A phase 2 multiple ascending dose trial of bapineuzumab in mild to moderate Alzheimer disease

BACKGROUND

Bapineuzumab, a humanized anti-amyloid-beta (Abeta) monoclonal antibody for the potential treatment of Alzheimer disease (AD), was evaluated in a multiple ascending dose, safety, and efficacy study in mild to moderate AD.

METHODS

The study enrolled 234 patients, randomly assigned to IV bapineuzumab or placebo in 4 dose cohorts (0.15, 0.5, 1.0, or 2.0 mg/kg). Patients received 6 infusions, 13 weeks apart, with final assessments at week 78. The prespecified primary efficacy analysis in the modified intent-to-treat population assumed linear decline and compared treatment differences within dose cohorts on the Alzheimer's Disease Assessment Scale-Cognitive and Disability Assessment for Dementia. Exploratory analyses combined dose cohorts and did not assume a specific pattern of decline.

RESULTS

No significant differences were found in the primary efficacy analysis. Exploratory analyses showed potential treatment differences (p < 0.05, unadjusted for multiple comparisons) on cognitive and functional endpoints in study "completers" and APOE epsilon4 noncarriers. Reversible vasogenic edema, detected on brain MRI in 12/124 (9.7%) bapineuzumab-treated patients, was more frequent in higher dose groups and APOE epsilon4 carriers. Six vasogenic edema patients were asymptomatic; 6 experienced transient symptoms.

CONCLUSIONS

Primary efficacy outcomes in this phase 2 trial were not significant. Potential treatment differences in the exploratory analyses support further investigation of bapineuzumab in phase 3 with special attention to APOE epsilon4 carrier status.

CLASSIFICATION OF EVIDENCE

Due to varying doses and a lack of statistical precision, this Class II ascending dose trial provides insufficient evidence to support or refute a benefit of bapineuzumab.

Peripherally administered antibodies against amyloid beta-peptide enter the central nervous system and reduce pathology in a mouse model of Alzheimer disease

One hallmark of Alzheimer disease is the accumulation of amyloid beta-peptide in the brain and its deposition as plaques. Mice transgenic for an amyloid beta precursor protein (APP) mini-gene driven by a platelet-derived (PD) growth factor promoter (PDAPP mice), which overexpress one of the disease-linked mutant forms of the human amyloid precursor protein, show many of the pathological features of Alzheimer disease, including extensive deposition of extracellular amyloid plaques, astrocytosis and neuritic dystrophy. Active immunization of PDAPP mice with human amyloid beta-peptide reduces plaque burden and its associated pathologies. Several hypotheses have been proposed regarding the mechanism of this response. Here we report that peripheral administration of antibodies against amyloid beta-peptide, was sufficient to reduce amyloid burden. Despite their relatively modest serum levels, the passively administered antibodies were able to enter the central nervous system, decorate plaques and induce clearance of preexisting amyloid. When examined in an ex vivo assay with sections of PDAPP or Alzheimer disease brain tissue, antibodies against amyloid beta-peptide triggered microglial cells to clear plaques through Fc receptor-mediated phagocytosis and subsequent peptide degradation. These results indicate that antibodies can cross the blood-brain barrier to act directly in the central nervous system and should be considered as a therapeutic approach for the treatment of Alzheimer disease and other neurological disorders.

Purification and cloning of amyloid precursor protein beta-secretase from human brain

Proteolytic processing of the amyloid precursor protein (APP) generates amyloid beta (Abeta) peptide, which is thought to be causal for the pathology and subsequent cognitive decline in Alzheimer's disease. Cleavage by beta-secretase at the amino terminus of the Abeta peptide sequence, between residues 671 and 672 of APP, leads to the generation and extracellular release of beta-cleaved soluble APP, and a corresponding cell-associated carboxy-terminal fragment. Cleavage of the C-terminal fragment by gamma-secretase(s) leads to the formation of Abeta. The pathogenic mutation K670M671-->N670L671 at the beta-secretase cleavage site in APP, which was discovered in a Swedish family with familial Alzheimer's disease, leads to increased beta-secretase cleavage of the mutant substrate. Here we describe a membrane-bound enzyme activity that cleaves full-length APP at the beta-secretase cleavage site, and find it to be the predominant beta-cleavage activity in human brain. We have purified this enzyme activity to homogeneity from human brain using a new substrate analogue inhibitor of the enzyme activity, and show that the purified enzyme has all the properties predicted for beta-secretase. Cloning and expression of the enzyme reveals that human brain beta-secretase is a new membrane-bound aspartic proteinase.

Cellular mechanisms of beta-amyloid production and secretion

The major constituent of senile plaques in Alzheimer's disease is a 42-aa peptide, referred to as beta-amyloid (Abeta). Abeta is generated from a family of differentially spliced, type-1 transmembrane domain (TM)-containing proteins, called APP, by endoproteolytic processing. The major, relatively ubiquitous pathway of APP metabolism in cell culture involves cleavage by alpha-secretase, which cleaves within the Abeta sequence, thus precluding Abeta formation and deposition. An alternate secretory pathway, enriched in neurons and brain, leads to cleavage of APP at the N terminus of the Abeta peptide by beta-secretase, thus generating a cell-associated beta-C-terminal fragment (beta-CTF). A pathogenic mutation at codons 670/671 in APP (APP "Swedish") leads to enhanced cleavage at the beta-secretase scissile bond and increased Abeta formation. An inhibitor of vacuolar ATPases, bafilomycin, selectively inhibits the action of beta-secretase in cell culture, suggesting a requirement for an acidic intracellular compartment for effective beta-secretase cleavage of APP. beta-CTF is cleaved in the TM domain by gamma-secretase(s), generating both Abeta 1-40 (90%) and Abeta 1-42 (10%). Pathogenic mutations in APP at codon 717 (APP "London") lead to an increased proportion of Abeta 1-42 being produced and secreted. Missense mutations in PS-1, localized to chromosome 14, are pathogenic in the majority of familial Alzheimer's pedigrees. These mutations also lead to increased production of Abeta 1-42 over Abeta 1-40. Knockout of PS-1 in transgenic animals leads to significant inhibition of production of both Abeta 1-40 and Abeta 1-42 in primary cultures, indicating that PS-1 expression is important for gamma-secretase cleavages. Peptide aldehyde inhibitors that block Abeta production by inhibiting gamma-secretase cleavage of beta-CTF have been discovered.

