Alzheimer's disease amyloid precursor protein is present in senile plaques and cerebrospinal fluid: immunohistochemical and biochemical characterization

Assessment of Neurodegenerative Changes in Turkeys Fed Diets with Different Proportions of Arginine and Methionine Relative to Lysine

Simple Summary

It is important to take care of a properly balanced amino acid composition in the diet in order to inhibit or delay the occurrence of processes and changes related to the destruction of nervous tissue. Therefore, an attempt was made in this manuscript to evaluate the effect of different ratios of the key amino acids arginine and methionine, relative to lysine, in relation to two turkey feeding standards. The amino acid guidelines formulated by British United Turkeys (BUT) suggest higher levels of lysine (Lys) in turkey diets than those recommended by the National Research Council (NRC). In order to assess the impact of such supplementation, we analyzed the level of indicators informing the presence or degree of advancement of neurodegenerative processes in the nervous tissue (the level of acetylcholinesterase and amyloid-β; the concentration of AChE complexes with amyloid-β and Tau protein, called glycosylated acetylcholinesterase (GAChE), indicative of the destruction of neurons). The level of low-density lipoprotein receptor-related protein 1, or LRP-1, which facilitates the breakdown of toxic amyloid-β, was also assessed. In addition, the effect of different doses of these amino acids on neurodegenerative changes in DNA, especially the degree of methylation of histone proteins resulting from covalent modifications was compared between lysine and arginine residues.

Abstract

We postulated that the use of optimal levels and proportions of Arg and Met relative to a low or high concentration of Lys in diets for meat turkeys would reduce the occurrence of metabolic disturbances in the nervous tissue that can lead to neurodegenerative changes. The aim of the study was to determine the effect of various proportions of Lys, Arg, and Met in diets for turkeys, with a low content of Lys in accordance with NRC (Experiment 1) recommendations, and in diets with high Lys levels that are close to the recommendations of breeding companies (Experiment 2) on selected indicators of potential neurodegenerative effects in the brain and liver of turkeys. The Experiment 1 and Experiment 2 was conducted using 864 day-old turkey chicks randomly assigned to six groups, in eight replicates (6 groups × 18 birds × 8 replicates). A full description of the methodology can be found in previously published papers using the same experimental design. Indicators informing about the presence or advancement of neurodegenerative processes in the nervous tissue were determined in the brain and liver (level of: AChE, amyloid-β, GAChE, Tau protein, LRP1, and the degree of DNA methylation). It was established that in the case of both a low (National Research Council, NRC) and a high (British United Turkeys, BUT) level of Lys in the diet of turkeys, the Arg level can be reduced to 90% of the Lys level and Met to 30% of the Lys level, because this does not cause neurodegenerative changes in turkeys. Unfavorable neurodegenerative changes may appear if the Arg level is increased from 100 to 110% of the Lys level recommended by the NRC. However, due to the lack of such a relationship when Arg is increased from 100 to 110% of the Lys level recommended by BUT, at this stage of research no definitive conclusions can be drawn regarding the risk of neurodegenerative changes caused by increasing Arg in the diet of turkeys.

BACE1 Overexpression Reduces SH-SY5Y Cell Viability Through a Mechanism Distinct from Amyloid-β Peptide Accumulation: Beta Prime-Mediated Competitive Depletion of sAβPPα

BACKGROUND

The Alzheimer's disease (AD)-associated amyloid-beta protein precursor (AβPP) can be cleaved by β-site AβPP cleaving enzyme 1 (BACE1) and the γ-secretase complex to yield neurotoxic amyloid-β (Aβ) peptides. However, AβPP can also be cleaved in a 'non-amyloidogenic' manner either by α-secretase to produce soluble AβPP alpha (sAβPPα) (a fragment with neuroprotective/neurogenic functions) or through alternative BACE1-mediated 'beta prime' activity yielding soluble AβPP beta prime (sAβPPβ').

OBJECTIVE

To determine whether sAβPPα depletion, as opposed to Aβ peptide accumulation, contributes to cytotoxicity in AD-relevant SH-SY5Y neuroblastoma cell models.

METHODS

AβPP proteolysis was characterized by immunoblotting in mock-, wild-type AβPP (wtAβPP)-, BACE1-, and Swedish mutant AβPP (SweAβPP)-transfected cells. AβPP beta prime cleavage was confirmed through secretase inhibitor studies and C-terminal fragment analysis. The roles of sAβPPα and sAβPPβ' in cell viability were confirmed by overexpression studies.

RESULTS

Despite producing enhanced Aβ peptide levels, wtAβPP- and SweAβPP-transfected cells did not exhibit reduced viability whereas BACE1-transfected cells did. sAβPPα generation in SH-SY5Y-BACE1 cells was virtually ablated in lieu of BACE1-mediated sAβPPβ' production. sAβPPα overexpression in SH-SY5Y-BACE1 cells restored viability whereas sAβPPβ' overexpression decreased viability further. The anti-AβPP 6E10 antibody was shown to cross-react with sAβPPβ'.

CONCLUSION

sAβPPα depletion and/or sAβPPβ' accumulation, but not elevated Aβ peptide levels, represent the cytotoxic mechanism following BACE1 overexpression in SH-SY5Y cells. These data support the novel concept that competitive sAβPPα depletion by BACE1 beta prime activity might contribute to AD. The cross-reactivity of 6E10 with AβPPβ'also questions whether previous studies assessing sAβPPα as a biomarker using this antibody should be revisited.

Significance of Blood and Cerebrospinal Fluid Biomarkers for Alzheimer's Disease: Sensitivity, Specificity and Potential for Clinical Use

Alzheimer's disease (AD) is the most common type of dementia, affecting more than 5 million Americans, with steadily increasing mortality and incredible socio-economic burden. Not only have therapeutic efforts so far failed to reach significant efficacy, but the real pathogenesis of the disease is still obscure. The current theories are based on pathological findings of amyloid plaques and tau neurofibrillary tangles that accumulate in the brain parenchyma of affected patients. These findings have defined, together with the extensive neurodegeneration, the diagnostic criteria of the disease. The ability to detect changes in the levels of amyloid and tau in cerebrospinal fluid (CSF) first, and more recently in blood, has allowed us to use these biomarkers for the specific in-vivo diagnosis of AD in humans. Furthermore, other pathological elements of AD, such as the loss of neurons, inflammation and metabolic derangement, have translated to the definition of other CSF and blood biomarkers, which are not specific of the disease but, when combined with amyloid and tau, correlate with the progression from mild cognitive impairment to AD dementia, or identify patients who will develop AD pathology. In this review, we discuss the role of current and hypothetical biomarkers of Alzheimer's disease, their specificity, and the caveats of current high-sensitivity platforms for their peripheral detection.

