive impairment (MCI).

METHODS

This was a post-hoc exploratory analysis from a 3-4-year, randomized, placebo-controlled study of rivastigmine in participants with MCI (InDDEx study). Participants who consented to genetic testing were included in the current analyses. The incidence of progression to AD, cognitive decline and changes in MRI brain volumes were investigated in participants from the placebo arm of the InDDEx study.

RESULTS

Of the 1018 participants in the overall study, 464 were successfully genotyped for both APOE and butyrylcholinesterase. Of these, 68 (14.7%) carried > or =1 APOE epsilon4 and > or =1 BCHE-K allele. The presence of APOE epsilon4 was associated with a significantly higher incidence of progression to AD whereas the presence of BCHE-K had no independent effect on progression. A synergistic effect of the combined presence of APOE epsilon4 and BCHE-K on the time to clinical diagnosis of AD and on MRI brain volumes was seen. Progression to AD and hippocampal volumetric loss was greatest in participants who carried both APOE epsilon4 and BCHE-K alleles and lowest in BCHE-K carriers without the APOE epsilon4 allele.

CONCLUSION

In MCI, the risk of cognitive decline, hippocampal volumetric loss and progression to AD seems to be the greatest in individuals who carry at least one copy of both the BCHE-K and APOE epsilon4 alleles.

Analysis of Abeta interactions using ProteinChip technology

Abeta peptides are now acknowledged to play a central role in the pathogenesis of Alzheimer's disease. Their generation results from the sequential cleavage of amyloid precursor protein by beta and gamma secretases. The resulting peptide fragments impart toxicity via their ability to form soluble oligomers and bind to cell membranes. In this chapter we describe the use of ProteinChip technology to study the physicochemical behaviour of Abeta and its mechanisms of toxicity. These include analyzing (1) Abeta processing and quantitation of peptide fragments, (2) Abeta aggregation and the quantitation of oligomers, and (3) Abeta-lipid interactions.

Competitive ELISA studies of neural thread protein in urine in Alzheimer's disease

A specific and reliable competitive affinity assay kit has been developed to quantitatively measure neural thread protein (NTP) in first morning urine samples. This assay, called the urine neural thread protein test (UNTP), is a competitive enzyme-linked immunosorbent assay (ELISA) format affinity assay using 32-well microtiter plates. The assay detects UNTP in the 10-60 microg/mL range (an improvement over earlier assays of 10(3) x ), is linear and more reproducible (average coefficient of variation [CV] 6.2% in precision studies). The utility of the assay has been demonstrated in urine samples from patients with Alzheimer's disease (AD) and controls (sensitivity of 90% and specificity of 91%). Test-retest assays of subjects with AD and controls were comparatively stable at intervals of 2 days to 4.5 years, which suggests that positive (elevated) or negative (normal) NTP levels do not fluctuate significantly over time with respect to the cutoff.

Humoral immune responses to peptides derived from the beta-amyloid peptide C-terminal sequence

There is a continuing interest in the immunochemical quantification of isoforms of amyloid beta-peptide (Abeta) in body fluids of patients with Alzheimer's disease (AD); however, at present there is no general procedure to produce and test the required antibodies. We examined various methods to generate rabbit anti-Abeta; antibodies that are specific for Abeta(38), Abeta(40) and Abeta(42), and we tested their specificity and sensitivity by ELISA and Western blotting. To produce high-affinity antibodies required repeated inoculations of small doses of peptide conjugates over a period of at least 6 months. Antibodies generated to peptides derived from the Abeta(42) sequence showed some cross-reactivity with Abeta(40), but antibodies generated to Abeta4 peptides did not cross-react with Abeta(42). The shortest peptide capable of generating antibodies of moderate affinity possessed the sequence Met(35)-Ala(42); however, antibodies raised to the peptide Gly(33)-Ala(42) possessed the greatest affinity (K(D) = 1 nM) and specificity for Abeta(42). The latter antibodies were over 50,000-fold more reactive with Abeta(42) than with Abeta(40). They can detect Abeta isoforms in extracts of normal brain, where the peptides are present at levels below one part per billion. Our results provide methods to generate and characterize the specificity and affinity of anti-Abeta antibodies. This information is necessary to develop sensitive and specific immunoassays to quantify Abeta isoforms in brain extracts and in body fluids.

Proteomic identification of biomarkers in the cerebrospinal fluid in a rat model of nigrostriatal dopaminergic degeneration

We have performed proteomic analysis in the cerebrospinal fluid in an animal model of Parkinson's disease induced by axotomy of the medial forebrain bundle. In this model, the degeneration of dopaminergic neurons was completed in 14 days, with a loss of about 50% dopaminergic neurons in the substantia nigra and a loss of more than 80% dopamine terminals in the striatum, with a similar diminution of dopamine levels in both structures. Proteins were separated by 2D electrophoresis and identified by matrix-assisted laser desorption-ionization time-of-flight (MALDI-TOF). We found significant increases of haptoglobin and transthyretin along with a decrease of Apo E concentrations in the cerebrospinal fluid of axotomized animals. Changes for haptoglobin and transthyretin were further confirmed in cerebrospinal fluid and plasma by Western blotting. These results suggest that monitoring plasma levels of these signals appears to be a promising biological marker of neuronal degeneration of the nigrostriatal dopaminergic system.