Immunization with amyloid-beta attenuates Alzheimer-disease-like pathology in the PDAPP mouse

Amyloid-beta peptide (Abeta) seems to have a central role in the neuropathology of Alzheimer's disease (AD). Familial forms of the disease have been linked to mutations in the amyloid precursor protein (APP) and the presenilin genes. Disease-linked mutations in these genes result in increased production of the 42-amino-acid form of the peptide (Abeta42), which is the predominant form found in the amyloid plaques of Alzheimer's disease. The PDAPP transgenic mouse, which overexpresses mutant human APP (in which the amino acid at position 717 is phenylalanine instead of the normal valine), progressively develops many of the neuropathological hallmarks of Alzheimer's disease in an age- and brain-region-dependent manner. In the present study, transgenic animals were immunized with Abeta42, either before the onset of AD-type neuropathologies (at 6 weeks of age) or at an older age (11 months), when amyloid-beta deposition and several of the subsequent neuropathological changes were well established. We report that immunization of the young animals essentially prevented the development of beta-amyloid-plaque formation, neuritic dystrophy and astrogliosis. Treatment of the older animals also markedly reduced the extent and progression of these AD-like neuropathologies. Our results raise the possibility that immunization with amyloid-beta may be effective in preventing and treating Alzheimer's disease.

High cerebrospinal fluid tau and low amyloid beta42 levels in the clinical diagnosis of Alzheimer disease and relation to apolipoprotein E genotype

OBJECTIVE

To evaluate cerebrospinal fluid (CSF) levels of amyloid beta protein ending at amino acid 42 (Abeta42) and tau as markers for Alzheimer disease (AD) and to determine whether clinical variables influence these levels.

DESIGN

Cohort study.

SETTING

Six academic research centers with expertise in dementia.

SUBJECTS

Eighty-two patients with probable AD, including 24 with very mild dementia (Mini-Mental State Examination score >23/30) (AD group); 60 cognitively normal elderly control subjects (NC group); and 74 subjects with neurological disorders, including dementia (ND group).

MAIN OUTCOME MEASURES

Levels of Abeta42 and tau were compared among AD, NC, and ND groups. Relationships of age, sex, Mini-Mental State Examination score, and apolipoprotein E (Apo E) genotype with these levels were examined using multiple linear regression. Classification tree models were developed to optimize distinguishing AD from NC groups.

RESULTS

Levels of Abeta42 were significantly lower, and levels of tau were significantly higher, in the AD group than in the NC or ND group. In the AD group, Abeta42 level was inversely associated with Apo E epsilon4 allele dose and weakly related to Mini-Mental State Examination score; tau level was associated with male sex and 1 Apo E epsilon4 allele. Classification tree analysis, comparing the AD and NC subjects, was 90% sensitive and 80% specific. With specificity set at greater than 90%, the tree was 77% sensitive for AD. This tree classified 26 of 74 members of the ND group as having AD. They had diagnoses difficult to distinguish from AD clinically and a high Apo E epsilon4 allele frequency. Markers in CSF were used to correctly classify 12 of 13 patients who later underwent autopsy, including 1 with AD not diagnosed clinically.

CONCLUSIONS

Levels of CSF Abeta42 decrease and levels of CSF tau increase in AD. Apolipoprotein E epsilon4 had a dose-dependent relationship with CSF levels of Abeta42, but not tau. Other covariates influenced CSF markers minimally. Combined analysis of CSF Abeta42 and tau levels discriminated patients with AD, including patients with mild dementia, from the NC group, supporting use of these proteins to identify AD and to distinguish early AD from aging. In subjects in the ND group with an AD CSF profile, autopsy follow-up will be required to decide whether CSF results are false positive, or whether AD is a primary or concomitant cause of dementia.

Estrogen therapy in postmenopausal women: effects on cognitive function and dementia

CONTEXT

Several studies have suggested that estrogen replacement therapy in postmenopausal women improves cognition, prevents development of dementia, and improves the severity of dementia, while other studies have not found a benefit of estrogen use.

OBJECTIVE

To determine whether postmenopausal estrogen therapy improves cognition, prevents development of dementia, or improves dementia severity.

DATA SOURCES

We performed a literature search of studies published from January 1966 through June 1997, using MEDLINE, manually searched bibliographies of articles identified, and consulted experts.

STUDY SELECTION

Studies that evaluated biological mechanisms of estrogen's effect on the central nervous system and studies that addressed the effect of estrogen on cognitive function or on dementia.

DATA EXTRACTION

We reviewed studies for methods, sources of bias, and outcomes and performed a meta-analysis of the 10 studies of postmenopausal estrogen use and risk of dementia using standard meta-analytic methods.

DATA SYNTHESIS

Biochemical and neurophysiologic studies suggest several mechanisms by which estrogen may affect cognition: promotion of cholinergic and serotonergic activity in specific brain regions, maintenance of neural circuitry, favorable lipoprotein alterations, and prevention of cerebral ischemia. Five observational studies and 8 trials have addressed the effect of estrogen on cognitive function in nondemented postmenopausal women. Cognition seems to improve in perimenopausal women, possibly because menopausal symptoms improve, but there is no clear benefit in asymptomatic women. Ten observational studies have measured the effect of postmenopausal estrogen use on risk of developing dementia. Meta-analysis of these studies suggests a 29% decreased risk of developing dementia among estrogen users, but the findings of the studies are heterogeneous. Four trials of estrogen therapy in women with Alzheimer disease have been conducted and have had primarily positive results, but most have been small, of short duration, non-randomized, and uncontrolled.

CONCLUSIONS

There are plausible biological mechanisms by which estrogen might lead to improved cognition, reduced risk for dementia, or improvement in the severity of dementia. Studies conducted in women, however, have substantial methodologic problems and have produced conflicting results. Large placebo-controlled trials are required to address estrogen's role in prevention and treatment of Alzheimer disease and other dementias. Given the known risks of estrogen therapy, we do not recommend estrogen for the prevention or treatment of Alzheimer disease or other dementias until adequate trials have been completed.

Amyloid precursor protein processing and A beta42 deposition in a transgenic mouse model of Alzheimer disease

The PDAPP transgenic mouse, which overexpresses human amyloid precursor protein (APP717V-->F), has been shown to develop much of the pathology associated with Alzheimer disease. In this report, levels of APP and its amyloidogenic metabolites were measured in brain regions of transgenic mice between 4 and 18 months of age. While absolute levels of APP expression likely contribute to the rate of amyloid beta-peptide (Abeta) deposition, regionally specific factors also seem important, as homozygotic mice express APP levels in pathologically unaffected regions in excess of that measured in certain amyloid plaque-prone regions of heterozygotic mice. Regional levels of APP and APP-beta were nearly constant at all ages, while A beta levels dramatically and predictably increased in brain regions undergoing histochemically confirmed amyloidosis, most notably in the cortex and hippocampus. In hippocampus, A beta concentrations increase 17-fold between the ages of 4 and 8 months, and by 18 months of age are over 500-fold that at 4 months, reaching an average level in excess of 20 nmol of A beta per g of tissue. A beta1-42 constitutes the vast majority of the depositing A beta species. The similarities observed between the PDAPP mouse and human Alzheimer disease with regard to A beta42 deposition occurring in a temporally and regionally specific fashion further validate the use of the model in understanding processes related to the disease.

