A protease activity associated with acetylcholinesterase releases the membrane-bound form of the amyloid protein precursor of Alzheimer's disease

Repurposing of erythropoietin as a neuroprotective agent against methotrexate-induced neurotoxicity in rats

BACKGROUND

Methotrexate (MTX) is a cytotoxic drug that can trigger neurotoxicity via enhancing oxidative stress, apoptosis, and inflammation. On the other hand, erythropoietin (EPO) functions as an antioxidant, anti-apoptotic, and anti-inflammatory agent, in addition to its hematopoietic effects.

AIM

The present study was developed to examine the neuroprotective impact of EPO against MTX-provoked neurotoxicity in rats.

METHODS

Chemo fog was elicited in Wistar rats via injection of one dosage of MTX (20 mg/kg, i.p) on the sixth day of the study. EPO was injected at 500 IU/kg/day, i.p for 10 successive days.

RESULTS

MTX triggered memory and learning impairment as evidenced by Morris water maze, passive avoidance, and Y-maze cognitive tests. In addition, MTX induced oxidative stress as evident from the decline in hippocampal Nrf2 and HO-1 levels. MTX brought about apoptosis, as demonstrated by the elevation in p53, caspase-3, and Bax levels, as well as the decrease in Bcl2 levels. MTX also decreased Beclin-1, an autophagy-related marker, and increased P62 expression. In addition, MTX downregulated Sirt-1/AKT/FoxO3a pathway and increased miRNA-34a gene expression. Moreover, MTX increased acetylcholinesterase activity and reduced neurogenesis. EPO administration remarkably counteracted MTX-induced molecular and behavioral disorders in rat hippocampi.

CONCLUSION

Our findings impart preclinical indication for repurposing of EPO as a promising neuroprotective agent through modulating miRNA-34a, autophagy, and the Sirt-1/FoxO3a signaling pathway.

Variations in Acetylcholinesterase Activity within Human Cortical Pyramidal Neurons Across Age and Cognitive Trajectories

We described an extensive network of cortical pyramidal neurons in the human brain with abundant acetylcholinesterase (AChE) activity. Emergence of these neurons during childhood/adolescence, attainment of highest density in early adulthood, and virtual absence in other species led us to hypothesize involvement of AChE within these neurons in higher cortical functions. The current study quantified the density and staining intensity of these neurons using histochemical procedures. Few faintly stained AChE-positive cortical pyramidal neurons were observed in children/adolescents. These neurons attained their highest density and staining intensity in young adulthood. Compared with the young adult group, brains of cognitively normal elderly displayed no significant change in numerical density but a significant decrease in staining intensity of AChE-positive cortical pyramidal neurons. Brains of elderly above age 80 with unusually preserved memory performance (SuperAgers) showed significantly lower staining intensity and density of these neurons when compared with same-age peers. Conceivably, low levels of AChE activity could enhance the impact of acetylcholine on pyramidal neurons to counterbalance other involutional factors that mediate the decline of memory capacity during average aging. We cannot yet tell if elderly with superior memory capacity have constitutively low neuronal AChE levels or if this feature reflects adaptive neuroplasticity.

Erythrocyte acetylcholinesterase as biomarker of pesticide exposure: new and forgotten insights

Acetylcholinesterase (AChE) acts on the hydrolysis of acetylcholine, rapidly removing this neurotransmitter at cholinergic synapses and neuromuscular junctions as well as in neuronal growth and differentiation, modulation of cell adhesion ("electrotactins") and aryl-acylamidase activity (AAA). This enzyme is also found in erythrocyte, as 160 kDa dimer that anchors to the plasma membrane via glycophosphatidylinositol. The function of this enzyme in erythrocytes has not yet been elucidated; however, it is suspected to participate in cell-to-cell interactions. Here, a review on erythrocyte AChE characteristics and use as biomarker for organophosphorus and carbamate insecticides is presented since it is the first specific target/barrier of the action of these pesticides, besides plasma butyrylcholinesterase (BChE). However, some past and current methods have disadvantages: (a) not discriminating the activities of AChE and BChE; (b) low accuracy due to interference of hemoglobin in whole blood samples. On the other hand, extraction methods of hemoglobin-free erythrocyte AChE allows: (a) the freezing and transporting of samples; (b) samples free of colorimetric interference; (c) data from only erythrocyte AChE activity; (d) erythrocyte AChE specific activity presents higher correlation with the central nervous system AChE than other peripheral ChEs; (e) slow spontaneous regeneration against anti-ChEs agents of AChE in comparison to BChE, thus increasing the chances of detecting such compounds following longer interval after exposure. As monitoring perspectives, hemoglobin-free methodologies may be promising alternatives to assess the degree of exposure since they are not influenced by this interfering agent.