Amyloid precursor protein glycosylation is altered in the brain of patients with Alzheimer's disease

Background

The amyloid precursor protein (APP) is a transmembrane glycoprotein that undergoes alternative proteolytic processing. Its processing through the amyloidogenic pathway originates a large sAPPβ ectodomain fragment and the β-amyloid peptide, while non-amyloidogenic processing generates sAPPα and shorter non-fibrillar fragments. Hence, measuring sAPPα and sAPPβ has been proposed as a means to identify imbalances between the amyloidogenic/non-amyloidogenic pathways in the brain of Alzheimer’s disease (AD) patients. However, to date, no consistent changes in these proteolytic fragments have been identified in either the brain or cerebrospinal fluid of AD individuals.

Methods

In frontal cortex homogenates from AD patients (n = 7) and non-demented controls (NDC; n = 7), the expression of total APP mRNA and that of the APP isoforms generated by alternative splicing, APP695 and APP containing the Kunitz protease inhibitor (KPI), was analyzed by qRT-PCR using TaqMan and SYBR Green probes. The balance between the amyloidogenic/non-amyloidogenic pathways was examined in western blots estimating the sAPPα and sAPPβ fragments and their membrane-tethered C-terminal fragments CTFα and CTFβ. CHO-PS70 cells, stably over-expressing wild-type human APP, served to evaluate whether Aβ42 peptide treatment results in altered APP glycosylation. We determined the glycosylation pattern of sAPPα and sAPPβ in brain extracts and CHO-PS70 culture media by lectin-binding assays.

Results

In the cortex of AD patients, we detected an increase in total APP mRNA relative to the controls, due to an increase in both the APP695 and APP-KPI variants. However, the sAPPα or sAPPβ protein levels remained unchanged, as did those of CTFα and CTFβ. We studied the glycosylation of the brain sAPPα and sAPPβ using lectins and pan-specific antibodies to discriminate between the fragments originated from neuronal APP695 and glial/KPI variants. Lectin binding identified differences in the glycosylation of sAPPβ species derived from the APP695 and APP-KPI variants, probably reflecting their distinct cellular origin. Moreover, the lectin-binding pattern differed in the sAPPα and sAPPβ originated from all the variants. Finally, when the lectin-binding pattern was compared between AD and NDC groups, significant differences were evident in sAPPα glycosylation. Lectin binding of the soluble sAPPα and sAPPβ from CHO-PS70 cells were also altered in cells treated with the Aβ peptide.

Conclusion

Our analysis of the lectin binding to sAPPα and sAPPβ suggests that glycosylation dictates the proteolytic pathway for APP processing. Differences between the demented and controls indicate that changes in glycosylation may influence the generation of the different APP fragments and, consequently, the pathological progression of AD.

A method of predicting the in vitro fibril formation propensity of Aβ40 mutants based on their inclusion body levels in E. coli

Overexpression of recombinant proteins in bacteria may lead to their aggregation and deposition in inclusion bodies. Since the conformational properties of proteins in inclusion bodies exhibit many of the characteristics typical of amyloid fibrils. Based on these findings, we hypothesize that the rate at which proteins form amyloid fibrils may be predicted from their propensity to form inclusion bodies. To establish a method based on this concept, we first measured by SDS-PAGE and confocal microscopy the level of inclusion bodies in E. coli cells overexpressing the 40-residue amyloid-beta peptide, Aβ40, wild-type and 24 charge mutants. We then compared these results with a number of existing computational aggregation propensity predictors as well as the rates of aggregation measured in vitro for selected mutants. Our results show a strong correlation between the level of inclusion body formation and aggregation propensity, thus demonstrating the power of this approach and its value in identifying factors modulating aggregation kinetics.

Alterations in the Balance of Amyloid-β Protein Precursor Species in the Cerebrospinal Fluid of Alzheimer's Disease Patients

We recently demonstrated that soluble forms of the amyloid-β protein precursor (sAβPP) assemble into multimeric complexes in cerebrospinal fluid (CSF), which contributes to the underestimation of specific sAβPP species when assessed by ELISA. To circumvent this issue, we analyzed by SDS-PAGE large fragments of sAβPP and their variants in the CSF from Alzheimer's disease (AD; n = 20) and control (n = 20) subjects, probing with specific antibodies against particular domains. Similar levels of sAβPPα and sAβPPβ protein were found in CSF samples from AD and controls, yet there appeared to be a shift in the balance of the soluble full-length AβPP (sAβPPf) species in AD samples, with a decrease in the proportion of the lower (∼100 kDa) band relative to the upper (∼120 kDa) band. Similar differences were observed in the contribution of the major KPI-immunoreactive AβPP species. CSF samples also displayed differences in the correlations of AβPP species with classical AD biomarkers, particularly with respect to the Aβ42 peptide. The differences reveal alterations that probably reflect pathophysiological changes in the brain.

C-terminal fragments of the amyloid precursor protein in cerebrospinal fluid as potential biomarkers for Alzheimer disease

This study assesses whether C-terminal fragments (CTF) of the amyloid precursor protein (APP) are present in cerebrospinal fluid (CSF) and their potential as biomarkers for Alzheimer’s disease (AD). Immunoprecipitation and simultaneous assay by Western blotting using multiplex fluorescence imaging with specific antibodies against particular domains served to characterize CTFs of APP in human CSF. We demonstrate that APP-CTFs are detectable in human CSF, being the most abundant a 25-kDa fragment, probably resulting from proteolytic processing by η-secretase. The level of the 25-kDa APP-CTF was evaluated in three independent CSF sample sets of patients and controls. The CSF level of this 25-kDa CTF is higher in subjects with autosomal dominant AD linked to PSEN1 mutations, in demented Down syndrome individuals and in sporadic AD subjects compared to age-matched controls. Our data suggest that APP-CTF could be a potential diagnostic biomarker for AD.