Mutant presenilins of Alzheimer's disease increase production of 42-residue amyloid beta-protein in both transfected cells and transgenic mice

The mechanism by which mutations in the presenilin (PS) genes cause the most aggressive form of early-onset Alzheimer's disease (AD) is unknown, but fibroblasts from mutation carriers secrete increased levels of the amyloidogenic A beta 42 peptide, the main component of AD plaques. We established transfected cell and transgenic mouse models that coexpress human PS and amyloid beta-protein precursor (APP) genes and analyzed quantitatively the effects of PS expression on APP processing. In both models, expression of wild-type PS genes did not alter APP levels, alpha- and beta-secretase activity and A beta production. In the transfected cells, PS1 and PS2 mutations caused a highly significant increase in A beta 42 secretion in all mutant clones. Likewise, mutant but not wildtype PS1 transgenic mice showed significant overproduction of A beta 42 in the brain, and this effect was detectable as early as 2-4 months of age. Different PS mutations had differential effects on A beta generation. The extent of A beta 42 increase did not correlate with presenilin expression levels. Our data demonstrate that the presenilin mutations cause a dominant gain of function and may induce AD by enhancing A beta 42 production, thus promoting cerebral beta-amyloidosis.

Beta-secretase processing of the beta-amyloid precursor protein in transgenic mice is efficient in neurons but inefficient in astrocytes

Alzheimer's disease is characterized by the extracellular deposition of beta-amyloid peptide (Abeta) in cerebral plaques and evidence is accumulating that amyloid is neurotoxic. Abeta is derived from the beta-amyloid precursor protein (APP). Proteolytic processing of APP by the enzyme, beta-secretase, produces the N terminus of Abeta, and releases a secreted ectodomain of APP (beta-s-APP). To develop animal models for measuring beta-secretase activity in specific brain cells in vivo, we have targeted the expression of the full-length human APP to either neurons or astrocytes in transgenic mice using the neuron- specific enolase (NSE) promoter or a modified glial fibrillary acidic protein (GFAP) gene, respectively. The APP cDNAs expressed were mutated (KM to NL at 670/671) to encode amino acid substitutions that enhance amyloidogenic processing in vitro. Western analyses revealed abundant production of beta-s-APP in the brains of NSE-APP mice and enzyme-linked immunosorbent assay analyses showed production of Abeta in fetal primary mixed brain cultures and brain homogenates from these transgenic animals. Because the NSE promoter drives expression primarily in neurons, this provides in vivo evidence that the beta-secretase cleavage necessary for generation of beta-s-APP and Abeta is efficiently performed in neurons. In contrast, only little beta-s-APP was detected in brain homogenates of GFAP-APP mice, indicating that astrocytes show very little beta-secretase activity in vivo. This provides strong in vivo evidence that the major source of Abeta in brain is from neurons and not from astrocytes.

Production and increased detection of amyloid beta protein and amyloidogenic fragments in brain microvessels, meningeal vessels and choroid plexus in Alzheimer's disease

Recent advances indicate soluble amyloid beta (A beta) protein is produced constitutively during normal metabolism of the amyloid precursor protein (APP). This has not been directly examined in human brain vascular tissues. Using a panel of well-characterized antibodies, here we show that increased amounts of soluble A beta were found in isolated vascular tissues from AD subjects compared to age-matched controls without significant Alzheimer pathology. Immunocytochemical analyses of isolated vessel preparations showed characteristic transverse patterns of A beta deposits in large vessels with smooth muscle, however, fine A beta deposits were apparent even in capillaries. A proportion of such A beta protein and potentially amyloidogenic carboxyl terminal fragments were released by solubilization and disruption of the vascular basement membrane by collagenase treatments. We further demonstrated by in vitro metabolic labelling that soluble A beta or an A beta-like peptide is associated and produced by cerebral microvessels, meningeal vessels and the choroid plexus isolated postmortem from human as well as rat brain. Compared to those from young rats, cerebral microvessels from aging rats showed increased release of carboxyl terminal fragments of APP and A beta-like peptide. Our observations provide the first direct demonstration that human vascular tissues produce soluble A beta, a product of the secretory pathway in APP processing. Our findings also suggest that aging associated alterations in the basement membranes are a factor in A beta accumulation that results in vascular amyloid deposition, the principal feature of cerebral amyloid angiopathy.

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).

Detection of phosphorylated Ser262 in fetal tau, adult tau, and paired helical filament tau

Paired helical filaments (PHFs) are the major structural elements of Alzheimer's disease neurofibrillary lesions, and these filaments are formed from hyperphosphorylated brain tau known as PHF-tau. Recent studies showed that many previously identified phosphorylated residues in PHF-tau also are phosphate acceptor sites in fetal and rapidly processed adult brain tau. However, Ser262 has been suggested to be uniquely phosphorylated in PHF-tau and a key regulator of the binding of tau to microtubules. For these reasons, we generated a monoclonal antibody (12E8) specific for phosphorylated Ser262 and showed that 12E8 binds to PHF-tau, rat and human fetal brain tau, as well as to rapidly processed adult rat and biopsy-derived human brain tau. Further, phosphorylation Ser262 was developmentally regulated, and endogenous brain phosphatases rapidly dephosphorylated Ser262 in biopsy-derived brain tau isolates. Finally, the phosphorylation of Ser262 did not eliminate the binding of tau to microtubules. Thus, we speculate that the binding of tau to microtubules is regulated by phosphorylation at multiple sites and that the generation of PHF-tau in Alzheimer's disease results from the reduced efficiency of phosphatases leading to the incremental accumulation of hyperphosphorylated tau.

Chronic elevation of secreted amyloid precursor protein in subcortically lesioned rats, and its exacerbation in aged rats

Subcortically lesioned rats were used as an animal model of some of the neurochemical and behavioral deficits of Alzheimer's disease (AD) to investigate the in vivo expression and metabolism of amyloid precursor protein (APP). Previously, the rapid and persistent induction of APP was described in cerebral cortices after disruption of its cholinergic, serotonergic, or noradrenergic afferents. In the present study, this induction was found to lead to the elevated secretion of APP into the cerebrospinal fluid of lesioned animals. Lesions of the forebrain cholinergic system in aged rats caused an even greater increase in the CSF levels of secreted APP. Antibodies to the extracellular domain of APP detected the protein whereas antibodies to the cytoplasmic region did not, indicating that the APP present in CSF was of the soluble form. Immunoprecipitation with an A beta sequence-specific antibody followed by immunoblot analysis indicated that a significant portion of secreted APP was of the species that contains at least the first 28 amino acids of the A beta sequence (APP gamma or APPA beta). By contrast, very low levels of A beta peptide were detected in CSF. The secretion was accompanied by an elevation of cellular C-terminal fragments of the APP in the lesioned cortex. Consistent with our previous results, this increased APP secretion was caused by lesions of subcortical cholinergic and serotonergic systems. The postlesion time course of APP secretion showed an initial reduction of APP (1 hr postlesion) in CSF followed by an eventual twofold elevation 1-6 weeks later. These results indicate that the induction of APP in response to loss of subcortical innervation leads to elevated secretion of a soluble form of cortically derived APP that contains significant portions of the A beta sequence.