Problems associated with biological markers of Alzheimer's disease

The etiopathogenesis of Alzheimer's disease (AD) is still unclear, although clinical diagnostic criteria exist and the neuropathology of AD has been studied in great detail during the last 20 years. The present study addresses certain problems in the search for biological markers for the diagnosis, as well as in the follow-up of the course of AD and its differential diagnosis and reports some of our own observations in comparison with other studies. These include protein, genetic and neuroimaging markers. The definitions of biological markers and search strategies are also discussed.

In cerebrospinal fluid ER chaperones ERp57 and calreticulin bind beta-amyloid

The beta-amyloids (abetas) are the major components of the plaque observed in the brains of patients with Alzheimer's disease. The conundrum is that although they are produced in everyone during the posttranslational processing in the endoplasmic reticulum (ER) of the amyloid precursor protein (APP), deposits are only observed in the elderly. Our work suggests that normals have a carrier protein(s) keeping them in solution. Based on immunoblotting studies of cerebrospinal fluid (CSF) from normals, we find that the bulk of the abetas are bound to the ER chaperones, ERp57 and calreticulin, suggesting that these may be carrier proteins which prevent aggregation of the abetas and that the deposits are due to faulty ER posttranslational processing of APP with the failure to form this complex. If membrane protein synthesis is similarly affected, it could explain the neuronal dysfunction characteristic of Alzheimer's disease.

Antibodies from a DNA peptide vaccination decrease the brain amyloid burden in a mouse model of Alzheimer's disease

The neuropathology of Alzheimer's disease(AD) is characterized by the accumulation of amyloid peptide Abeta in the brain derived from proteolytic cleavage of the amyloid precursor protein (APP). Vaccination of mice with plasmid DNA coding for the human Abeta42 peptide together with low doses of preaggregated peptide induced antibodies with detectable titers after only 2 weeks. One serum was directed against the four aminoterminal amino acids DAEF and differs from previously described ones. Both immune sera and monoclonal antibodies solubilized preformed aggregates of Abeta42 in vitro and recognized amyloid plaques in brain sections of mice transgenic for human APP. Passive immunization of transgenic AD mice caused a significant and rapid reduction in brain amyloid plaques within 24 h. The combined DNA peptide vaccine may prove useful for active immunization with few inoculations and low peptide dose which may prevent the recently described inflammatory reactions inpatients. The monoclonal antibodies are applicable for passive immunization studies and may lead to a therapy of AD.

Differential degradation of amyloid beta genetic variants associated with hereditary dementia or stroke by insulin-degrading enzyme

Inherited amino acid substitutions at position 21, 22, or 23 of amyloid beta (Abeta) lead to presenile dementia or stroke. Insulin-degrading enzyme (IDE) can hydrolyze Abeta wild type, yet whether IDE is capable of degrading Abeta bearing pathogenic substitutions is not known. We studied the degradation of all of the published Abeta genetic variants by recombinant rat IDE (rIDE). Monomeric Abeta wild type, Flemish (A21G), Italian (E22K), and Iowa (D23N) variants were readily degraded by rIDE with a similar efficiency. However, proteolysis of Abeta Dutch (E22Q) and Arctic (E22G) was significantly lower as compared with Abeta wild type and the rest of the mutant peptides. In the case of Abeta Dutch, inefficient proteolysis was related to a high content of beta structure as assessed by circular dichroism. All of the Abeta variants were cleaved at Glu3-Phe4 and Phe4-Arg5 in addition to the previously described major sites within positions 13-15 and 18-21. SDS-stable Abeta dimers were highly resistant to proteolysis by rIDE regardless of the variant, suggesting that IDE recognizes a conformation that is available for interaction only in monomeric Abeta. These results raise the possibility that upregulation of IDE may promote the clearance of soluble Abeta in hereditary forms of Abeta diseases.

Abeta levels in serum, CSF and brain, and cognitive deficits in APP + PS1 transgenic mice

We compared beta-amyloid peptide (Abeta) levels in the serum, CSF and brain (hippocampus) and correlated these with spatial learning in APP+PS1 transgenic mice. Compared with non-transgenic littermates, male 14-month-old APP + PS1 mice were impaired in spatial learning in the water maze. Among the APP + PS1 mice, only the hippocampal insoluble Abeta42 level correlated with spatial memory (r = -0.44). The levels of insoluble Abeta40 and Abeta42 were highly correlated (r = 0.92), and also correlated with soluble hippocampal Abeta42 (r = 0.64/0.69), which further correlated with the CSF Abeta42 (r = 0.52). None of these parameters correlated with serum Abeta40 levels. These findings support the role of insoluble Abeta42 in memory dysfunction and suggest a model with several pools (insoluble, extracellular soluble, CSF) of Abeta being in partial equilibrium with each other.