Restoring Soluble Amyloid Precursor Protein α Functions as a Potential Treatment for Alzheimer's Disease

Soluble amyloid precursor protein α (sAPPα), a secreted proteolytic fragment of nonamyloidogenic amyloid precursor protein (APP) processing, is known for numerous neuroprotective functions. These functions include but are not limited to proliferation, neuroprotection, synaptic plasticity, memory formation, neurogenesis, and neuritogenesis in cell culture and animal models. In addition, sAPPα influences amyloid-β (Aβ) production by direct modulation of APP β-secretase proteolysis as well as Aβ-related or unrelated tau pathology, hallmark pathologies of Alzheimer's disease (AD). Thus, the restoration of sAPPα levels and functions in the brain by increasing nonamyloidogenic APP processing and/or manipulation of its signaling could reduce AD pathology and cognitive impairment. It is likely that identification and characterization of sAPPα receptors in the brain, downstream effectors, and signaling pathways will pave the way for an attractive therapeutic target for AD prevention or intervention. © 2016 Wiley Periodicals, Inc.

Transmembrane Amyloid-Related Proteins in CSF as Potential Biomarkers for Alzheimer's Disease

In the continuing search for new cerebrospinal fluid (CSF) biomarkers for Alzheimer’s disease (AD), reasonable candidates are the secretase enzymes involved in the processing of the amyloid precursor protein (APP), as well as the large proteolytic cleavage fragments sAPPα and sAPPβ. The enzymatic activities of some of these secretases, such as BACE1 and TACE, have been investigated as potential AD biomarkers, and it has been assumed that these activities present in human CSF result from the soluble truncated forms of the membrane-bound enzymes. However, we and others recently identified soluble forms of BACE1 and APP in CSF containing the intracellular domains, as well as the multi-pass transmembrane presenilin-1 (PS1) and other subunits of γ-secretase. We also review recent findings that suggest that most of these soluble transmembrane proteins could display self-association properties based on hydrophobic and/or ionic interactions leading to the formation of heteromeric complexes. The oligomerization state of these potential new biomarkers needs to be taken into consideration for assessing their real potential as CSF biomarkers for AD by adequate molecular tools.

Heteromers of amyloid precursor protein in cerebrospinal fluid

BACKGROUND

Soluble fragments of the amyloid precursor protein (APP) generated by α- and β-secretases, sAPPα and sAPPβ, have been postulated as promising new cerebrospinal fluid (CSF) biomarkers for the clinical diagnosis of Alzheimer's disease (AD). However, the capacity of these soluble proteins to assemble has not been explored and could be relevant. Our aim is to characterize possible sAPP oligomers that could contribute to the quantification of sAPPα and sAPPβ in CSF by ELISA, as well as to characterize the possible presence of soluble full-length APP (sAPPf).

RESULTS

We employed co-immunoprecipitation, native polyacrylamide gel electrophoresis and ultracentrifugation in sucrose density gradients to characterize sAPP oligomers in CSF. We have characterized the presence of sAPPf in CSF from NDC and AD subjects and demonstrated that all forms, including sAPPα and sAPPβ, are capable of assembling into heteromers, which differ from brain APP membrane-dimers. We measured sAPPf, sAPPα and sAPPβ by ELISA in CSF samples from AD (n = 13) and non-disease subjects (NDC, n = 13) before and after immunoprecipitation with antibodies against the C-terminal APP or against sAPPβ. We demonstrated that these sAPP heteromers participate in the quantification of sAPPα and sAPPβ by ELISA. Immunoprecipitation with a C-terminal antibody to remove sAPPf reduced by ~30% the determinations of sAPPα and sAPPβ by ELISA, whereas immunoprecipitation with an APPβ antibody reduced by ~80% the determination of sAPPf and sAPPα.

CONCLUSIONS

The presence of sAPPf and sAPP heteromers should be taken into consideration when exploring the levels of sAPPα and sAPPβ as potential CSF biomarkers.

The Ubiquitin-Proteasome System and Molecular Chaperone Deregulation in Alzheimer's Disease

One of the shared hallmarks of neurodegenerative diseases is the accumulation of misfolded proteins. Therefore, it is suspected that normal proteostasis is crucial for neuronal survival in the brain and that the malfunction of this mechanism may be the underlying cause of neurodegenerative diseases. The accumulation of amyloid plaques (APs) composed of amyloid-beta peptide (Aβ) aggregates and neurofibrillary tangles (NFTs) composed of misfolded Tau proteins are the defining pathological markers of Alzheimer's disease (AD). The accumulation of these proteins indicates a faulty protein quality control in the AD brain. An impaired ubiquitin-proteasome system (UPS) could lead to negative consequences for protein regulation, including loss of function. Another pivotal mechanism for the prevention of misfolded protein accumulation is the utilization of molecular chaperones. Molecular chaperones, such as heat shock proteins (HSPs) and FK506-binding proteins (FKBPs), are highly involved in protein regulation to ensure proper folding and normal function. In this review, we elaborate on the molecular basis of AD pathophysiology using recent data, with a particular focus on the role of the UPS and molecular chaperones as the defensive mechanism against misfolded proteins that have prion-like properties. In addition, we propose a rational therapy approach based on this mechanism.

Distinct patterns of APP processing in the CNS in autosomal-dominant and sporadic Alzheimer disease

Autosomal-dominant Alzheimer disease (ADAD) is a genetic disorder caused by mutations in Amyloid Precursor Protein (APP) or Presenilin (PSEN) genes. Studies from families with ADAD have been critical to support the amyloid cascade hypothesis of Alzheimer disease (AD), the basis for the current development of amyloid-based disease-modifying therapies in sporadic AD (SAD). However, whether the pathological changes in APP processing in the CNS in ADAD are similar to those observed in SAD remains unclear. In this study, we measured β-site APP-cleaving enzyme (BACE) protein levels and activity, APP and APP C-terminal fragments in brain samples from subjects with ADAD carrying APP or PSEN1 mutations (n = 18), patients with SAD (n = 27) and age-matched controls (n = 22). We also measured sAPPβ and BACE protein levels, as well as BACE activity, in CSF from individuals carrying PSEN1 mutations (10 mutation carriers and 7 non-carrier controls), patients with SAD (n = 32) and age-matched controls (n = 11). We found that in the brain, the pattern in ADAD was characterized by an increase in APP β-C-terminal fragment (β-CTF) levels despite no changes in BACE protein levels or activity. In contrast, the pattern in SAD in the brain was mainly characterized by an increase in BACE levels and activity, with less APP β-CTF accumulation than ADAD. In the CSF, no differences were found between groups in BACE activity or expression or sAPPβ levels. Taken together, these data suggest that the physiopathological events underlying the chronic Aβ production/clearance imbalance in SAD and ADAD are different. These differences should be considered in the design of intervention trials in AD.