Immunohistochemical analysis of hippocampal butyrylcholinesterase: Implications for regional vulnerability in Alzheimer's disease

Studies of acetylcholine degrading enzymes acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) in Alzheimer's disease (AD) have suggested their potential role in the development of fibrillar amyloid-β (Aβ) plaques (amyloid plaques). A recent genome-wide association study analysis identified a novel association between genetic variations in the BCHE locus and amyloid burden. We studied BChE immunoreactivity in hippocampal tissue sections from AD and control cases, and examined its relationship with amyloid plaques, neurofibrillary tangles (NFT), dystrophic neurites (DN) and neuropil threads (NT). Compared to controls, AD cases had greater BChE immunoreactivity in hippocampal neurons and neuropils in CA2/3, but not in the CA1, CA4 and dentate gyrus. The majority of amyloid plaques (> 80%, using a pan-amyloid marker X-34) contained discrete neuritic clusters which were dual-labeled with antibodies against BChE and phosphorylated tau (clone AT8). There was no association between overall regional BChE immunoreaction intensity and amyloid plaque burden. In contrast to previous reports, BChE was localized in only a fraction (~10%) of classic NFT (positive for X-34). A similar proportion of BChE-immunoreactive pyramidal cells were AT8 immunoreactive. Greater NFT and DN loads were associated with greater BChE immunoreaction intensity in CA2/3, but not in CA1, CA4 and dentate gyrus. Our results demonstrate that in AD hippocampus, BChE accumulates in neurons and plaque-associated neuritic clusters, but only in a small proportion of NFT. The association between greater neurofibrillary pathology burden and markedly increased BChE immunoreactivity, observed selectively in CA2/3 region, could reflect a novel compensatory mechanism. Since CA2/3 is generally considered more resistant to AD pathology, BChE upregulation could impact the cholinergic modulation of glutamate neurotransmission to prevent/reduce neuronal excitotoxicity in AD hippocampus.

Acetylcholinergic neurotransmission and the β-amyloid cascade: implications for Alzheimer’s disease

Alzheimer's disease is characterized by both decreases in acetylcholinergic neurotransmission and increases in beta-amyloid accumulation. Currently, available clinical psychopharmacologic treatment is focused on increasing acetylcholinergic neurotransmission, whereas no clinical treatments to directly reduce beta-amyloid accumulation are available. Cholinesterase inhibitors improve cognition, certain neuropsychiatric symptoms and functional impairment in patients with mild-to-moderate Alzheimer's disease, and it is believed that this is mainly symptomatic treatment. However, this review discusses various levels of interaction between acetylcholinergic neurotransmission and the beta-amyloid cascade, which suggest that some specific acetylcholinergic treatments may reduce beta-amyloid accumulation, and therefore may slow disease progression over the long term. Various suggestions are made on how such potential disease-modifying effects could be studied in the future.

ADAM10 as a biomarker for Alzheimer's disease: a study with Brazilian elderly

Alzheimer's disease (AD) is the most common cause of dementia in people above age 65. Platelet studies with ADAM10 have shown that its expression is reduced in AD patients. The aim of this research was to compare the platelet levels of ADAM10 protein in two Brazilian elderly groups, considering the stages of the disease. The SDS-PAGE technique followed by Western blotting was used. Data were analyzed using comparison, correlation and association statistical methods. The results showed reduced platelet ADAM10 levels in AD elderly compared to non-AD subjects. The disease progression intensified this reduction. ADAM10 was the only statistically significant variable (p = 0.01) to increase the AD occurrence probability. The cutoff value of 0.4212 in the receiver operating characteristic curve captured sensitivity and specificity of 70 and 80.77%, respectively. Together with other clinical criteria, ADAM10 seems to be a relevant biomarker tool for early and accurate AD diagnosis.

The functions of the amyloid precursor protein gene

The amyloid precursor protein (APP) gene and its protein products have multiple functions in the central nervous system and fulfil criteria as neuractive peptides: presence, release and identity of action. There is increased understanding of the role of secretases (proteases) in the metabolism of APP and the production of its peptide fragments. The APP gene and its products have physiological roles in synaptic action, development of the brain, and in the response to stress and injury. These functions reveal the strategic importance of APP in the workings of the brain and point to its evolutionary significance.

Butyrylcholinesterase-Mediated enhancement of the enzymatic activity of trypsin

1. Acetylcholinesterase (AChE, EC 3.1.1.7) and butyrylcholinesterase (BuChE, EC 3.1.1.8) are enzymes that catalyze the hydrolysis of esters of choline. 2. Both AChE and BuChE have been shown to copurify with peptidases. 3. BuChE has also been shown to copurify with other proteins such as transferrin, with which it forms a stable complex. In addition, BuChE is found in association with beta-amyloid protein in Alzheimer brain tissues. 4. Since BuChE copurifies with peptidases, we hypothesized that BuChE interacts with these enzymes and that this association had an influence on their catalytic activities. One of the peptidases that copurifies with cholinesterases has specificity similar to trypsin, hence, this enzyme was used as a model to test this hypothesis. 5. Purified BuChE causes a concentration-dependent enhancement of the catalytic activity of trypsin while trypsin does not influence the catalytic activity of BuChE. 6. We suggest that, in addition to its esterase activity, BuChE may assume a regulatory role by interacting with other proteins.