Amyloid-beta: an antioxidant that becomes a pro-oxidant and critically contributes to Alzheimer's disease

Elevated production of amyloid-beta (A beta) as a preventive antioxidant for brain lipoproteins under the action of increased oxidative stress in aging is postulated to represent a major event in the development of Alzheimer's disease (AD). Increase in A beta production is followed by chelation of transition metal ions by A beta, accumulation of A beta-metal lipoprotein aggregates, production of reactive oxygen species and neurotoxicity. Chelation of copper by A beta is proposed to be a most important part of this pathway, because A beta binds copper stronger than other transition metals and because copper is a more efficient catalyst of oxidation than other metals. This amyloid-binds-copper (ABC) model does not remove A beta peptide from its central place in our current thinking of AD, but rather places additional factors in the center of discussion. Most importantly, they embrace pathological mechanisms known to develop in aging (which is the major risk factor for AD), such as increased production of reactive oxygen species by mitochondria, that are positioned upstream relative to the generation of A beta.

Recent developments in Alzheimer's disease

Alzheimer's disease is the most frequent form of dementia and it is estimated that its prevalence will quadruple by the year 2050. In the past decade, a number of important new developments have provided insight in the pathogenesis, improved diagnosis and allowed therapy of dementia. Several new mutations in the amyloid protein precursor gene, presenilin-1 and -2 genes and the influence of the apolipoprotein E gene isotypes on the disease phenotype have been described. The role of secretases in the generation of amyloid in senile plaques has been determined and this may provide important new therapeutic approaches in the future. The role of vascular lesions in the development of dementia and relationship with the Lewy body variant of Alzheimer's disease have been refined. Acetylcholine is deficient in Alzheimer's disease and can be supplemented in part by treatment with acetylcholinesterase inhibitors. Recently, surprising results of vaccination with amyloid in a transgenic mouse model have opened a completely new perspective in the prevention and treatment of Alzheimer's disease.

Treatment with the selective muscarinic agonist talsaclidine decreases cerebrospinal fluid levels of total amyloid beta-peptide in patients with Alzheimer's disease

Brain amyloid load in Alzheimer's disease (AD) is, at least in genetic forms, associated with overproduction of amyloid beta-peptides (A beta). Thus, lowering A beta production is a central therapeutic target in AD and may be achieved by modulating such key enzymes of amyloid precursor protein (APP) processing as beta-, gamma-, and alpha-secretase activities. Talsaclidine is a selective muscarinic M1 agonist that stimulates the nonamyloidogenic alpha-secretase pathway in model systems. Talsaclidine was administered double-blind, placebo-controlled, and randomized to 24 AD patients and cerebrospinal fluid (CSF) levels of total A beta were quantitated before and after 4 weeks of drug treatment. We observed that talsaclidine decreases CSF levels of A beta significantly over time within the treatment group (n = 20) by a median of 16% as well as compared to placebo (n = 4) by a median of 27%. We conclude that treatment with selective M1 agonists may reduce A beta production and may thus be further evaluated as a potential amyloid-lowering therapy of AD.

The levels of cerebrospinal fluid Abeta40 and Abeta42(43) are regulated age-dependently

Decreased levels of cerebrospinal fluid (CSF) Abeta42 is a diagnostic marker of Alzheimer's disease. To clarify the biological basis of this marker, the physiological alterations of CSF Abeta40 and Abeta42 by aging were studied. CSF samples from 92 normal subjects between 8 and 89 years old were measured using a specific ELISA for Abeta40 and Abeta42(43). High concentrations of Abeta40 and Abeta42(43) in the young group, under 29 years old, changed to be at low concentrations in the adult group between 30 and 59 years old. Subsequently, the levels increased again with age. Third order regression analysis showed a significant correlation between the levels of Abeta40 and age (Y = - 169 X(3) + 3.1X(2)- 0.02X + 4135; P < 0.034) and between the levels of Abeta42(43) and age (Y = - 46 X(3) + 0.9 X(2)- 0.005X + 992; P < 0.005). The levels of CSF Abeta40 and Abeta42(43) were physiologically regulated to show a U-shaped natural course in normal aging. These findings suggested that the physiological increase of Abeta42(43) over 59 years of age is selectively inhibited in Alzheimer's disease.