Biomarkers of Alzheimer's disease

Although a battery of neuropsychological tests is often used in making a clinical diagnosis of Alzheimer's disease (AD), definitive diagnosis still relies on pathological evaluation at autopsy. The identification of AD biomarkers may allow for a less invasive and more accurate diagnosis as well as serve as a predictor of future disease progression and treatment response. Importantly, biomarkers may also allow for the identification of individuals who are already developing the underlying pathology of AD such as plaques and tangles yet who are not yet demented, i.e. "preclinical" AD. Attempts to identify biomarkers have included fluid and imaging studies, with a number of candidate markers showing significant potential. More recently, better reagent availability and novel methods of assessment have further spurred the search for biomarkers of AD. This review will discuss promising fluid and imaging markers to date.

Evidence for binding of the ectodomain of amyloid precursor protein 695 and activated high molecular weight kininogen

To identify ligands that bind to the N-terminal portion of human amyloid precursor protein (APP), we sought binding partners for a fragment of the ectodomain of human APP695 (sAPP(695)T). The probe bound to fragments of high molecular weight kininogen (HK) in rat cortical membrane preparations in vitro. Laser confocal microscopy indicated that APP and HK colocalize near cerebral blood vessels, in the neuropil, and in many neurons of rat brain. sAPP(695)T bound to human activated kininogen (HKa) (K(d)=0.3+/-0.1 nM), but not to inactivated or low molecular weight kininogen. Binding was specific for the light chain sequence of HKa. Biotinylated human HKa also bound to sAPP(695) (K(d)=0.3+/-0.5 nM). sAPP(695) and HKa form tight complexes in solution that can be coimmunoprecipitated. These results support the hypothesis that forms of APP and kininogen can interact in brain tissue. Considering the implications of APP in neurite outgrowth, the APP-HKa interaction could modulate neurogenesis.

Levels of alpha- and beta-secretase cleaved amyloid precursor protein in the cerebrospinal fluid of Alzheimer's disease patients

Alternative cleavage of the amyloid precursor protein (APP) results in generation and secretion of both soluble APP (sAPP) and beta-amyloid (Abeta). Abeta is the main component of the amyloid depositions in the brains of Alzheimer's disease (AD) patients. Using Western blotting, we compared the levels of alpha-secretase cleaved sAPP, beta-secretase cleaved sAPP and total sAPP, in cerebrospinal fluid (CSF) from 13 sporadic AD patients and 13 healthy controls. Our findings show significant amounts of beta-secretase cleaved sAPP in CSF. There was no statistically significant difference in the levels of beta-secretase cleaved sAPP between AD patients and controls. The levels of alpha-secretase cleaved sAPP and total sAPP were, however, found to be significantly lower in the AD patients than in the controls.

Down-regulation of the amyloid protein precursor of Alzheimer's disease by antisense oligonucleotides reduces neuronal adhesion to specific substrata

The hallmark of Alzheimer's disease is the cerebral deposition of amyloid which is derived from the amyloid precursor protein (APP). The function of APP is unknown but there is increasing evidence for the role of APP in cell-cell and/or cell-matrix interactions. Primary cultures of murine neurons were treated with antisense oligonucleotides to down-regulate APP. This paper presents evidence that APP mediates a substrate-specific interaction between neurons and extracellular matrix components collagen type I, laminin and heparan sulphate proteoglycan but not fibronectin or poly-L-lysine. It remains to be determined whether this effect is the direct result of APP-matrix interactions, or whether an intermediatry pathway is involved.

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

Cerebrospinal fluid (CSF) soluble amyloid beta-protein (sA beta) concentrations from 69 patients with Alzheimer's disease (AD), 23 patients with vascular dementia (VD), and 76 non-demented controls were measured by a sandwich enzyme linked immunosorbent assay using two monoclonal antibodies (4G8 and 6E10) specific for A beta. sA beta concentrations were lower in CSF from patients with AD or VD compared to those in controls. CSF sA beta concentrations did not correlate with the Mini-Mental State Examination scores in patients with AD. VD patients with moderate to severe dementia had lower CSF sA beta concentrations than those with mild dementia. Because a considerable overlap of CSF sA beta levels existed between AD and control groups, the assay is not useful as a diagnostic test for AD.

Characterization of a novel processing pathway for Alzheimer's amyloid beta precursor protein

Amyloid beta (A beta) is a normal proteolytic fragment of a large precursor protein (beta PP) which undergoes altered conformation, leading to fibril formation. Two main beta PP processing pathways have been described, and we are now reporting the characterization of a third beta PP pathway. A membrane-associated 16 kDa component identified in human platelets isolated from normal donors. Based on size, immunoreactivity and amino acid sequence analysis, the fragment is a C-terminal beta PP component which starts at position 642 (APP770 numbering) and contains the intact A beta sequence. The presence of this novel pathway of beta PP processing in resting platelets suggest that it occurs as a normal event.

Differential distribution of amyloid protein precursor immunoreactivity in the rat brain studied by using five different antibodies

The beta-amyloid or A4 protein is found deposited in neuritic plaques and neurofibrillary tangles in Alzheimer's disease (AD) affected brains and in the brains of adults with Down's Syndrome. The precursor to this 42 amino acid protein is the 695 amino acid long amyloid protein precursor (APP-695). Two additional APP species, APP-751 and APP-770, each contain a 56-amino-acid insert sequence that is analogous to Kunitz protease inhibitors. APP mRNA is widely distributed in both the human and rat brain, although the adult rat does not develop mature amyloid pathology. In this study we used antibodies against the N-terminus, junction site (unique to APP-695) insert sequence (unique to APP-751,-770), A4 region, and C-terminus of APP to immunolabel sections from throughout the young adult rat brain. From these results we constructed maps of the staining pattern of each antibody. We found that APP is widely distributed throughout the brain, that labelling is predominantly neuronal in character, and that there is marked variation among the antibodies in the extent of labelling, the particular cell populations stained, and the structures labelled within individual cells. The differential staining patterns observed with the five different antibodies suggest that the way APP is processed differs from one region to another and within different compartments in the cell. The specificity of the antibodies was established by Western blot analysis, in which APP species of approximately 95 and 110 kD were found. Our findings on the distribution of APP provide a foundation for further investigations into the normal role of APP and the pathogenesis of AD.