Structural roles of acetylcholinesterase variants in biology and pathology

Apart from its catalytic function in hydrolyzing acetylcholine, acetylcholinesterase (AChE) affects cell proliferation, differentiation and responses to various insults, including stress. These responses are at least in part specific to the three C-terminal variants of AChE which are produced by alternative splicing of the single ACHE gene. 'Synaptic' AChE-S constitutes the principal multimeric enzyme in brain and muscle; soluble, monomeric 'readthrough' AChE-R appears in embryonic and tumor cells and is induced under psychological, chemical and physical stress; and glypiated dimers of erythrocytic AChE-E associate with red blood cell membranes. We postulate that the homology of AChE to the cell adhesion proteins, gliotactin, glutactin and the neurexins, which have more established functions in nervous system development, is the basis of its morphogenic functions. Competition between AChE variants and their homologs on interactions with the corresponding protein partners would inevitably modify cellular signaling. This can explain why AChE-S exerts process extension from cultured amphibian, avian and mammalian glia and neurons in a manner that is C-terminus-dependent, refractory to several active site inhibitors and, in certain cases, redundant to the function of AChE-like proteins. Structural functions of AChE variants can explain their proliferative and developmental roles in blood, bone, retinal and neuronal cells. Moreover, the association of AChE excess with amyloid plaques in the degenerating human brain and with progressive cognitive and neuromotor deficiencies observed in AChE-transgenic animal models most likely reflects the combined contributions of catalytic and structural roles.

Secretion of protease nexin-II/amyloid beta protein precursor by human colorectal carcinoma cells and its modulation by cytokines/growth factors and proteinase inhibitors

Trypsin inhibitors secreted by human colorectal adenocarcinoma cell lines were analyzed by reverse zymography. Among eleven cell lines analyzed, the major inhibitor secreted was protease nexin-II (PN-II), a secreted form of amyloid beta protein precursor (APP) containing a Kunitz-type serine proteinase inhibitor domain. Expression of the APP gene was also confirmed in the cell lines and the main APP mRNA species were PN-II types. The APP gene expression was constant during cell growth in vitro. On the other hand, the rate of extracellular PN-II accumulation markedly increased after long-term serum-free maintenance of the confluent culture. The extracellular accumulation of PN-II was also strongly stimulated either by interleukin-1beta (IL-1beta) treatment or to a lesser extent by basic fibroblast growth factor, tumor necrosis factor-alpha, hepatocyte growth factor or epidermal growth factor. Neither serum depletion- nor IL-1beta-induced stimulation of extracellular PN-II accumulation were accompanied by obvious alteration of the levels of APP mRNA and cellular APP holoprotein, suggesting that the enhanced extracellular accumulation of PN-II might result from up-regulation of the secretory pathway of APP. The IL-1beta-induced PN-II secretion was significantly inhibited by relatively high concentrations (50-200 microg/ml) of aprotinin, a serine proteinase inhibitor, in a dose-dependent manner without obvious cell-toxic effects.

The role of the amyloid protein precursor (APP) in Alzheimer's disease: does the normal function of APP explain the topography of neurodegeneration?

Alzheimer's disease (AD) is the most common form of dementia in the aged population. Early-onset familial AD (FAD) involves mutations in a gene on chromosome 21 encoding the amyloid protein precursor or on chromosomes 14 or 1 encoding genes known as presenilins. All mutations examined have been found to increase the production of amyloidogenic forms of the amyloid protein (A beta), a 4 kDa peptide derived from APP. Despite the remarkable progress in elucidating the biochemical mechanisms responsible for AD, little is known about the normal function of APP. A model of how APP and A beta are involved in pathogenesis is presented. This model may explain why certain neuronal populations are selectively vulnerable in AD. It is suggested that those neurons which more readily undergo neuritic sprouting and synaptic remodelling are more vulnerable to A beta neurotoxicity.

Stabilization of recombinant Drosophila acetylcholinesterase

The uses of pure and stable acetylcholinesterase can range from simple basic research to applications in environment quality assessment. In order to satisfy some of these needs its recombinant expression is routinely performed. Affinity-purified recombinant Drosophila melanogaster acetylcholinesterase proved to be instable; an apparent cause of this seemed to be the presence of contaminants with protease activity as evidenced by SDS-PAGE. The elimination of these accompanying products was achieved by anion-exchange, hydrophobic interaction, and cibacron blue affinity chromatography applied downstream from procainamide affinity chromatography. The utilization of a parallel affinity acting via an engineered histidine tail permitted the elimination of the copurified proteases as well. Despite the elimination of the contaminants, the apparently pure extracts were still unstable. It is shown that such instability can be counterbalanced by provoking protein-protein interactions, either between enzyme molecules or with other molecules such as bovine serum albumin. Another way to reduce instability is the addition of a reversible inhibitor or polyethylene glycol 3350.

Brain acetylcholinesterase promotes amyloid-beta-peptide aggregation but does not hydrolyze amyloid precursor protein peptides

It has been suggested that acetylcholinesterase (AChE) has both a putative proteolytic activity against the amyloid precursor protein (APP), and a capacity to accelerate the assembly of amyloid-beta-peptide (Abeta) into Alzheimer's fibrils. Here, we have studied the ability of bovine brain AChE to share both activities. Results indicate that AChE purified through acridinium was able to process the APP peptides, however after further purification by an edrophonium column, the protease activity was lost. Under both conditions the capacity of the enzyme to promote amyloid formation was maintained. Kinetic studies of the Abeta aggregation process using edrophonium-AChE, indicated that the lag phase of the aggregation process was smaller than the one observed with the esterase purified by acridinium alone. Considering that the total amount of amyloid formed, measured by thioflavine-T fluorescence, was similar for both AChE preparations, our results suggest that the edrophonium-AChE possesses an higher intrinsic capacity to stimulate the aggregation of Abeta(1-40) peptide.