Standardization of measurement of beta-amyloid(1-42) in cerebrospinal fluid and plasma

The standardization and clinical validation of the measurement of beta-amyloid(1-42) (Abeta42) in cerebrospinal fluid (CSF), plasma and urine is described using a commercially available sandwich-type ELISA with 21F12 and 3D6 as monoclonal antibodies. The INNOTEST beta-amyloid(1-42) allows the specific and reliable measurement of(1-42) amyloid peptides in CSF and plasma. The Abeta42 concentrations in serum and urine were below the detection limit. In plasma, no differences were found in Abeta42 levels between controls and patients with different neurodegenerative disorders (Alzheimer's disease (AD), Lewy body disease (LBD), others). In contrast, CSF-Abeta42 concentrations were lower in AD and LBD patients as compared to controls. No correlation was found in AD patients between CSF and plasma concentrations of Abeta42 or between CSF Abeta42 levels and blood-brain-barrier function. The quantitative outcome of the test is in part dependent on confounding factors such as tube type, freeze/thaw cycles, temperature of incubation, standard preparation protocol, and antibody selection. Notwithstanding these aspects, it emerged that Abeta42 is a useful biochemical marker for the diagnosis of AD patients, but there is a need for an international Abeta standard, a universally accepted protocol for CSF preparation, and a thorough evaluation of assay performance in function of the boundary conditions.

Cerebrospinal fluid levels of amyloid precursor protein and amyloid beta-peptide in Alzheimer's disease and major depression - inverse correlation with dementia severity

Alzheimer's disease (AD) is the most common neurodegenerative disorder characterized by progressive dementia that ultimately leads to death. Histopathological hallmarks of AD include brain amyloid deposits and neurofibrillary tangles. Major depression is a frequent diagnosis in every gerontopsychiatric clinic that sees patients with both cognitive and affective disorders. Many depressed patients, in fact, are clinically characterized by cognitive impairments. Thus, an assay that excludes - or confirms - probable AD in cognitively impaired patients is desirable. Such assays may use protein markers that are derived from such histopathologically relevant molecules as the amyloid precursor protein (APP) and its derivatives including the amyloid beta-peptides (Abeta). To evaluate the differential diagnostic properties of cerebrospinal fluid (CSF) Abeta and secreted soluble ectodomain (APPs), we quantitated CSF levels of these measures in AD patients and compared them to age-matched control patients with major depression. CSF levels of APPs and Abeta were similar in patients with AD or major depression, and the apolipoprotein E genotype had no influence on CSF levels of Abeta in AD patients. Measurement of Abeta peptide using a novel zinc/copper capture ELISA that detects aggregated Abeta peptides as well demonstrated similar levels in AD and major depression. In AD patients, CSF levels of total Abeta (Abeta1-40 plus Abeta1-42) were inversely correlated with a functional measure of dementia severity (NOSGER), suggesting that CSF levels of Abeta decrease with advancing severity of AD. Thus, CSF levels of Abeta are not useful for the differentiation of AD from major depression. However, CSF levels of Abeta reflect the severity of dementia and may be useful as biological markers of the stage of the disease.

Glutamine synthetase, hemoglobin alpha-chain, and macrophage migration inhibitory factor binding to amyloid beta-protein: their identification in rat brain by a novel affinity chromatography and in Alzheimer's disease brain by immunoprecipitation

Proteins binding to amyloid beta-protein (Abeta) may modulate the accumulation of Abeta in Alzheimer's disease (AD) brain. We developed a monomeric Abeta column for isolation of the proteins binding to Abeta from rat brain. By amino acid sequence analysis and immunoreactivity with specific antibodies, we identified three new Abeta-binding proteins, glutamine synthetase, hemoglobin alpha-chain, and macrophage migration inhibitory factor as well as serum albumin, beta-tubulin, and glyceraldehyde-3-phosphate dehydrogenase already identified as proteins bound to amyloid beta-protein precursor. In addition, the retained fraction contained both apolipoprotein E and alpha(1)-antichymotrypsin already known as Abeta binding proteins. Furthermore, we detected the complexes of these new binding proteins with Abeta in a soluble fraction of the cerebral cortex of AD brain by immunoprecipitation. Our results suggest that these binding proteins also associate with Abeta, leading to the clearance or the accumulation of Abeta and the neuronal cell damage in human brain.

Cerebrospinal fluid A beta42 is increased early in sporadic Alzheimer's disease and declines with disease progression

All mutations known to cause familial Alzheimer's disease (AD) act by increasing the levels of soluble beta-amyloid peptide (A beta), especially the longer form, A beta42. However, in vivo elevation of soluble A beta in sporadic AD has so far not been shown. In the present study, we used enzyme-linked immunosorbent assays specific for A beta42 and A beta40 to investigate cerebrospinal fluid from sporadic AD at different stages of disease severity, to clarify the roles of A beta42 and A beta40 during disease progression. We also evaluated three other groups--one group of patients with mild cognitive impairment who were at risk of developing dementia, a cognitively intact, nondemented reference group diagnosed with depression, and a perfectly healthy control group. We found that A beta42 is strongly elevated in early and mid stages of AD, and thereafter it declines with disease progression. On the contrary, A beta40 levels were decreased in early and mid stages of AD. The group of cognitively impaired patients and the depression reference group had significantly higher levels of A beta42 than the healthy control group, implying that A beta42 is increased not only in AD, but in other central nervous system conditions as well. Our data also point out the importance of having thoroughly examined control material. The initial increase and subsequent decrease of A beta42 adds a new biochemical tool to follow the progression of AD and might be important in the monitoring of therapeutics.