Cerebrospinal fluid levels of amyloid beta-protein in Alzheimer's disease: inverse correlation with severity of dementia and effect of apolipoprotein E genotype

Alzheimer's disease (AD) is characterized by formation in brain of neurofibrillary tangles and of amyloid deposits. The major protein component of the former is tau, while the latter are composed of amyloid beta-peptides (A beta), which are derived by proteolytic cleavage of the amyloid beta-protein precursor (APP). Both tau and various secretory APP derivatives including A beta and APPS are present in human cerebrospinal fluid (CSF). To investigate whether clinical signs of AD are paralleled by changes in CSF levels of these proteins, we correlated quantitative measures of dementia severity with CSF concentrations of A beta, of APPS, and of tau. We found that levels of A beta in CSF of AD patients were inversely correlated both to cognitive and to functional measures of dementia severity. In contrast, levels of APPS and of tau did not correlate with dementia severity. Apolipoprotein E (apoE) genotype did not influence CSF levels of A beta, APPS, or tau, which were similar among AD patients with Apo E epsilon 3/3, epsilon 3/4, and epsilon 4/4 alleles. These data indicate that CSF levels of A beta decrease with advancing severity of dementia in AD and suggest that they are independent of a patient's Apo E genotype.

Elevation of microtubule-associated protein tau in the cerebrospinal fluid of patients with Alzheimer's disease

Currently, there is no biochemical marker clinically available to test for the presence of Alzheimer's disease (AD). Recent studies suggest that the core component of AD-associated neurofibrillary tangles (NFTs), the microtubule-associated protein tau, might be present in CSF. This study focuses on establishing both the presence of tau in CSF and its potential utility in the diagnosis of AD. We obtained CSF from 181 individuals; 71 of these were diagnosed as having probable AD by NINCDS-ADRDA criteria. The remaining 110 individuals were divided into three groups: (1) age-matched demented non-AD patients (n = 25), (2) neurologic controls (n = 59), and (3) other controls (n = 26). We developed a sensitive enzyme-linked immunosorbent tau assay using monoclonal antibodies prepared against recombinant human tau. We confirmed specificity of the antibodies by a combination of immunoprecipitation and immunoblot results. By this assay we measured that the AD population has a mean level of tau 50% greater than the non-AD dementia patients. Comparing AD patients with all other groups, the difference in tau levels as analyzed by one-way ANOVA is highly statistically significant (p < 0.001). Postmortem analysis of two AD patients with high levels of CSF tau revealed a high density of NFTs in the hippocampus. There was no significant correlation between tau and age in the non-AD groups. This study suggests that CSF tau is elevated in AD and might be a useful aid in antemortem diagnosis.

Channel formation in planar lipid bilayers by a neurotoxic fragment of the beta-amyloid peptide

Alzheimer's disease (AD) pathology is characterized by plaques, tangles, and neuronal cell loss. The main constituent of plaques is beta-amyloid peptide (A beta), a 39-42 residue peptide which has been linked to disruption of calcium homeostasis and neurotoxicity in vitro. We demonstrate that a neurotoxic fragment of A beta, A beta (25-35) spontaneously inserted into planar lipid membranes to form weakly selective, voltage dependent, ion-permeable channels. We suggest that channel formation may be involved in the pathogenesis of AD and that A beta (25-35) may be the active channel forming segment.

Identification of secreted beta-amyloid precursor protein binding sites on intact human fibroblasts

Based upon recent evidence that the secreted form of APP can cause the release of cytokines and elicit other biological activities, we sought to identify whether a receptor could be identified on the surface of cells. The secreted amyloid precursor protein containing the Kunitz domain (scAPP751) is identical to protease nexin II, a protease inhibitor which has been shown to form complexes with labeled EGF binding protein that subsequently binds to cells. Results of [125I]scAPP751-trypsin complex incubated with intact fibroblast cells show that the complex appears to bind in a saturable time-dependent and reversible manner. The kinetic constants from the binding studies demonstrate a k1 = 2.5 x 10(7) M-1 s-1 and k2 = 4.7 x 10(-4) s-1 and thus a KD (= k2/k1) = 20 pM. Furthermore, the complex formation of [125I]scAPP751 with a protease appears to be a requirement for optimal binding. The binding affinity of secreted APP demonstrated in this study is consistent with its potency in eliminating a range of biological efforts that have been documented.

Secondary structure of amyloid beta peptide correlates with neurotoxic activity in vitro

Amyloid beta peptide (A beta), the major protein constituent of senile plaques in patients with Alzheimer's disease, is believed to facilitate the progressive neurodegeneration that occurs in the latter stages of this disease. Early attempts to characterize the structure-activity relationship of A beta toxicity in vitro were compromised by the inability to reproducibly elicit A beta-dependent toxicity across different lots of chemically equivalent peptides. In this study we used CD spectroscopy to demonstrate that A beta secondary structure is an important determinant of A beta toxicity. Solubilized A beta was maximally toxic when the peptide adopted a beta-sheet conformation. Three of the four A beta lots tested had a random coil conformation and were weakly toxic or inactive, whereas the single A beta lot exhibiting toxic activity at low peptide concentrations had significant beta-sheet structure. Incubation of the weakly toxic A beta lots in aqueous stock solutions for several days before use induced a time-dependent conformational transition from random coil to beta-sheet and increased A beta toxicity in three different toxicity assays. Furthermore, the secondary structure of preincubated A beta was dependent upon peptide concentration and pH, so that beta-sheet structures were attenuated when peptide solutions were diluted or buffered at neutral and basic pH. Our data could explain some of the variable toxic activity that has been associated with A beta in the past and provide additional support for the hypothesis that A beta can have a causal role in the molecular neuropathology of Alzheimer's disease.

Serine phosphorylation of the secreted extracellular domain of APP

The phosphorylation status of full-length APP (FL-APP) and secreted APP (s-APP) was investigated in stably transfected cells. 32P incorporation was detected in the mature full-length APP both in the absence and presence of phorbol ester. Surprisingly, 32P-phosphate was incorporated in the secreted ectodomain, and this was stable to treatment of the [32P]-phospho-s-APP with a large excess of PNGase F, suggesting that N-linked oligosaccharide sites do not account for phosphate incorporation. Phosphoamino acid analysis of the [32P]-phospho-s-APP resulted in the recovery of [32P]-phosphoserine as the preponderant species. Brefeldin A completely inhibited the release of [32P]-phospho s-APP, but did not inhibit the incorporation of 32P into the FL-APP, suggesting that phosphorylation occurs early in the central vacuolar pathway. It is possible that ectodomain phosphorylation by a novel luminal or extracellular protein kinase may play a role in regulating the metabolic fate of APP.