Interaction between Alzheimer's disease beta A4 precursor protein (APP) and the extracellular matrix: evidence for the participation of heparan sulfate proteoglycans

The interaction between the Alzheimer amyloid precursor protein (APP) and an intact extracellular matrix (ECM), matrigel, obtained from Engelbreth-Holm-Swarm tumors was evaluated. Based on quantitative analyses of the binding data obtained from solid phase binding assays, two binding sites on the ECM were identified for [125I]-APP (with apparent Kd1 of 1.0 x 10(-11) M and Kd2 of 1.6 x 10(-9) M respectively). Over 70% of [125I]-APP was displaced by heparin and N-desulfated heparin but not by chondroitin sulfate. Pretreatment of matrigel with heparitinase decreased the binding of [125I]-APP by 80%. beta-amyloid peptides (residues 1-40, 1-28, and 1-16) containing a heparin binding domain also displaced 80% of bound [125I]-APP, which was totally displaced by intact APP. The binding of [125I]-APP to matrigel increased by 210% with a decrease in the pH. These observations suggest that [125I]-APP interacts mainly with heparan sulfate proteoglycan present in the ECM. The binding of [125I]-APP to individual ECM components was also analyzed. [125I]-APP was found to bind laminin and collagen type IV but not fibronectin. However, when these ECM constituents were combined, the extent of APP-binding decreased significantly, to levels comparable to those obtained with intact matrigel, suggesting that multiple interactions may occur between ECM constituents and [125I]-APP. The results are discussed in terms of APP function and amyloidogenesis.

Amyloidogenic processing of Alzheimer's amyloid precursor protein in vitro and its modulation by metal ions and tacrine

Recent studies implicate that excessive amyloidogenic pathway of amyloid precursor protein (APP) processing may be the final common pathway involved in the pathogensis of AD. In attempts to identify the proteases or factors leading to excessive amyloid deposition, we evaluated the potential role of acethylcholinesterase (AChE) and its associated protease for amyloidogenic processing of APP in vitro. Prolonged incubation of a recombinant APP770 with AChE produced several amyloidogenic fragments accumulating a relatively stable a 18 kDa A beta (amyloid beta-protein) bearing carboxy terminal peptide, which was further degraded by an increased concentration of AChE. Protease inhibitory profiles confirmed the trypsin-like serine protease activity present in AChE preparation. This observed APP processing was significantly enhanced by Ca2+, Mg2+, or Mn2+ at 1 mM concentration and modulated in concentration dependent manners by metal ions such as Ca2+, Zn2+, Fe2+/Fe3+, Al3+, or a tacrine, a centrally active cholinesterase inhibitor. Our data imply that AChE and its associated protease may be involved in the generation a 18 kDa amyloidogenic peptide under certain physiological condition in vivo and that the gradual changes in their proteolytic activities or locations and the locally disturbed metal homeostasis could be factors associated with abnormal accumulation of APP, eventually leading to amyloid deposition in AD brain. In addition, zinc or tacrine treatment of AD patients with high dosage or in the long term may have effects on the process of amyloidogensis.

Non-classical actions of cholinesterases: role in cellular differentiation, tumorigenesis and Alzheimer's disease

The cholinesterases are members of the serine hydrolase family, which utilize a serine residue at the active site. Acetylcholinesterase (AChE) is distinguished from butyrylcholinesterase (BChE) by its greater specificity for hydrolysing acetylcholine. The function of AChE at cholinergic synapses is to terminate cholinergic neurotransmission. However, AChE is expressed in tissues that are not directly innervated by cholinergic nerves. AChE and BChE are found in several types of haematopoietic cells. Transient expression of AChE in the brain during embryogenesis suggests that AChE may function in the regulation of neurite outgrowth. Overexpression of cholinesterases has also been correlated with tumorigenesis and abnormal megakaryocytopoiesis. Acetylcholine has been shown to influence cell proliferation and neurite outgrowth through nicotinic and muscarinic receptor-mediated mechanisms and thus, that the expression of AChE and BChE at non-synaptic sites may be associated with a cholinergic function. However, structural homologies between cholinesterases and adhesion proteins indicate that cholinesterases could also function as cell-cell or cell-substrate adhesion molecules. Abnormal expression of AChE and BChE has been detected around the amyloid plaques and neurofibrillary tangles in the brains of patients with Alzheimer's disease. The function of the cholinesterases in these regions of the Alzheimer brain is unknown, but this function is probably unrelated to cholinergic neurotransmission. The presence of abnormal cholinesterase expression in the Alzheimer brain has implications for the pathogenesis of Alzheimer's disease and for therapeutic strategies using cholinesterase inhibitors.