Age and damage induced changes in amyloid protein precursor immunohistochemistry in the rat brain

Alzheimer's disease (AD) is characterized by the extensive deposition of the 42-amino-acid beta-amyloid or A4 protein in neuritic plaques and neurofibrillary tangles within the brain. This protein is liberated from the much larger amyloid protein precursor (APP). Multiple species of APP have been proposed, including several forms that contain a 56 amino acid insert sequence analogous to the Kunitz protease inhibitors. Although expression of APP mRNA is reportedly altered in AD brain and various roles for APP have been proposed, the pathogenesis of amyloid deposition and AD remains unclear. AD is also characterized by specific memory impairments associated with decreased cholinergic activity. While aging rats do not develop mature amyloid pathology, behaviorally impaired aged rats demonstrate an analogous cholinergic decline. In this study, we examined behaviorally characterized aged rats and normal young controls for changes in APP immunohistochemistry by using anti-APP antibodies, which detect N- or C-terminal regions and which distinguish APP species with or without the Kunitz protease inhibitor domain. The results show specific age- and behavior-related changes in cortical APP immunoreactivity as well as limited numbers of APP immunoreactive deposits in the aged rats. Additionally, we found that lesions of the fimbria-fornix pathway, which in part mimic the memory impairments and loss of cholinergic activity seen in AD, result in the marked accumulation of APP immunoreactive material in the region of cholinergic fiber degeneration in the hippocampus. These findings are discussed in relation to the pathogenesis of AD in humans.

Identification and regulation of the high affinity binding site of the Alzheimer's disease amyloid protein precursor (APP) to glycosaminoglycans

The specific binding of the amyloid protein precursor (APP) to glycosaminoglycans (GAG) suggests that APP is a cell adhesion molecule (CAM) and/or substrate adhesion molecule (SAM). In order to characterize this activity of APP in the brain at the molecular level, we have purified and characterized the major APP species from rat brain. The major isoform isolated was sequenced and found to be APP695. In a solid-phase binding assay, the specificity of this brain-specific APP isoform-GAG interaction was analysed. The binding of APP to the glycosaminoglycan heparin was found to be time-dependent and saturable. A strong heparin-binding site within a region conserved in rodent and human APP, APLP1 and APLP2, was identified. Saturable binding to heparin through this binding site was found to occur at nmol concentrations of APP. This putative high-affinity site was then located within a sequence of 22 amino acids in length corresponding to residues 316-337 of APP695. This sequence is encoded by APP exon 9 and the first three codons of exon 10. Since all APP and L-APP isoforms so far described include these exons, the strong heparin binding site is a ubiquitous feature of all APP and L-APP isoforms strongly suggesting that the brain-specific and neuronal, as well as the non-neuronal and peripheral APPs and L-APPs do have CAM- and SAM-like activities. Certain metal ions including zinc (II) have been proposed as risk factors in Alzheimer's disease (AD). Recently we showed that APP binds zinc (II) at higher nmol concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Morphological and biochemical analyses of amyloid plaque core proteins purified from Alzheimer disease brain tissue

Amyloid plaque cores were purified from Alzheimer disease brain tissue. Plaque core proteins were solubilized in formic acid which upon dialysis against guanidinium hydrochloride (GuHCl) partitioned into soluble (approximately 15%) and insoluble (approximately 85%) components. The GuHCl-soluble fraction contained beta-amyloid1-40, whereas the GuHCl-insoluble fraction was fractionated into six components by size exclusion HPLC: S1 (> 200 kDa), S2 (200 kDa), S3 (45 kDa), S4 (15 kDa), S5 (10 kDa), and S6 (5 kDa). Removal of the GuHCl reconstituted 10-nm filaments composed of two intertwined 5-nm strands. Fractions S5 and S6 also yielded filamentous structures when treated similarly, whereas fractions S1-S4 yielded amorphous aggregates. Chemical analysis identified S4-S6 as multimeric and monomeric beta-amyloid. Immunochemical analyses revealed alpha 1-antichymotrypsin and non-beta-amyloid segments of the beta-amyloid precursor protein within fractions S1 and S2. Several saccharide components were identified within plaque core protein preparations by fluorescence and electron microscopy, as seen with fluorescein isothiocyanate- and colloidal gold-conjugated lectins. We have shown previously that this plaque core protein complex is more toxic to neuronal cultures than beta-amyloid. The non-beta-amyloid components likely mediate this additional toxicity, imposing a significant influence on the pathophysiology of Alzheimer disease.

A 109-amino-acid C-terminal fragment of Alzheimer's-disease amyloid precursor protein contains a sequence, -RHDS-, that promotes cell adhesion

Amyloid beta (A beta), the major constituent of the fibrils composing senile plaques and vascular amyloid deposits in Alzheimer's disease (AD) and related disorders, is a 39-42-residue self-aggregating degradation peptide of a larger multidomain membrane glycoprotein designated amyloid precursor protein (APP). An array of biological functions has been assigned to different APP domains, including growth regulation, neurotoxicity, inhibitory activity of serine proteinases and promotion of cell-cell and cell-matrix interactions. A beta is generated through an as-yet-unknown catabolic pathway that by-passes or inhibits the cleavage of APP within the A beta sequence. We have identified a 16 kDa intermediate APP C-terminal fragment containing A beta in leptomeningeal vessels of aged normal individuals and AD patients by means of its immunoreactivity with a panel of four different anti-(APP C-terminal) antibodies, indicating a different pathway of APP processing. Previous studies have indicated that the APP C-terminal domain is the most likely to be involved in cell-matrix interactions. A 109-amino-acid construct C109 with a sequence analogous to the C-terminal of APP (positions 587-695 of APP695), similar in length and immunoreactivity to the 16 kDa fragment, was found to promote cell adhesion. By use of synthetic peptides, this activity was initially located to the extracellular 28 residues of A beta. Inhibition studies demonstrated that the sequence RHDS (amino acids 5-8 of A beta, corresponding to residues 601-604 of APP695 was responsible for the adhesion-promoting activity. The interaction is dependent on bivalent cations and can be blocked either by the tetrapeptides RHDS and RGDS or by an anti-(beta 1 integrin) antibody. Thus, through integrin-like surface receptors, APP or its derivative proteolytic fragments containing the sequence RHDS may modulate cell-cell or cell-matrix interactions.