Regulated cleavage of Alzheimer beta-amyloid precursor protein in the absence of the cytoplasmic tail

Alzheimer beta-amyloid precursor protein can be phosphorylated on residues Thr654, Ser655 and Thr668 on its cytoplasmic domain. Proteolytic cleavage of the amyloid precursor protein and release of the amyloid precursor protein ectodomain into the medium of cultured cells can be activated by phorbol esters which stimulate protein kinase C. In the present study, using mutated amyloid precursor protein, we show that phosphorylation of cytoplasmic residues is not required for the phorbol ester-activated cleavage and release of the amyloid precursor protein ectodomain. Remarkably, deletion of the entire amyloid precursor protein cytoplasmic tail had no effect on the phorbol ester-activated cleavage/release. The results indicate that activation of amyloid precursor protein cleavage/release by protein kinase C involves phosphorylation of some component of the processing pathway, instead of or in addition to the cytoplasmic tail of the amyloid precursor protein.

Characterization of beta-amyloid peptide from human cerebrospinal fluid

beta-Amyloid peptide (A beta) is one of the main components of senile plaques in the brain tissue of Alzheimer's disease (AD) patients. A beta is proteolytically cleaved from the amyloid precursor protein (APP), an integral membrane protein possessing a large extracellular N-terminal domain followed by a single membrane-spanning region and a short cytoplasmic C-terminal tail. A beta has been isolated from senile plaques and cerebral vascular tissue of AD brain and characterized as a heterogeneous peptide containing 28-43 amino acids whose sequence begins in the extracellular domain of APP and extends into the putative transmembrane sequence. It has long been speculated that A beta may also be present in body fluids, such as CSF, that contact neuritic plaques. Recently using a specific enzyme-linked immunosorbent assay we were able to quantify one form of A beta in CSF. In this report, using one of these antibodies covalently bound as an affinity matrix, multiple complex forms of A beta have been isolated and characterized from CSF derived from patients with either meningitis or other neurological disorders. Amino acid sequencing reveals A beta species with N-termini of Asp1, Glu3, His6, Glu11, and Val12, although on a molar basis, Asp1 represents the predominant aminoterminus. Laser desorption mass spectrometry confirmed the presence in CSF of A beta species containing 27, 28, 30, 34, 35, 40, 42, and 43 amino acids, all beginning at Asp1; two stable trimers, (Asp1-Met35)3 and (His6-Ala42)3; and one stable dimer containing (Asp1-Val40)2.(ABSTRACT TRUNCATED AT 250 WORDS)

beta-Amyloid precursor protein metabolites and loss of neuronal Ca2+ homeostasis in Alzheimer's disease

Recent findings link altered processing of beta-amyloid precursor protein (beta APP) to disruption of neuronal Ca2+ homeostasis and an excitotoxic mechanism of cell death in Alzheimer's disease. A major pathway of beta APP metabolism results in the release of secreted forms of beta APP, APPss. These secreted forms are released in response to electrical activity and can modulate neuronal responses to glutamate, suggesting roles in developmental and synaptic plasticity. beta APP is upregulated in response to neural injury and APPss can protect neurons against excitotoxic or ischemic insults by stabilizing the intracellular Ca2+ concentration [Ca2+]i. An alternative beta APP processing pathway liberates intact beta-amyloid peptide, which can form aggregates that disrupt Ca2+ homeostasis and render neurons vulnerable to metabolic or excitotoxic insults. Genetic abnormalities (e.g. certain beta APP mutations or Down syndrome) and age-related changes in brain metabolism (e.g. reduced energy availability or increased oxidative stress) may favor accumulation of [Ca2+]i-destabilizing beta-amyloid peptide and diminish the release of [Ca2+]i-stabilizing, neuroprotective APPss.

Cells with a familial Alzheimer's disease mutation produce authentic beta-peptide

Cells overexpressing the beta-amyloid precursor protein possessing a mutation found in familial Alzheimer's disease overproduce beta-amyloid peptide (A beta). Because these findings were based on immunological identification, we have chemically characterized the peptides produced. Purified A beta fragments from the conditioned media of these cells were found to have N-terminal sequence consistent with the A beta found in cerebral plaques. Mass spectrometric data demonstrated a series of A beta fragments consistent with those found in Alzheimer's disease (AD); the major species corresponding to A beta(1-40). Significantly, a longer fragment corresponding to A beta(1-42) was found. These findings suggest that this cellular system may be useful for mechanistic studies of A beta generation and possibly for the development of therapeutic agents to treat AD.

Altered calcium signaling and neuronal injury: stroke and Alzheimer's disease as examples

Several cellular signaling systems have been implicated in the neuronal death that occurs both in development ("natural" cell death) or in pathological conditions such as stroke and Alzheimer's disease (AD). Here we consider the possibility that neuronal degeneration in an array of disorders including stroke and AD arises from one or more alterations in calcium-regulating systems that result in a loss of cellular calcium homeostasis. A long-standing hypothesis of neuronal injury, the excitatory amino acid (EAA) hypothesis, is revisited in light of new supportive data concerning the roles of EAAs in stroke and the neurofibrillary degeneration in AD. Two quite new concepts concerning mechanisms of neuronal injury and death are presented, namely: 1) growth factors normally "stabilize" intracellular free calcium levels ([Ca2+]i) and protect neurons against ischemic/excitotoxic injury, and 2) aberrant processing of beta-amyloid precursor protein (APP) can cause neurodegeneration by impairing a neuroprotective function of secreted forms of APP (APPs) which normally regulate [Ca2+]i. Altered APP processing also results in the accumulation of beta-amyloid peptide which contributes to neuronal damage by destabilizing calcium homeostasis; in AD beta-amyloid peptide may render neurons vulnerable to excitotoxic conditions that accrue with increasing age (e.g., altered glucose metabolism, ischemia). Growth factors may normally protect neurons against the potentially damaging effects of calcium influx resulting from energy deprivation and overexcitation. For example, bFGF, NGF and IGFs can protect neurons from several brain regions against excitotoxic/ischemic insults. Growth factors apparently stabilize [Ca2+]i by several means including: a reduction in calcium influx; enhanced calcium extrusion or buffering; and maintenance or improvement of mitochondrial function. For example, bFGF can suppress the expression of a N-methyl-D-aspartate (NMDA) receptor protein that mediates excitotoxic damage in hippocampal neurons. Growth factors may also prevent the loss of neuronal calcium homeostasis and the increased vulnerability to neuronal injury caused by beta-amyloid peptide. Since elevated [Ca2+]i can elicit cytoskeletal alterations similar to those seen in AD neurofibrillary tangles, we propose that neuronal damage in AD results from a loss of calcium homeostasis. The data indicate that a variety of alterations in [Ca2+]i regulation may contribute to the neuronal damage in stroke and AD, and suggest possible means of preventing neuronal damage in these disorders.