Gelatinase A possesses a beta-secretase-like activity in cleaving the amyloid protein precursor of Alzheimer's disease

The ability of the 72 kDa gelatinase A to cleave the amyloid protein precursor (APP) was investigated. HeLa cells were transfected with an APP695 plasmid. The cells were incubated with gelatinase A, which cleaved the 110 kDa cell-surface APP, releasing a 100 kDa form of the protein. A peptide homologous to the beta-secretase site was cleaved by gelatinase A adjacent to a glutamate residue at position -3 (beta A4 numbering system). A peptide homologous to the alpha-secretase site was not cleaved. The results demonstrate that 72 kDa gelatinase A is not an alpha-secretase, but that it may have a beta-secretase activity.

Membrane alterations as causes of impaired signal transduction in Alzheimer's disease and aging

Changes in cell-membrane composition in normal aging and in Alzheimer's and other age-related diseases appear to result in impaired neurotransmitter-triggered signal transduction. The impaired signal transduction seems to be related to dysfunctions in the coupling of G proteins to their receptors and effectors. Direct demonstration of altered physiochemical properties of brain tissue of patients with Alzheimer's disease has been achieved by small-angle X-ray diffraction. In this disease, thinner membranes correlate with a 30% decrease in moles of cholesterol:phospholipid. Such changes can affect directly the coupling and uncoupling properties of G proteins, and can account for signal transduction deficits. These findings offer a complementary alternative to the beta-amyloid hypothesis, and an opportunity to consider new types of therapeutic interventions.

Reversibility of the effect of tacrine on the secretion of the beta-amyloid precursor protein in cultured cells

The amyloid beta-protein (A beta) of Alzheimer's disease is derived from a family of large integral membrane glycoproteins, beta-amyloid precursor proteins (beta APP). Two secretory proteolytic pathways are involved in the metabolism of beta APP. The major pathway involves cleavage within the A beta sequence and generates carboxyl-truncated derivatives of beta APP which are secreted into the conditioned medium of cells. The minor 'amyloidogenic' pathway results in the production of A beta. Here, cell cultures were used to examine the metabolism of beta APP by tacrine, a centrally active cholinesterase inhibitor reported to improve cognitive deficits. Treatment with tacrine in cells resulted in the drastic inhibition of secretion of the major isoforms of beta APP into the medium. The effect of tacrine can be reversed by washing away the drug from the cells. Treatment with tacrine did not change the level of either HSP-70 or LDH. Thus, the inhibitory effect of tacrine on the secretion of beta APP was not due to the permanent damage or loss of cells as normal release of beta APP could be restored when the drug was washed away.

Cholinesterase activity in the plaques, tangles and angiopathy of Alzheimer's disease does not emanate from amyloid

Previous histochemical observations in our laboratory have demonstrated the presence of butyrylcholinesterase and an enzymatically altered form of acetylcholinesterase activity in the plaques, tangles and amyloid-containing vessels of Alzheimer's disease. These findings suggested possible interactions between amyloid and cholinesterases. In this study we employed a cholinesterase biochemical assay to determine whether the amyloid precursor protein either had cholinesterase activity itself or influenced the enzymatic activity of cholinesterases. None of the three amyloid precursor sequences used (695, 751, 770, up to 16 micrograms/ml) exhibited any acetylcholinesterase or butyrylcholinesterase activity that could be detected by our method. In addition, none of the amyloid precursor proteins influenced the enzymatic activity of purified acetylcholinesterase or butyrylcholinesterase in a specific manner. It is therefore quite unlikely that amyloid can, by itself, account for the intense cholinesterase activity associated with the pathological lesions of AD.

Tacrine alters the secretion of the beta-amyloid precursor protein in cell lines

The characteristic features of Alzheimer's disease (AD) include a high density of beta-amyloid-containing plaques in the cerebral cortex and the loss of basal forebrain cholinergic neurons. Amyloid beta-protein (A beta, Mr. approximately 4.5 kDa) is derived from a family of large (Mr. approximately 110-140 kDa) beta-amyloid precursor proteins (APP) which are integral membrane glycoproteins consisting of a large extracytoplasmic domain, a transmembrane domain, and a short cytoplasmic tail. Secreted derivatives of APP lacking the cytoplasmic tail, transmembrane domain, and a small portion of the extracellular domain are generated by the proteolytic processing of full length APP by a family of proteolytic enzymes known as APP secretases. Using cell cultures, we investigated the possibility that APP processing can be regulated by a centrally active cholinesterase inhibitor, tacrine, which has recently been shown to improve memory and cognitive functions in patients with AD. We analyzed the level of APP in glial, fibroblast, pheochromocytoma (PC12), and neuroblastoma cells by immunoblotting cell lysates and conditioned media. Normal levels of secretion of soluble APP derivatives by cells into conditioned media were severely inhibited by treating cells with tacrine. A similar decrease after treatment with tacrine was observed when neuroblastoma and PC12 cells were pretreated with either growth factors, phorbol ester, or retinoic acid. To determine whether the effect of tacrine on APP levels was specific or a more general phenomenon affecting other proteins, we measured the level of heat shock protein-70 (HSP-70) and another secretory protein, protease nexin-1 (PN-1). Tacrine treatment did not alter the level of HSP-70 in cell extracts and tacrine affected mildly the secretion of PN-1. Thus, the processing of HSP and PN-1, unlike APP, was not severely affected by treating cells with tacrine. Our results suggest that tacrine may inhibit an acetylcholinesterase-associated proteolytic activity involved in the secretion of APP, which results in less secretion of soluble APP into the conditioned media from tacrine treated cells. These results demonstrate that tacrine regulates APP secretion in cell cultures and suggest the possibility that tacrine therapy of Alzheimer's disease may, in the longer term, have effects on the process of A beta deposition.