Senile plaques in cerebral amyloid angiopathy show accumulation of amyloid precursor protein without cytoskeletal abnormalities

The abnormal neurites that surround beta-amyloid in senile plaques (SP) in Alzheimer disease contain beta-amyloid precursor protein (beta APP) or abnormal filaments which react with antibodies to tau. Occasionally, beta APP and abnormal filaments are present in the same neurite. Whether both types of abnormal neurites are reactive to the presence of beta-amyloid or they are instead independent from each other is unknown. To begin to clarify this issue, we comparatively studied beta APP and tau-epitopes in SP from cases of classical Alzheimer disease and cases of cerebral amyloid angiopathy, with SP but without neurofibrillary pathology. In subjects with cerebral amyloid angiopathy, about one-third of SP, the same percentage as in Alzheimer disease, were beta APP reactive in the absence of tau-reactivity. beta APP epitopes were ultrastructurally localized in dense bodies of probable lysosomal origin, adjacent to the core of SP. These results demonstrate that beta APP and tau-reactive cytoskeletal alterations occur independently in the neurites of SP. The presence of beta APP in dystrophic neurites of SP and the localization of beta APP in lysosomes suggest that beta APP containing dystrophic neurites may play a role in the extracellular deposition of amyloid.

A soluble form of prion protein in human cerebrospinal fluid: implications for prion-related encephalopathies

The cellular prion protein (PrPc) is a 33-35 kDa sialoglycoprotein anchored to the external surface of neural and non-neural cells by a glycosyl phosphatidylinositol moiety. In addition, a secretory form of PrPc has been found in cell-free translation systems and in cell cultures. On this basis, we investigated human cerebrospinal fluid for the presence of soluble PrP and identified a protein whose molecular weight, antigenic determinants, N-terminal amino acid sequence and sensitivity to protease digestion corresponded to those of PrPc. In prion-related encephalopathies of humans and animals, the secretory form of PrPc might be converted into the abnormal isoform PrPSc and play a role in the dissemination of the disease process and amyloid formation.

Amyloid beta protein precursors with kunitz-type inhibitor domains and acetylcholinesterase in cerebrospinal fluid from patients with dementia of the Alzheimer type

We used the ELISA to measure the concentration of amyloid protein precursor with Kunitz type trypsin inhibitor domains (APPI) in CSF of dementia of the Alzheimer type (DAT) and examined the correlation of APPI with acetylcholinesterase (AChE) and somatostatin (SRIF). We found the APPI concentration in CSF of DAT to be significantly elevated compared with that of multi-infarct dementia and controls. We could significantly correlate APPI with AChE, but not correlate APPI with SRIF. The present results suggest that measurement of CSF APPI levels may be useful for diagnosis of DAT and the change of APPI may closely be associated with abnormality of acetylcholine system in DAT that has been reported.

Epitope map of two polyclonal antibodies that recognize amyloid lesions in patients with Alzheimer's disease

Two synthetic peptides with sequences identical with those of fragments of the extracellular domain of the Alzheimer's-disease amyloid precursor protein (APP) were used to raise antibodies. SP28 comprises positions 597-624 of the APP695 isoform, whereas SP41 extends towards the N-terminus (amino acids 584-624) and contains the entire SP28 peptide. Using e.l.i.s.a. and inhibition experiments we identified the two beta-turn-containing segments 602-607 and 617-624 as the epitopes recognized by anti-SP41 and anti-SP28 respectively. Both antibodies immunolabelled amyloid lesions in brains from Alzheimer's-disease patients and patients with related disorders, whereas they were unreactive in control brains. However, when probed on immunoblots, anti-SP28 failed to detect full-length APP from baculovirus-infected Sf9 cells, and anti-SP41 reacted weakly compared with other anti-APP antisera. The data suggest that these antibodies are directed to conformational epitopes not existent in the native molecules but present after alternative APP processing.

Decreased levels of soluble amyloid beta-protein precursor in cerebrospinal fluid of live Alzheimer disease patients

The amyloid beta-protein is deposited in senile plaques and the cerebrovasculature in Alzheimer disease (AD). Since it is derived from proteolytic processing of its parent protein, the amyloid beta-protein precursor (APP), we investigated whether levels of the secreted forms of APP are altered in cerebrospinal fluid (CSF) of AD patients. Quantitative immunoblotting studies with the anti-APP monoclonal antibody P2-1 revealed that probable AD patients had markedly lower CSF APP levels than did demented non-Alzheimer-type patients and healthy control subjects. Using antibody P2-1 in an enzyme-linked immunosorbent assay, we measured CSF levels of APP in a larger population consisting of 13 patients diagnosed with probable AD, 18 patients diagnosed with dementia (non-Alzheimer type), and 16 nondemented, healthy controls. Mean CSF levels of APP were approximately 3.5-fold lower in the live patients diagnosed with probable AD compared to the demented non-Alzheimer-type controls or the nondemented, healthy individuals. These findings suggest that abnormal metabolism of APP is reflected in the extracellular fluids of the central nervous system and that CSF levels of soluble APP provide a useful biochemical marker to assist in the clinical diagnosis of AD.

Immunohistochemical localization of beta-amyloid precursor protein sequences in Alzheimer and normal brain tissue by light and electron microscopy

Immunohistochemical staining with antibodies directed against four segments of the amyloid precursor protein (APP) was studied by light and electron microscopy in normal and Alzheimer (AD) brain tissue. The segments according to the Kang et al. sequence were: 18-38 (T97); 527-540 (R36); 597-620 (1-24 of beta-amyloid protein [BAP], R17); and 681-695 (R37) (Kang et al. [1987]: Nature 325:733-736). The antibodies recognized full length APP in Western blots of extracts of APP transfected cells. They stained cytoplasmic granules in some pyramidal neurons in normal appearing tissue from control and AD cases. In AD affected tissue, the antibodies to amino terminal sections of APP stained tangled neurons and neuropil threads, and intensely stained dystrophic neurites in senile plaques. By electron microscopy, this staining was localized to abnormal filaments. The antibody to the carboxy terminal segment failed to stain neurofibrillary tangles or neuropil threads; it did stain some neurites with globular swellings. It also stained globular and elongated deposits in senile plaque areas. The antibody against the BAP intensely stained extracellular material in senile plaques and diffuse deposits. By electron microscopy, the antibodies all stained intramicroglial deposits. Some of the extracellular and intracellular BAP-positive deposits were fibrillary. Communication between intramicroglial and extracellular fibrils was detected in plaque areas. These data suggest the following sequence of events. APP is normally concentrated in intraneuronal granules. In AD, it accumulates in damaged neuronal fibers. The amino terminal portion binds to abnormal neurofilaments. Major fragments of APP are phagocytosed and processed by microglia with the BAP portion being preserved. The preserved BAP is then extruded and accumulates in extracellular tissue.