Evidence for excitoprotective and intraneuronal calcium-regulating roles for secreted forms of the beta-amyloid precursor protein

The beta-amyloid precursor protein (beta APP) is a membrane-spanning glycoprotein that is the source of the beta-amyloid peptide (beta AP) which accumulates as senile plaques in the brains of patients with Alzheimer's disease. beta APP is normally processed such that a cleavage occurs within the beta AP, liberating secreted forms of beta APP (APPss) from the cell. The neuronal functions of these forms are unknown. We now report that APPss have a potent neuroprotective action in cultured rat hippocampal and septal neurons and in human cortical neurons. APPs695 and APPs751 protected neurons against hypoglycemic damage, and the neuroprotection was abolished by antibodies to a specific region common to both APPs695 and APPs751. APPss caused a rapid and prolonged reduction in [Ca2+]i and prevented the rise in [Ca2+]i that normally mediated hypoglycemic damage. APPss also protected neurons against glutamate neurotoxicity, effectively raising the excitotoxic threshold. APPss may normally play excitoprotective and neuromodulatory roles. Alternative processing of APPss in Alzheimer's disease may contribute to neuronal degeneration by compromising the normal function of APPss and by promoting the deposition of beta AP.

Microinjection of synthetic amyloid beta-protein in monkey cerebral cortex fails to produce acute neurotoxicity

The cerebral deposition of amyloid beta protein (A beta P) is an early pathogenetic event in Alzheimer's disease (AD). Recent studies suggest both neurotoxic and neurotrophic effects of A beta P in vitro. Because progressive A beta P deposition and surrounding neuritic dystrophy occur spontaneously in primates, we evaluated the in vivo effects of synthetic A beta P in monkey cortex. Experimental and control (reverse or substituted) peptides were stereotactically injected into multiple neocortical sites of adult rhesus monkeys in a vehicle of either artificial cerebrospinal fluid or acetonitrile. After 2 weeks, all injection sites were identified and characterized. A beta P antibodies specifically detected the injected A beta P1-40 peptide. Serial sections stained with silver and antineurofilament protein demonstrated comparable degrees of degenerating neurons, dystrophic neurites, and axonal spheroids associated with both experimental and control peptide injections. Alz 50 staining was sparse or absent in all sites. Similar results were obtained in an animal killed 3 months after injection. We conclude that specific cellular changes closely resembling the pathology of Alzheimer's disease were not detected in these acute experiments, and that control and experimental A beta P peptides produced indistinguishable effects.

Mutation of the beta-amyloid precursor protein in familial Alzheimer's disease increases beta-protein production

Progressive cerebral deposition of the 39-43-amino-acid amyloid beta-protein (A beta) is an invariant feature of Alzheimer's disease which precedes symptoms of dementia by years or decades. The only specific molecular defects that cause Alzheimer's disease which have been identified so far are missense mutations in the gene encoding the beta-amyloid precursor protein (beta-APP) in certain families with an autosomal dominant form of the disease (familial Alzheimer's disease, or FAD). These mutations are located within or immediately flanking the A beta region of beta-APP, but the mechanism by which they cause the pathological phenotype of early and accelerated A beta deposition is unknown. Here we report that cultured cells which express a beta-APP complementary DNA bearing a double mutation (Lys to Asn at residue 595 plus Met to Leu at position 596) found in a Swedish FAD family produce approximately 6-8-fold more A beta than cells expressing normal beta-APP. The Met 596 to Leu mutation is principally responsible for the increase. These data establish a direct link between a FAD genotype and the clinicopathological phenotype. Further, they confirm the relevance of the continuous A beta production by cultured cells for elucidating the fundamental mechanism of Alzheimer's disease.

Beta/A4 domain of APP: antigenic differences between cell lines

The expression of amyloid precursor protein (APP) in olfactory neuroblasts has been examined with a panel of antibodies directed against varied regions of the APP molecule. The pattern of reactivity was compared to that in the transformed human glial cell line SVG, human cortical brain tissue, and in kidney epithelial 293 cells containing stably transfected and overexpressed human APP751. Antibodies directed against the C-terminus and extracellular domains of amyloid precursor protein (APP) react more strongly on immunoblot with transfected 293 cells and brain tissue than with olfactory neuroblasts (ON) or SVG cells. Antibodies directed against the beta/A4 region of APP show a contrasting pattern of reactivity, yielding greater reactivity with ON and SVG cells than with transfected 293 cells or brain tissue. Analysis of the APP transcripts using polymerase chain reaction indicates that ON and SVG both make predominantly APP770 and 751, as does the transfected 293 cell line. In the absence of any differences in APP transcripts among the cell lines, the difference in availability of the beta/A4 region appears likely to be due to posttranslational modification. These data therefore indicate that processing of APP varies among cell lines and thus may vary from tissue to tissue.

Amyloid beta-peptide is produced by cultured cells during normal metabolism

Alzheimer's disease is characterized by the extracellular deposition in the brain and its blood vessels of insoluble aggregates of the amyloid beta-peptide (A beta), a fragment, of about 40 amino acids in length, of the integral membrane protein beta-amyloid precursor protein (beta-APP). The mechanism of extracellular accumulation of A beta in brain is unknown and no simple in vitro or in vivo model systems that produce extracellular A beta have been described. We report here the unexpected identification of the 4K (M(r) 4,000) A beta and a truncated form of A beta (approximately 3K) in media from cultures of primary cells and untransfected and beta-APP-transfected cell lines grown under normal conditions. These peptides were immunoprecipitated readily from culture medium by A beta-specific antibodies and their identities confirmed by sequencing. The concept that pathological processes are responsible for the production of A beta must not be reassessed in light of the observation that A beta is produced in soluble form in vitro and in vivo during normal cellular metabolism. Further, these findings provide the basis for using simple cell culture systems to identify drugs that block the formation or release of A beta, the primary protein constituent of the senile plaques of Alzheimer's disease.

Lack of Alzheimer pathology after beta-amyloid protein injections in rat brain

In order to establish a direct relationship between beta-amyloid protein (beta AP) and in vivo neurotoxicity, we made intraparenchymal injections and Alzet pump infusions of beta AP into the hippocampus and cortex of adult rats. We tested a number of synthetic beta AP peptides (beta AP 1-40, 1-38, and 25-35) and peptide controls (scrambled and reversed 1-40, and scrambled and reversed 25-35) over a wide range of concentrations and in a variety of vehicles. The rats were sacrificed from 2-35 days following the implant, and the brains examined by standard immunohistochemical and histological methods used to evaluate the pathologies associated with Alzheimer's disease. We report the lack of Alzheimer related pathology and no significant morphological differences between the beta AP peptide and the peptide and vehicle control injections. These observations indicate that the simple intraparenchymal injection of beta AP in the rat brain is not an appropriate model of Alzheimer-related neurotoxicity.