A sandwich enzyme immunoassay for measuring AD7C-NTP as an Alzheimer's disease marker: AD7C test

A simple, fast, reliable, and specific immunoassay has been developed to detect and measure AD7C-NTP, a biochemical marker for Alzheimer's disease, in cerebrospinal fluid (CSF). This assay, called the AD7C Test, is an enzyme-linked sandwich immunoassay (ELSIA) using 96 well microtiter plates. The plate surface is coated with a monoclonal antibody (N3I4) which has a high affinity and specificity for AD7C-NTP, capturing it effectively from CSF samples. The detection was achieved using a polyclonal antibody (ADRI). Both N3I4 and ADRI were generated using recombinantly produced AD7C-NTP. The assay is highly sensitive (30-50 pg), linear to 2.0 ng (r2 > 0.99), and reproducible (C.V. < 10%). The utility of the assay has been demonstrated using CSF specimens from early Alzheimer's disease patients and age matched controls (sensitivity of 89% and specificity of 89%).

Biochemical assay for AD7C-NTP in urine as an Alzheimer's disease marker

A reliable and specific immunoassay has been developed to detect and measure AD7C-NTP, a biochemical marker for Alzheimer's disease, in urine. The urine samples are first processed by centrifugation and ultrafiltration to fractionate and concentrate AD7C-NTP. The urinaryAD7C-NTP has the same molecular weight asAD7C-NTP in brain and cerebrospinal fluid by size exclusion chromatography. It has also retained the binding properties to the monoclonal and polyclonal antibodies developed against recombinantly produced AD7C-NTP. This assay is an enzyme linked sandwich immunoassay (ELSIA) using 96 well microtiter plates. The plate surface is coated with a monoclonal antibody (N314) which has a high affinity and specificity for AD7C-NTP, capturing it effectively from the samples. The detection was achieved using a polyclonal antibody (ADRI). The utility of the assay has been demonstrated using urine specimens from Alzheimer's disease (AD) patients and non-Alzheimer's controls. UrinaryAD7C-NTP in the AD group (2.5 ng/mL, n=66) was significantly higher than the non-AD group (0.8 ng/mL, n=134). Using 1.5 ng/mL as cut off, in this patient population, specificity and sensitivity of urinary AD7C-NTP were comparable to CSFAD7C-NTP.

Combination of the different biological markers for increasing specificity of in vivo Alzheimer's testing

In view of existing drugs (acetylcholine esterase inhibitors) and emerging therapeutic compounds (e.g. neuroprotective and anti-inflammatory compounds), CSF markers would be of great use to improve the clinical diagnostic accuracy of Alzheimer's disease (AD). Correct identification of AD would be especially important early in the course of the disease, when the clinical diagnosis is difficult, and drugs have the greatest potential of being effective. Biochemical markers for AD include ApoE genotyping, where the ApoE epsilon 4 allele has proven to have a high predictive value for AD. Biochemical markers for AD also include several potential cerebrospinal fluid (CSF) markers: beta-amyloid(1-42), possibly reflecting amyloid deposition and formation of senile plaques; PHFtau protein a marker for the phosphorylation state of tau, and formation of neurofibrillary tangles; (total)tau protein, a normal axonal protein, as a marker for ongoing neuronal and axonal degeneration; synaptic vesicle proteins, e.g. synaptotagmin, a synaptic vesicle protein which is found in the CSF, as markers for synaptic activity or degeneration; neuromodulin or growth-associated protein GAP-43, as a marker for synaptic degeneration, and the CSF/serum albumin ratio, as a marker for blood-brain barrier damage, used to exclude patients with concomitant cerebrovascular pathology. However, although CSF markers may identify different pathogenic processes in AD, there is no such process that is specific for AD, and thus little hope of ever finding a single CSF biochemical marker that gives an absolute discrimination between AD and other dementia disorders. Instead, combination of several CSF biochemical markers, each reflecting a pathogenic process, may increase both the sensitivity and specificity. Further, the accuracy of the clinical diagnosis of AD may increase if the diagnosis is based on the summarised information gained from the clinical examination, brain-imaging techniques (SPECT, CT/MRT scans), and biochemical markers. Using this approach, CSF markers have a large potential to help to differentiate AD from the most problematic differential diagnoses, especially age-associated memory impairment, depressive pseudo-dementia, Parkinson's disease, and frontal lobe dementia.