Blood markers in Alzheimer disease: subnormal acetylcholinesterase and butyrylcholinesterase in lymphocytes and erythrocytes

In patients with the clinical diagnosis of Alzheimer disease (AD), we searched for systemic changes in components of the blood as a diagnostic tool. The acetylcholine-related enzymes acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) were measured in plasma, erythrocytes, platelets and lymphocytes. Results did not show a general effect; notwithstanding, specific cell types presented alterations either in AChE or BuChE but not in both enzymatic activities. In AD patients, AChE of lymphocytes was reduced by 60% compared with the age-matched controls. However, when patients were divided, the sporadic but not the familial subgroup exhibited a significant reduction. In erythrocytes the BuChE activity was reduced by 45% in sporadic AD. The molecular forms of the lymphocyte AChE were characterized by velocity sedimentation. Both globular forms were subnormal, more so the tetrameric G4 AChE form than the G2 form.

Cholinesterases colocalize with sites of neurofibrillary degeneration in aged and Alzheimer's brains

Acetylcholinesterase and butyrylcholinesterase have been associated with structures undergoing neurofibrillary degeneration, as well as with all types of senile plaques, in non-demented aged and Alzheimer's brains. At the electron microscope level, the reaction product of both enzymes, appeared to decorate paired helical filaments, straight filaments and beta A4 amyloid fibrils. Recent studies showed that cholinesterases were associated with amyloid at early stages, e.g., in diffuse plaques. In the present study, the interrelationship of cholinesterases to structures undergoing neurofibrillary degeneration was analyzed further. Tau immunoreactivity was compared to the staining pattern observed with the two esterases. Double protocols consecutively performed on the same sections, and counterstaining with thioflavin-S, confirmed the presence of cholinesterases in all structures with neurofibrillary degeneration. The conclusion that cholinesterases consistently colocalize with both neurofibrillary bundles and beta A4 amyloid fibrils at all stages of their accumulation, allows us to speculate on the possible role that these enzymes may play in either the formation or the consolidation of fibrillary aggregates.

Acetylcholinesterase activation of peritoneal macrophages is independent of catalytic activity

1. In diverse tissues, acetylcholinesterase appears to play a critical role in the functional state of cells completely dependent of cholinergic transmission. However, very little is known about the mechanisms and actual molecular structures mediating the fundamental interactions between this protein and the cellular membrane. 2. In this study, peritoneal macrophages were used as a model system to study the possible interaction between acetylcholinesterase, acting in a non-cholinergic capacity, and the cellular membrane. 3. When acetylcholinesterase was incubated with macrophages harvested from rat peritoneum, the rate of oxygen consumption was increased in a concentration-dependent manner that was independent of mitochondrial block with sodium cyanide. Furthermore, heat inactivation of enzymatic activity or application of BW 284C51 at a concentration which totally blocks catalytic activity did not eliminate the effect. 4. In contrast, incubation with bovine serum albumin or butyrylcholinesterase actually retarded oxygen consumption. 5. The effect of acetylcholinesterase depended on the presence of divalent cations and was inhibited by mannan and D-mannose, but not D-galactose. It is concluded that acetylcholinesterase can induce a "respiratory burst" in macrophages independent of its conventional catalytic site but involving either the mannose receptor of the monocyte-derived macrophage or a possible sugar binding site on acetylcholinesterase itself.

Expression of acetylcholinesterase messenger RNA in human brain: an in situ hybridization study

The distribution of messenger RNA coding for acetylcholinesterase was studied in human post mortem brain and rhesus monkey by in situ hybridization histochemistry and compared to the distribution of acetylcholinesterase activity. Acetylcholinesterase messenger RNA had--similar to acetylcholinesterase enzymatic activity--a widespread distribution in human bain. Acetylcholinesterase messenger RNA positive cells corresponded to perikarya rich in acetylcholinesterase activity in most but not all regions. Examples for mismatches included the inferior olive and human cerebellar cortex. The presence of hybridization signals in cerebral cortex and an enrichment in layer III and V of most isocortical areas confirmed that perikaryal acetylcholinesterase in cerebral cortex is of postsynaptic origin and not derived from cholinergic projections. In striatum the expression of high levels of acetylcholinesterase messenger RNA was restricted to a small population of large striatal neurons. In addition, low levels of expression were found in most medium sized striatal neurons. Cholinergic neurons tended to express high levels of acetylcholinesterase messenger RNA whereas in cholinoceptive neurons the levels were moderate to low. However, some noncholinergic neurons like dopaminergic cells in substantia nigra, noradrenergic cells in locus coeruleus, serotoninergic cells in raphé dorsalis, GABAergic cells in thalamic reticular nucleus, granular cells in cerebellar cortex and pontine relay neurons expressed levels comparable to cholinergic neurons in basal forebrain. It is suggested that neurons expressing high levels of acetylcholinesterase messenger RNA may synthesize acetylcholinesterase for axonal transport whereas neurons with an expression of acetylcholinesterase confined to somatodendritic regions tend to contain lower levels of acetylcholinesterase messenger RNA.