Alzheimer's amyloid precursor protein accumulates within axonal swellings in human brain lesions

By using a monoclonal antibody and polyclonal antibodies to human beta amyloid precursor protein (APP), we immunohistochemically examined axonal swellings (spheroids). The immunoreactivity of APP was observed in numerous spheroids, which were found around the areas of old cerebral infarctions, in the globus pallidus with Hallervorden-Spatz disease as well as in or around brain abscesses. The axonal swellings were not immunolabeled with beta protein. These results implied that APP accumulated in spheroids regardless of beta protein depositions.

Distribution of Alzheimer's disease amyloid protein precursor in normal human and rat nervous system

Alzheimer's disease and cerebral amyloid angiopathies (CAA) are clinically heterogeneous diseases, but pathogenically related by the deposition of beta A4-amyloid in the brain in the form of neuritic plaques and/or vascular infiltrates. Antibodies directed against the N-terminal region of the predicted sequence of the beta A4 amyloid protein precursor (APP) were used to investigate the cellular distribution of this protein in the brain of normal humans and rats. We found a widespread presence of APP throughout the nervous tissue, including neurons, blood vessels, meningeal membranes, choroid plexus and ependymal cells. The highest APP immunoreactivity in both species was found in neuronal cell bodies and their processes, and around blood vessels. These findings may account for the clinical, pathological and aetiological differences found among the beta A4-amyloidosis.

A critical analysis of postulated pathogenetic mechanisms in amyloidogenesis

This review has examined several of the major thrusts in amyloid research, past and present. The data concerning amyloid precursor quantity, primary protein and gene structure, and precursor proteolysis have shown that there are contradictions that must be resolved before these elements can be reamalgamated into a unified view of amyloidogenesis. One possibility is presented in Figure 2. A general hypothesis of amyloid formation that accounts for the uniformity of fibril structure, amyloid staining properties, and the specific selection of precursors and their specific anatomic localization in each form of amyloid has yet to be proposed. Some of these questions may be answered by an analysis of common structural constituents in amyloid deposits. Analyzing amyloid generation in the context of these common elements separates amyloid research into several specific areas (Figure 2). The first area concerns factors that govern the expression of amyloid precursor protein genes, thus providing adequate quantities of the precursor, if such a precursor pool does not already exist. Without such a pool, amyloid deposition clearly cannot occur. The second area concerns information as to where these precursors usually bind and/or exert their normal function. Once determined, this information will likely indicate the site or sites where the particular precursor may give rise to amyloid deposits. The last area concerns factors at these local sites that govern the interaction of the precursor with basement membrane or related extracellular matrix elements that would define both the site and the final common pathway for amyloid deposition.

Beta A4 amyloid protein and its precursor in Alzheimer's disease

The beta A4 amyloid protein is now understood to play a pivotal role in the development of Alzheimer's disease. This protein is generated by the abnormal processing of the amyloid protein precursor, a large membrane glycoprotein. Insights into the mechanisms of this abnormal processing will give information relevant to the design of new therapeutic strategies for Alzheimer's disease.

Amyloid beta protein deposition in brains from elderly subjects with leukoaraiosis

We studied immunocytochemically with 2 amyloid beta protein (ABP) antisera brains from 5 non-demented elderly subjects with evidence of diffuse periventricular white matter hypodensity on computed tomography. In periventricular white matter of all brains we found ABP reactive deposits arranged around small vessels walls. Furthermore ABP reactive deposits, identical to those currently called diffuse plaques, were detected in neocortex in amount proportional to that of white matter deposits. We suggest that ABP microangiopathy and parenchymal deposition is responsible of white matter rarefaction in a subset of cases with a diffuse hypodensity on CT scan which has been called leukoaraiosis.

Peptides homologous to the amyloid protein of Alzheimer's disease containing a glutamine for glutamic acid substitution have accelerated amyloid fibril formation

beta-Amyloid (A beta) deposition in fibril form is the central event in a number of diseases, including Alzheimer's disease (AD) and hereditary cerebral hemorrhage with amyloidosis - Dutch type (HCHWA-D). A beta is produced by degradation of a larger amyloid precursor protein (APP). Recently a mutation in the APP gene has been found in HCHWA-D causing a glutamine for glutamic acid substitution at residue 22 of A beta. The influence of this mutation on fibrillogenesis is not known, although it is clear that affected patients have accelerated cerebrovascular amyloid deposition, with disease symptoms early in life. We report the in vitro demonstration of accelerated fibril formation in a 28 residue synthetic peptide homologous to the Dutch variant A beta. Furthermore, in eight residue peptides homologous to A beta the presence of the mutation is necessary for fibril formation. These findings provide a mechanism for accelerated amyloid formation in the Dutch variant of APP.

Alzheimer's disease: beta-amyloid precursor protein expression in plaques varies among cytoarchitectonic areas of the medial temporal lobe

The anatomic distributions of beta-amyloid peptide (beta AP) and beta-amyloid precursor protein (beta APP) in the medial temporal lobe were examined with immunocytochemistry in Alzheimer's disease. beta AP-containing plaques were found most frequently in the cortical and basal regions of the amygdala, and in the hippocampal CA1, subiculum, and dentate molecular layer. beta APP expression in plaques was found in a similar distribution, with some, but not all beta AP plaques also showing beta APP. In the cortical and basal amygdala, some cases showed beta APP in the centers of plaques, whereas in the hippocampus, all cases displayed beta APP mainly in plaque neurites. The lateral regions of the amygdala contained mainly diffuse beta AP plaques which had little beta APP. These findings suggest that although beta APP expression and beta AP deposition generally colocalize, processing of beta APP may vary among closely interconnected anatomic regions.