The effects of age, apolipoprotein E phenotype and gender on the concentration of amyloid-beta (A beta) 40, A beta 4242, apolipoprotein E and transthyretin in human cerebrospinal fluid

OBJECTIVE

To test whether the concentrations of amyloid-beta (A beta) 40, A beta 42, apolipoprotein E (apoE) and transthyretin in the CSF of normal individuals, are linked to three factors which modulate the risk of Alzheimer's disease (AD): chronological age, gender, and the presence of the apoE4 allele.

METHODS AND RESULTS

Proteins were measured by enzyme-linked immunosorbent assays except for transthyretin, which was assayed by radial immunodiffusion. The apoE phenotype was determined by isoelectric focusing. While the CSF levels of A beta 42, apoE, and transthyretin are reported to be reduced in AD, we found no relationship between age, gene, or apoE phenotype and the level of any of these proteins in the CSF of nondemented individuals. The concentration of A beta 40 was not modulated by gender or apoE phenotype, but did decline significantly with age.

CONCLUSION

These results indicate that the changes observed in the CSF of AD patients are specific to the disease itself rather than the known risk factors.

Longitudinal study of cerebrospinal fluid amyloid proteins and apolipoprotein E in patients with probable Alzheimer's disease

Levels of soluble amyloid beta protein (sAbeta), amyloid beta precursor protein (APP) and apolipoprotein E (apoE) were examined in cerebrospinal fluid (CSF) obtained twice, at baseline and after 3-year follow-up, from 25 patients with probable Alzheimer's disease (AD). Levels of sAbeta and apoE from patients with the apoE4 allele decreased with time, whereas the levels were similar in patients without apoE4 allele. Changes of sAbeta and apoE concentrations correlated significantly with those of mini-mental state examination (MMSE) scores. Levels of sAbeta did not change with time in patients with mild dementia, whereas they decreased significantly in patients with moderate dementia. ApoE concentrations decreased in both groups whereas APP levels were similar. We conclude that measurements of CSF sAbeta and apoE levels may be helpful in monitoring progression of the disease.

Amyloid beta-protein deposition in the leptomeninges and cerebral cortex

To further investigate the process of amyloid beta-protein (Abeta) deposition, we determined, using sensitive enzyme immunoassays, the levels of Abeta40 and Abeta42 (Abetas) in the soluble and insoluble fractions of the leptomeninges (containing arachnoid mater and leptomeningeal vessels) and cerebral cortices from elderly control subjects showing various stages of Abeta deposition and from patients affected by Alzheimer's disease (AD). In both locations, insoluble Abeta levels were higher by orders of magnitude than soluble Abeta levels. Soluble Abeta levels in cortices were much lower than those in leptomeninges. In insoluble Abeta in the cortex, Abeta42 was by far the predominant species, and Abeta42 in AD cortices was characterized by the highest degree of modifications in the amino terminus. In contrast, this Abeta42 predominance was not observed in insoluble Abeta in the leptomeninges, which were found to be able to accumulate Abetas to an extent similar to that in the cortex, on a weight basis. The levels of insoluble Abeta in the leptomeninges or cortex generally correlated with the degree of cerebral amyloid angiopathy or the abundance of senile plaque, respectively. However, the presence of plaque-free cortical samples showing significant levels of insoluble Abeta42 suggests that biochemically detectable Abeta accumulation precedes immunocytochemically detectable Abeta deposition in the cortex.

Amyloid beta protein 42(43) in cerebrospinal fluid of patients with Alzheimer's disease

To investigate the pathomechanism of amyloid beta protein (A beta) deposition in brains with Alzheimer's disease (AD), cerebrospinal fluid (CSF) levels of A beta species (CSF-A beta) with different carboxy termini, i.e. A betaX-40 and A betaX-42(43) as well as A beta1-40 and A beta1-42(43), were measured in patients with AD and age-matched controls without dementia (CTR) using sandwich enzyme-linked immunosorbent assays (ELISAs). The present study revealed that both CSF-A betaX-42(43) and A beta1-42(43) levels were significantly lower in the AD patients (P<0.005) than in the CTR group, whereas neither CSF-A betaX-40 nor CSF-A beta1-40 levels showed any differences between the two groups. In addition, although there was no difference between the ratios of A betaX-40 to A beta1-40 in the AD and CTR groups, the ratios of A betaX-42(43) to A beta1-42(43) were increased in the AD group compared with those in the CTR group (P<0.05). Therefore, it can be assumed that the ratios of amino terminal truncations and/or modifications of CSF-A beta42(43) with carboxy termini ending at residue 42(43) were more increased in the AD group than in the CTR group. Increased adsorption of A beta42(43) to A beta deposition in AD brains, decreased secretion of A beta42(43) to CSF and/or increased clearance of A beta42(43) from CSF might explain the diminished levels of A beta42(43) in the CSF of AD patients. In addition, CSF-A beta42(43) could reflect increased amino terminal truncations and/or modifications of A beta42(43) in AD brains.