Developmentally transient expression of acetylcholinesterase within cortical pyramidal neurons of the rat brain

Using a histochemical method for the visualization of cholinesterase activity in neurons, we have observed developmentally transient expression of acetylcholinesterase (AChE) in cortical pyramidal neurons of the rat brain. Depending on the extent of the deposition of AChE reaction product, several types of cortical neurons could be visualized. We designated neurons with moderate-to-high staining intensity as AChEH and neurons with relatively lower staining intensity as AChEL. At birth (P0), very little AChE activity was found within cortical neurons. Between P1-P4, there was a gradual emergence of AChE-stained cortical neurons. At this stage, the majority of these neurons were of the AChEL type. At P5-P7 we observed an abrupt increase in AChE-stained cortical neurons. The number and the staining intensity of these neurons was at a peak at P8-P10. At this age range, the majority of these neurons were of the AChEH variety and displayed morphological characteristics of cortical pyramidal neurons. At P11-P15, there was an abrupt decrease in the number of AChEH neurons. After P15, the density and staining intensity of cortical AChE-positive (cholinergic) axons gradually increased. Nevertheless, AChEL pyramidal neurons were detected through these fibers up to P21. At P21, a dense plexus of AChE-positive axons was observed in all cortical areas while very little AChE reaction product was visible in pyramidal neurons, and this pattern continued into adult life. When the adult cortex was denervated from its AChE-positive axons by lesions of the nucleus basalis magnocellularis, many AChEL pyramidal neurons were uncovered.(ABSTRACT TRUNCATED AT 250 WORDS)

Characterisation of an epitope specific to the neuron-specific isoform of human enolase recognised by a monoclonal antibody raised against a synthetic peptide corresponding to the C-terminus of beta/A4-protein

Antibodies to synthetic peptides corresponding to different regions of beta/A4-protein recognize deposits of amyloid in the brains of patients with Alzheimer's disease. Down's syndrome cases and in the normal ageing brain. We have prepared a monoclonal antibody, mAb 22.212, raised against a synthetic C-terminal peptide of beta/A4 protein (residues 28-40) which labelled senile plaques in Alzheimer's disease after proteolytic treatment of tissue sections. In addition to recognising synthetic beta/A4-peptides that include the C-terminal residues 28-42, the mAb 22.212 was found to cross-react with a soluble, 47 kDa protein found in brain homogenates. This protein was shown, by amino acid sequence analysis and immunoassay, to be neuron-specific enolase (NSE). The mAb 22.212 did not recognize the non-neuronal enolase (NNE) or muscle-specific enolase (MSE) isoforms and its epitope was mapped to a short stretch of amino-acids unique to NSE, near the C-terminus. The cross-reactive NSE epitope is sited between residues 402-423 in NSE and shows no common sequence with beta/A4, perhaps suggesting that it is a conformational epitope. The significance and applications of these findings are discussed.

The role of extracellular matrix in the processing of the amyloid protein precursor of Alzheimer's disease

Alzheimer's disease (AD) is characterized by the presence of extracellular amyloid plaques, which contain a protein referred to as the amyloid or beta A4 protein. The beta A4 protein is derived from a larger precursor protein (APP). Studies of autosomal-dominant forms of AD have established the central role of APP in the pathogenesis of the disease. Despite considerable research, the function of APP is unknown. APP can be processed by at least two separate routes. The first route involves a protease known as "APP secretase," which cleaves within the amyloid sequence, thereby mitigating amyloid formation. The second route may result in the production of potentially amyloidogenic fragments. Our studies suggest that following release from the cell membrane, APP interacts with components of the extracellular matrix (ECM) such as the heparan sulfate proteoglycans (HSPG's). The interaction of APP with HSPG's may be important for the function of APP. Substratum-bound APP was found to dramatically increase neurite outgrowth and survival of chick sympathetic neurons in vitro. This effect was dependent upon the presence of substratum-bound HSPG. The results suggest that normally, when bound to the ECM, APP functions to promote neurite outgrowth and/or cell survival. Loss of this normal trophic function might occur in AD, when APP is proteolytically processed via the amyloidogenic pathway.

Protease inhibitors and indolamines selectively inhibit cholinesterases in the histopathologic structures of Alzheimer's disease

Neurofibrillary tangles and amyloid plaques express acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) activity in Alzheimer's disease. We had found that traditional AChE inhibitors such as BW284C51, tacrine and physostigmine were more potent inhibitors of the AChE in normal axons and cell bodies than of the AChE in plaques and tangles. We now report that the reverse pattern is seen with indolamines, carboxypeptidase inhibitor, and the nonspecific protease inhibitor bacitracin. These substances are more potent inhibitors of the cholinesterases in plaques and tangles than of those in normal axons and cell bodies. These results show that the enzymatic properties of plaque and tangle-associated cholinesterases diverge from those of normal axons and cell bodies. The selective susceptibility to bacitracin and carboxypeptidase inhibitor indicates that the catalytic sites of plaque and tangle-bound cholinesterases are more closely associated with peptidase or protease-like properties than the catalytic sites of cholinesterases in normal neurons and axons. This shift in enzymatic affinity may lead to the abnormal protein processing which is thought to play a major role in the pathogenesis of AD. The availability of pharmacological and dietary means for altering brain indolamines raises novel therapeutic possibilities for inhibiting the abnormal cholinesterase activity associated with Alzheimer's disease.