Aged non-human primates: an animal model of age-associated neurodegenerative disease

Aged non-human primates develop age-associated behavioral and brain abnormalities similar to those that occur in aged humans and, to a greater extent, in individuals with Alzheimer's disease. Declines in performance on cognitive and memory tasks begin at the monkey equivalent of late-middle life. As occurs in elderly humans, significant differences have been demonstrated in levels of performance between animals within older age groups. The brains of old monkeys show degenerative changes in neurons, abnormal axons and neurites (particularly in telencephalic areas), and deposits of amyloid in senile plaques and around blood vessels. Moreover, in some older animals, decrements occur in markers of specific neurotransmitter circuits, including the basal forebrain cholinergic system. It has been suggested that alterations in these cholinergic neurons contribute to the memory deficits that occur in older individuals. Because axotomy-induced retrograde degeneration of these neurons can be prevented by the administration of nerve growth factor, we have begun studies to determine whether administration of nerve growth factor improves performance of aged animals on memory tasks. This review describes the complementary nature of studies of non-human primates and human subjects, illustrating how these investigations can clarify factors that influence behavior and brain biology in age-associated diseases.

Analysis and quantitation of the beta-amyloid precursor protein in the cerebrospinal fluid of Alzheimer's disease patients with a monoclonal antibody-based immunoassay

One of the major clinical findings in Alzheimer's disease (AD) is the formation of deposits of beta-amyloid protein in amyloid plaques, derived from the beta-amyloid precursor protein (beta-APP). To determine the possible use of beta-APP as a diagnostic marker for AD in CSF, a monoclonal antibody-based immunoassay specific for this protein was developed. The assay does not differentiate between beta-APP695 and beta-APP751 forms but does preferentially recognize beta-APP751 complexed with a protease. Of the two sets of CSF samples tested, one set, obtained from living patients, gave a slightly lower level of beta-APP in AD and Parkinson's disease patients relative to controls, whereas the other set, composed of postmortem samples, showed no significant differences between the AD and control groups.

Accumulation of amyloid precursor fragment in Alzheimer plaques

Regenerative and degenerative neurites are components of classical senile plaques found in brain tissue of patients with Alzheimer's disease (AD). Amyloid beta/A4-protein derived from its precursor, amyloid beta/A4-protein precursor (APP/ABPP), constitutes the major portion of the amyloid core of senile plaques. A large N-terminal portion of APP (approximately Mr 100,000) is released from cells, leaving a minor C-terminal portion (approximately Mr 15,000) behind. A series of antisera against various sequences of APP were prepared and used to study the localization of each sequence in brain tissue. Plaque neurites stained as intensely as neuronal cell bodies with three antisera against the N-terminal portion of APP (N-terminal to a.a. 225), whereas five other antisera directed against the other C-terminal portions of APP (a.a. 284 to C-terminal) and antisera against the Kunitz-type protease inhibitor portion of APP stained plaque neurites less intensely than neuronal cell bodies in the hippocampus. These results suggest that a major part of the APP present in the neuritic component of senile plaques is a fragment representing the N-terminal one-third of the molecule.

Intact Alzheimer amyloid precursor protein (APP) is present in platelet membranes and is encoded by platelet mRNA

Using antibodies directed against N-terminal and C-terminal epitopes we have immunologically detected APP species in the membrane and saline-soluble fractions of unstimulated platelets, and in the conditioned medium of thrombin-stimulated platelets. These studies demonstrate an intact 140 kD membrane-associated form of APP that is released on degranulation. Evidence that platelets synthesize at least one form of APP (APP751) was obtained by enzymatic amplification of specific mRNA using Polymerase Chain Reaction (PCR) and direct sequence analysis of PCR product. Processing of APP for release may occur via successive C-terminal truncations, and/or by the release and proteolysis of an intact membrane associated form. An intact form of APP in platelets provides a circulating substrate upon which proteases from many tissues may act to produce beta protein (AB) during pathologic conditions.

Colocalization of amino terminal and A4 (beta-amyloid) antigens in Alzheimer plaques: evidence for coordinated processing of the amyloid precursor protein

The mechanism by which the A4 (beta-amyloid) domain of the Alzheimer amyloid precursor protein (APP) is deposited in plaques is unknown, and limited information is available concerning the extent to which other APP sites are associated with plaques. To address these issues, we prepared antiserum to a peptide adjacent to the N-terminus of the APP (referred to as N1) and examined its distribution in brain relative to A4 by double-immunostaining techniques. Anti-N1 localized to both neurons and glia in control and Alzheimer patients. In the Alzheimer brain, anti-N1 detected plaques. Quantitation revealed that 85% of thioflavin-positive plaques, and 91% of A4-positive plaques were also N1 positive. Double-staining methods directly demonstrated colocalization of distant APP sites. The data suggest that suggest that proposed mechanisms for amyloid deposition during plaque formation must take into account the extracytoplasmic domain, in addition to the A4 region, rather than be confined exclusively to the A4 site.

Cleavage of amyloid beta peptide during constitutive processing of its precursor

The amyloid beta peptide (A beta P) is a small fragment of the much larger, broadly distributed amyloid precursor protein (APP). Abundant A beta P deposition in the brains of patients with Alzheimer's disease suggests that altered APP processing may represent a key pathogenic event. Direct protein structural analyses showed that constitutive processing in human embryonic kidney 293 cells cleaves APP in the interior of the A beta P, thus preventing A beta P deposition. A deficiency of this processing event may ultimately prove to be the etiological event in Alzheimer's disease that gives rise to senile plaque formation.

Mutation of the Alzheimer's disease amyloid gene in hereditary cerebral hemorrhage, Dutch type

An amyloid protein that precipitates in the cerebral vessel walls of Dutch patients with hereditary cerebral hemorrhage with amyloidosis is similar to the amyloid protein in vessel walls and senile plaques in brains of patients with Alzheimer's disease, Down syndrome, and sporadic cerebral amyloid angiopathy. Cloning and sequencing of the two exons that encode the amyloid protein from two patients with this amyloidosis revealed a cytosine-to-guanine transversion, a mutation that caused a single amino acid substitution (glutamine instead of glutamic acid) at position 22 of the amyloid protein. The mutation may account for the deposition of this amyloid protein in the cerebral vessel walls of these patients, leading to cerebral hemorrhages and premature death.