Human A beta-amyloid and amyloid precursor protein accumulates in rat brain cells after cultured human leptomeningeal fibroblast implants

Cultured human leptomeningeal fibroblasts grafted into rat frontal cortex were localized to the implant pocket and to adjacent host leptomeninges. Immunohistochemical studies using a panel of human-specific and domain-specific APP antibodies revealed that all grafted cells expressed both APP and A beta in situ. Remarkably, these antibodies also labeled rat pial and ependymal cells as well as reactive astrocytes adjacent to vessels. In addition, apical projections and cell bodies of many cortical pyramidal neurons contained human-specific APP immunoreactive material. Groups of subcortical neurons, particularly those of the amygdala, hippocampal formation and suprachiasmatic nuclei, were similarly labeled. The presence of human APP in host brains was confirmed by immunoblotting. Birefringent Congo Red staining was observed in the cortical neuropil and in leptomeningeal vessels. These data indicate that grafted leptomeningeal fibroblasts hyperexpress APP and A beta which can diffuse into parenchyma and be taken up by specific rat cells.

Apolipoprotein E genotype and amyloid load in Alzheimer disease and control brains

We investigated the effect of apolipoprotein E (apoE) genotype on amyloid load in the frontal and cerebellar cortices of 24 patients with definite Alzheimer disease (AD) and 19 controls. Amyloid load was examined by using two methods: 1) acid-extractable amyloid beta-protein (A beta) and insoluble A beta levels of frontal and cerebellar cortices were measured by using enzyme-linked immunosorbent assay, and 2) all types of amyloid plaques and neurofibrillary tangles (NFT) in the frontal cortices were counted after silver staining. Acid-extractable A beta and insoluble A beta levels were higher in AD brains than controls, although there was an overlap between the groups. Acid-extractable A beta and insoluble A beta levels were higher from AD and controls with the apoE epsilon 4 alleles than those without such alleles. However, the differences did not reach statistical significance in AD group. There was no correlation between acid-extractable A beta or insoluble A3 levels and the number of amyloid plaques in AD and control brains. However, insoluble A beta levels correlated positively with the number of NFT in AD brains. Our results show that although apoE epsilon 4 influences the accumulation of A beta, multiple processes may be involved in deposition of A beta in the brain.

Muscarinic therapies in Alzheimer's disease; from palliative treatments to disease modification

Cholinergic therapies for Alzheimer's disease (AD) have been developed following painstaking neuropathological and neurochemical studies. Drugs based upon this approach are in development and it is hoped that these compounds will be of some use as palliative therapy. However, increasing evidence from molecular biology suggests that increasing cholinergic neurotransmission might not only alter cognition but also modify disease progression. The evidence that muscarinic-induced increases in protein kinase C activity favourably alter amyloid precursor protein metabolism and tau phosphorylation is reviewed. A unifying hypothesis of AD pathogenesis brings together plaque and tangle formation, suggesting that cholinergic therapies in development may have far-reaching implications as treatments for AD. (Int J Psych Clin Pract 1997; 1: 15-20).

Effect of cerebrospinal fluid from normal and Alzheimer's patients with different apolipoprotein E phenotypes on in vitro aggregation of amyloid beta-protein

We examined the effect of cerebrospinal fluid (CSF) from 23 Alzheimer's disease (AD) patients and 22 age-matched non-demented controls with apolipoprotein E4/4, 3/3, or 3/2 phenotypes on in vitro aggregation of amyloid beta-protein (A beta) 1-40 by Thioflavin T fluorescence spectroscopy. CSF from both AD and control groups inhibited A beta aggregation, as compared to that of phosphate buffered saline, in agreement with an earlier report (Wisniewski et al., 1993). However, there was significantly less aggregation of A beta in presence of CSF from AD than that from non-demented controls. The presence of CSF from controls with apoE3/3 phenotype resulted in higher A beta aggregation as compared to other phenotypes. There was a positive correlation between CSF apoE concentrations and A beta aggregation; whereas age, CSF soluble A beta levels or severity of dementia did not correlate with A beta aggregation. These results suggest that mechanism of sequestration of A beta in CSF may not be defective in AD. Amyloid formation in AD may be impact of altered balance of other factors such as amyloid-associated proteins/extracellular matrix components that can immobilize A beta in the brain, and promote its fibrillogenesis in AD.