Synthetic amyloid beta-protein fails to produce specific neurotoxicity in monkey cerebral cortex

Because progressive amyloid beta-protein (A beta P) deposition and surrounding neuritic dystrophy occur spontaneously in primates, we evaluated the in vivo effects of synthetic A beta P in monkey cortex. Experimental and control A beta P were stereotactically injected into multiple neocortical sites of adult rhesus monkeys in a vehicle of either artificial cerebrospinal fluid or acetonitrile. After 2 weeks or 3 months, injection sites were identified and characterized histologically and immunocytochemically. A beta P antibodies specifically detected the injected A beta P1-40 peptide. Serial sections stained with silver and antineurofilament protein demonstrated comparable degrees of degenerating neurons, dystrophic neurites, and axonal spheroids associated with both experimental and control peptide injections. Alz 50 staining was sparse or absent in all sites. We conclude that specific cellular changes closely resembling AD pathology were not detected in these experiments, and that control and experimental A beta P peptides produced indistinguishable effects. Methodological concerns regarding the in vivo modeling of A beta P bioactivity are discussed.

Evidence for a nonsecretory, acidic degradation pathway for amyloid precursor protein in 293 cells. Identification of a novel, 22-kDa, beta-peptide-containing intermediate

We have analyzed the metabolic pathway of maturation of APP751 in stably transfected 293 cells, in the presence of either of the cysteine protease inhibitors leupeptin or E-64. Metabolic labeling, followed by immunoprecipitation at various times in the chase with a rabbit polyclonal antibody (anti-BX6) specific to the carboxyl-terminal end of amyloid precursor protein (APP), revealed the accumulation of a novel approximately 22-kDa carboxyl-terminal fragment (22-CTF) in the inhibitor-treated cells. This fragment, which was not detectable in untreated cells, was immunoprecipitated by four separate antibodies to the carboxyl-terminal region of APP as well as by polyclonal and monoclonal antibodies specific to the first 16 amino acids of the beta-peptide domain. Antibodies to the amino-terminal end of APP do not, however, recognize the fragment. Co-treatment of the inhibitor-treated cells with either of the lysosomotropic agents chloroquine or ammonium chloride completely blocked the generation of this fragment but did not significantly affect APP maturation or secretion. All, however, slowed the intracellular turnover of the cell-associated, approximately 9-kDa carboxyl-terminal fragment (c-CTF) produced during constitutive secretion. Densitometric analyses of these results suggest that this non-secretory pathway of APP degradation, mediated by cysteine proteases in an intracellular acidic compartment, accounts for approximately 70% of total APP metabolism and that a key processing intermediate in this pathway is a 22-kDa, beta-peptide-containing APP carboxyl-terminal fragment. It is possible that inefficient degradation of such an intermediate leads to the formation of aggregating beta-peptide.

Detection of distinct isoform patterns of the beta-amyloid precursor protein in human platelets and lymphocytes

Cerebral deposition of the amyloid beta-protein (A beta P), approximately 40 residue fragment of the integral membrane protein, beta-amyloid precursor protein (beta APP), has been implicated as the probable cause of some cases of familial Alzheimer's disease (AD). The parallels between A beta P deposition in AD and the deposition of certain plasma proteins in systemic amyloid diseases has heightened interest in the analysis of beta APP in circulating cells and plasma. Here, we describe distinct isoform patterns of beta APP in peripheral platelets and lymphocytes. PCR-mediated amplification of mRNA from purified platelets demonstrated the expression of all three major beta APP transcripts (beta APP770,751,695). The full-length, approximately 140 kDa form of beta APP751,770 was detected in membranes of resting and activated platelets but very little immature, approximately 122 kDa beta APP751,770 was found, suggesting a different processing of beta APP in platelets than that described in a variety of cultured cells and tissues. Platelets stimulated with thrombin, calcium ionophore, or collagen released the soluble, carboxyl-truncated form of beta APP (protease nexin-II), but no evidence for the shedding of full-length beta APP associated with platelet microparticles was found, in contrast to previous reports. As a positive control marker for microparticles, the fibrinogen receptor subunit, GPIIIa, was readily detected in platelet releasates. Resting and activated platelets contained similar amounts of the approximately 10 kDa carboxyl terminal beta APP fragment that is retained in platelet membranes following the constitutive cleavage of protease nexin-II. Nonstimulated peripheral B and T lymphocytes contained small amounts of membrane-associated mature and immature beta APP751,770. The potentially amyloidogenic full-length beta APP molecules present in circulating platelets and lymphocytes but not in microparticles could serve as a source of the microvascular A beta P deposited during aging and particularly in AD.

Identification of a stable fragment of the Alzheimer amyloid precursor containing the beta-protein in brain microvessels

Altered proteolysis of the beta-amyloid precursor protein (beta APP) resulting in release of the approximately 40-residue amyloid beta-protein (A beta P) may be a seminal pathogenetic event in Alzheimer disease. Using region-specific beta APP antibodies, we searched for stable proteolytic intermediates containing the intact A beta P region in brain tissue. A 22-kDa beta APP fragment was selectively detected in microvessels purified from cerebral cortex and other brain regions. On immunoblots, the 22-kDa band is labeled by five distinct antisera to beta APP carboxyl-terminal peptides and by affinity-purified antibodies to the recombinant proteins beta APP444-592 and beta APP592-695, which flank the A beta P region. The protein is virtually undetectable in whole-brain homogenates or microvessel-free fractions of brain. The protein is extractable from microvessels in Triton X-100 and other detergents, indicating its membrane association. In comparison with cortical microvessels, microvessels purified from white matter, cerebellum, and nonneural tissues contain lower amounts of the 22-kDa protein. The protein is found in microvessels of both normal and Alzheimer disease brains and occurs in low amounts in microvessels from fresh bovine brain. The size and specific immunoreactivity of the 22-kDa protein indicate that it is a stable fragment of beta APP containing the intact A beta P. The occurrence of this potentially amyloidogenic intermediate in microvessels is consistent with a vascular or hematogenous origin for some A beta P deposits in Alzheimer disease.

beta-Amyloid peptides destabilize calcium homeostasis and render human cortical neurons vulnerable to excitotoxicity