The effects of aspartic acid-bond isomerization on in vitro properties of the amyloid beta-peptide as modeled with N-terminal decapeptide fragments

The 42-amino acid A beta, the major constituent of the senile plaque deposits of the brains of Alzheimer's disease patients, exhibits a high degree of heterogeneity at its N-terminus. Isomerization of aspartic acid bonds at residues 1 and 7 renders A beta more prone to aggregate and form extended structure as it was shown by in vivo and in vitro studies. We recently demonstrated the ability of mid-chain aspartic acid-bond isomerization to break the dominant helical structure of the N-terminal decapeptide fragment by CD. In the current study we use molecular modeling to show that insertion of the extra -CH2-group into the decapeptide backbone results in the formation of stable reverse-turns and destabilizes the helical conformer that competes with the extended structure at the full-sized peptide level. The molecular modeling also reveals a limited propensity of the diisomerized peptide to form extended structure directly. Anti-A beta pAb 2332 is more sensitive for the non-isomerized status of the decapeptide than that of the full-sized peptide. mAb 6E10, raised against unmodified A beta recognizes only the unmodified decapeptide or the peptide isomerized at the first aspartic acid in a conformation-dependent manner, but does not recognize the mid-chain isomerized or diisomerized decapeptide in any circumstance. The diisomerized decapeptide was used as immunogen to generate polyclonal antibody 14943 that is not selective for the isomerized status of either the full-size peptide or the decapeptide, but recognizes the isomerized peptides preferentially when the peptide antigen structures are conserved during the enzyme-linked immunoassay procedure. Owing to the aberrant behavior of the full-sized A beta peptide during standard RP-HPLC, serum stability studies that indicate extracellular stability can be more effectively performed on the decapeptide fragments. Remarkably, the diisomerized peptide exhibits a significantly increased stability towards serum peptidases compared with the unmodified or monoisomerized peptides, suggesting a possible mechanism of the retention of the isomerized A beta peptide in the affected brains.

Cholesterol modulates alpha-secretase cleavage of amyloid precursor protein

Amyloid precursor protein (APP) and cholesterol metabolism are genetically linked to Alzheimer's disease, the latter through apolipoprotein E, a lipid and cholesterol transport protein. We have examined the hypothesis that the processing of APP is disrupted by elevated cholesterol, which is known to modulate the activity of several transmembrane proteins. In the current study, cholesterol, solubilized by methyl- beta-cyclodextrin or ethanol, was added to the culture media of APP 751 stably transfected HEK 293 cells. Radiolabeled APP and APPsol (the soluble N-terminal derivative following alpha-secretase cleavage) were precipitated from lysates and conditioned media of stably transfected HEK 293 cells; the relative levels were determined by quantitative densitometry following separation by SDS-polyacrylamide gel electrophoresis. The data show that cholesterol, solubilized by methyl-beta-cyclodextrin, greatly reduced the levels of APPsol. Low doses of ethanol-solubilized cholesterol similarly caused a dramatic reduction of APPsol. By contrast, levels of APP holoprotein remained the same or increased. The large decrease seen in APPsol production was not due to nonspecific inhibition of secretion because several secreted proteins increased in level. Cholesterol, which impedes membrane fluidity, may lower APPsol production by impeding the interaction of the substrate with its protease(s). If APPsol were to function trophically, as suggested by other studies, the current conclusion suggests that changes in cellular cholesterol levels in Alzheimer's disease could contribute to neuronal degeneration by decreasing the production of APPsol.

Amyloid beta-peptide in cerebrospinal fluid in individuals with the Swedish Alzheimer amyloid precursor protein mutation

The neuropathological hallmarks of Alzheimer's disease (AD) are amyloid-containing plaques and neurofibrillary tangles. The main constituent of senile plaques is amyloid beta-peptide (A beta) and in recent years, pathogenic mutations in the amyloid precursor protein (APP) gene have been discovered in some AD families. The APP670/671 mutation, found in a Swedish AD family, has revealed over-production of A beta as one pathogenic mechanism for the development of AD. In the present study we have used an immunoassay to measure A beta levels in cerebrospinal fluid (CSF) from APP670/671 mutation-carriers and non-carriers. A correlation was seen between decrease in A beta levels and duration of disease although no difference was found in levels of A beta between the groups (14.5 +/- 3.3 ng/ml versus 14.9 +/- 2.3 ng/ml).

Amyloid beta binding proteins in vitro and in normal human cerebrospinal fluid

A major neuropathological feature of Alzheimer's disease (AD) is the deposition of amyloid beta (A beta) in the form of senile plaques. The A beta peptide exists both in a beta-pleated sheet fibrillar form in amyloid deposits and as a normal soluble protein in biological fluids. Numerous proteins have been identified immunohistochemically to be associated with senile plaques, where A beta is the major constituent. Some of the latter have also been suggested to be carriers of the normal soluble A beta (sA beta) including apolipoprotein J (apoJ), apolipoprotein E (apoE) and transthyretin (TTR). We have found, using several different methods, that numerous proteins can bind synthetic A beta peptides when high concentrations are used; however, using an affinity anti-sA beta column we confirm that apoJ is the major binding protein in pooled human cerebrospinal fluid. On the other hand it is known that apoE co-purifies with A beta biochemically extracted from senile plaques. In AD tissue there may be a change in the major apolipoprotein binding A beta from apoJ to apoE.