In Alzheimer's disease (AD), abnormal accumulations of beta-amyloid are present in the brain and degenerating neurons exhibit cytoskeletal aberrations (neurofibrillary tangles). Roles for beta-amyloid in the neuronal degeneration of AD have been suggested based on recent data obtained in rodent studies demonstrating neurotoxic actions of beta-amyloid. However, the cellular mechanism of action of beta-amyloid is unknown, and there is no direct information concerning the biological activity of beta-amyloid in human neurons. We now report on experiments in human cerebral cortical cell cultures that tested the hypothesis that beta-amyloid can destabilize neuronal calcium regulation and render neurons more vulnerable to environmental stimuli that elevate intracellular calcium levels. Synthetic beta-amyloid peptides (beta APs) corresponding to amino acids 1-38 or 25-35 of the beta-amyloid protein enhanced glutamate neurotoxicity in cortical cultures, while a peptide with a scrambled sequence was without effect. beta APs alone had no effect on neuronal survival during a 4 d exposure period. beta APs enhanced both kainate and NMDA neurotoxicity, indicating that the effect was not specific for a particular subtype of glutamate receptor. The effects of beta APs on excitatory amino acid (EAA)-induced neuronal degeneration were concentration dependent and required prolonged (days) exposures. The beta APs also rendered neurons more vulnerable to calcium ionophore neurotoxicity, indicating that beta APs compromised the ability of the neurons to reduce intracellular calcium levels to normal limits. Direct measurements of intracellular calcium levels demonstrated that beta APs elevated rest levels of calcium and enhanced calcium responses to EAAs and calcium ionophore. The neurotoxicity caused by EAAs and potentiated by beta APs was dependent upon calcium influx since it did not occur in calcium-deficient culture medium. Finally, the beta APs made neurons more vulnerable to neurofibrillary tangle-like antigenic changes induced by EAAs or calcium ionophore (i.e., increased staining with tau and ubiquitin antibodies). Taken together, these data suggest that beta-amyloid destabilizes neuronal calcium homeostasis and thereby renders neurons more vulnerable to environmental insults.

The beta-amyloid precursor protein is not processed by the regulated secretory pathway

The beta-amyloid peptide is derived from a larger membrane bound protein and accumulates as amyloid in Alzheimer's diseased brains. beta-amyloid precursor protein (beta APP) proteolytically processed during constitutive secretion cannot be a source of deposited amyloid because this processing results in cleavage within the amyloidogenic peptide. To see if other secretory pathways could be responsible for generating potentially amyloidogenic molecules we tested the possibility that beta APP is targeted to the regulated secretory pathway. Stable AtT20 cell lines expressing exogenous human beta APP were genetically engineered. These cells were labeled with [35S]-methionine, and chased in the presence or absence of secretagogue. The beta APP both inside the cells and released from the cells was analyzed by immunoprecipitation and gel analysis. Quantitation of autoradiograms showed that virtually all of the synthesized beta APP was secreted by the constitutive pathway, and that no detectable (less than 1%) beta APP was targeted to the regulated secretory pathway.

DNA diagnosis for hereditary cerebral hemorrhage with amyloidosis (Dutch type)

Hereditary cerebral hemorrhage with amyloidosis of the Dutch type (HCHWA-D) is tightly linked to the Alzheimer amyloid precursor protein gene on chromosome 21, which codes for the amyloid beta-protein. A point mutation detected at position 1852 of the amyloid precursor protein gene in four HCHWA-D patients was hypothesized to be the basic defect. This study proves that 22 HCHWA-D patients from three pedigrees all carry this point mutation, whereas the mutation is absent in escapees from the HCHWA-D families as well as in randomly selected Dutch individuals. A mutation-specific oligonucleotide is now available for the confirmation of the HCHWA-D diagnosis. Therefore, presymptomatic testing and prenatal evaluation of individuals at risk in the HCHWA-D families is now feasible.

Exact cleavage site of Alzheimer amyloid precursor in neuronal PC-12 cells

We have identified the secretory cleavage site in the Alzheimer amyloid precursor (APP) in a non-transfected neuronal cell line, using cyanogen bromide digests of APP purified from medium conditioned by PC-12 cells which were differentiated to a neuronal phenotype. The results obtained are most consistent with proteolysis of the Lys16-Leu17 bond in the beta amyloid peptide, followed by partial removal of Lys16 by a basic carboxypeptidase.

A possible mechanism of action of the neurotoxic agent iminodipropionitrile (IDPN): a selective aggregation of the medium and heavy neurofilament polypeptides (NF-M and NF-H)

Juvenile male rats treated acutely with the neurotoxic agent, iminodipropionitrile showed no changes in the levels of total neurofilament subunit mRNA or protein for up to 28 days. However, the drug promoted aggregation of the neurofilaments, both spontaneously upon isolation and in an in vitro reassembly assay. This observation correlated with a basic pI shift of the heavy neurofilament subunit, due to a yet to be identified modification. Because of the crucial involvement of this neurofilament subunit in axonal integrity, it is likely that iminodipropionitrile produces a major portion of its neurotoxicity through this mechanism.

Isolation of baculovirus-derived secreted and full-length beta-amyloid precursor protein

We have expressed two forms of the Alzheimer's beta-amyloid precursor protein (beta APP), the 695-amino acid form (695 beta APP), and the 751-amino acid form (751 beta APP) in a baculovirus system. Both forms were expressed as full-length precursor, and were subsequently processed in vivo to release extracellular secreted proteins. The secreted forms were cleaved from the full-length beta APP in a manner analogous to the cleavage of beta APP during constitutive secretion in mammalian cells (Weidemann, A., König, G., Bunke, D., Fischer, P., Salbaum, J. M., Masters, C. L., Beyreuther, K. (1989) Cell 57, 115-126; Oltersdorf, T., Ward, P. J., Henriksson, T., Beattie, E. C., Neve, R., Lieberburg, I., and Fritz, L. J. (1990) J. Biol. Chem. 265, 4492-4497). High levels of expression of 20-50 mg/liter were achieved. Both full-length and secreted forms of the beta-amyloid precursor proteins were purified using a combination of ion-exchange and immunoaffinity chromatography using a monoclonal antibody directed against beta APP. The 751 beta APP-derived full-length and secreted forms, which contain the Kunitz protease inhibitor domain, were shown to be as active in the inhibition of trypsin as is mammalian-derived secreted beta APP. The availability of purified full-length beta APP from the baculovirus system will be valuable for biochemical and cell biological analyses that may elucidate the mechanism of the inappropriate processing that leads to beta-amyloid formation in Alzheimer's disease.

The protease inhibitory properties of the Alzheimer's beta-amyloid precursor protein

We have expressed the 57-amino acid Kunitz domain of the Alzheimer's beta-amyloid precursor protein (APP751) as a bacterial fusion protein. The protease inhibitory properties of the purified fusion protein, BX9, were virtually identical in all respects tested to those of purified secreted APP751. Both proteins strongly inhibited pancreatic trypsin (Kis = 0.2 and 0.3 nM) and less well epidermal growth factor-binding protein (Kis = 1 and 3.5 nM), alpha-chymotrypsin (Kis = 3 and 6 nM), and the gamma-subunit of nerve growth factor (Kis = 8 and 9 M). Neither protein appreciably inhibited plasma and pancreatic kallikreins, thrombin, lung tryptase, neutrophil elastase, or cathepsin G. The remarkable similarity of the protease inhibitory profile of BX9 to that of secreted APP751 suggests that proper intramolecular disulfide bond formation has occurred in the bacterial fusion protein and leads to the conclusion that the amyloid precursor protein Kunitz domain is a relatively specific inhibitor of only a few trypsin-like arginine esterases.

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.