Neurological cholinesterases in the normal brain and in Alzheimer's disease: relationship to plaques, tangles, and patterns of selective vulnerability

Butyrylcholinesterase (BChE) and an altered form of acetylcholinesterase (AChE) accumulate in the plaques and tangles of Alzheimer's disease (AD). The sources for these plaque- and tangle-bound cholinesterases have not been identified. We now report that AChE and BChE activities with pH preferences and inhibitor selectivities identical to those of plaque- and tangle-bound cholinesterases are found in the astrocytes and oligodendrocytes of control and AD brains. These glial-type cholinesterases are selectively inhibited by indolamines and protease inhibitors. In control brains glial-type cholinesterases appear confined to the intracellular space, whereas in patients with AD they decorate plaques and tangles as well. In control and AD brains AChE-positive glia are distributed throughout the cortical layers and subcortical white matter, whereas BChE-positive glia reach high densities only in the deep cortical layers and white matter. In non-AD control brains, the ratio of BChE to AChE glia was higher in entorhinal and inferotemporal cortex, two regions with a high susceptibility to the pathology of AD, than in primary somatosensory and visual cortex, two areas with a relatively lower susceptibility to the disease process. There was no age-related differences in the density or distribution of cholinesterase-positive glia. In comparison with age-matched control specimens, AD brains had a significantly higher density of BChE glia and a lower density of AChE glia in entorhinal and inferotemporal regions but not in the primary somatosensory or visual areas. These results suggest that glia constitute a likely source for the cholinesterase activity of plaques and tangles and that a high ratio of BChE- to AChE-positive glia may play a permissive or causative role in the neuropathology of AD.

Proteolysis at the secretase and amyloidogenic cleavage sites of the beta-amyloid precursor protein by acetylcholinesterase and butyrylcholinesterase using model peptide substrates

1. It was recently proposed that acetylcholinesterase (AChE), in addition to its esteratic activity, has proteolytic activity such that it may cleave the beta-amyloid precursor (beta-APP) within the beta-amyloid sequence. The purpose of this paper was to examine further whether AChE or butyrylcholinesterase (BuChE) had associated proteinase activity that was involved in the metabolism of beta-APP. 2. The ability of various preparations of AChE and BuChE to hydrolyze two synthetic fragments of beta-APP695 as model substrates containing the normal and aberrant cleavage sites was studied. 3. Digestion of these synthetic substrates with commercial preparations of Electrophorus electricus AChE indicated the presence of a trypsin-like proteolytic activity cleaving each peptide at the carboxy-terminal side of an internal lysine residue. 4. Purification of the trypsin-like proteinase activity by aminobenzamidine affinity chromatography yielded a preparation that was devoid of AChE activity but retained all of the proteinase activity. 5. Amino-terminal sequence analysis of this preparation showed that the first 13 amino acid residues were identical to beta-pancreatic trypsin. 6. These data indicate that the proteinase activity found in these commercial preparations of AChE is due to contamination with trypsin.

A protease is recovered with a dimeric form of acetylcholinesterase in fetal bovine serum

A protease activity which co-purified with affinity-purified fetal bovine serum acetylcholinesterase (AChE) has been shown to release the amyloid protein precursor (APP) of Alzheimer's disease from cell membranes. The nature of this protease and its relationship to AChE have not been established. In this study, the protease activity was found to be recovered with a minor dimeric form of AChE. This minor form (AChEII) was distinguished from the more abundant tetrameric form (AChEI) by a higher catalytic subunit relative molecular mass (M(r)) of 80,000 (80K), and by a lower affinity for edrophonium-Sepharose. The difference in subunit M(r) was due to differing degrees of glycosylation, as deglycosylation of both AChEI and AChEII gave rise to a similar subunit M(r) of 62K. The protease activity recovered with AChEII was not an intrinsic property of the esterase, as it was separated from the esterase by anion-exchange chromatography, and by immunoprecipitation with anti-AChE antibodies. AChEI possessed a similar subunit M(r) to the tetrameric form of AChE secreted from the bovine adrenal gland, while AChEII possessed a similar subunit molecular weight to the dimeric membrane-bound form of bovine erythrocyte AChE. Thus, it is possible that AChEII may be a solubilised form of a dimeric glycosylphosphatidyl inositol-linked AChE.

Distribution of acetylcholinesterase in the hippocampal region of the mouse. III. The area dentata

The distribution of acetylcholinesterase (AChE) was examined in the area dentata of the adult mouse (Mus musculus domesticus). A distinctly stratified distribution of the enzyme was observed and was compared in detail with cytoarchitectural fields and layers. In the stratum moleculare, bands of relatively high AChE activity were seen immediately beneath the pia, at the borders between the outer, middle, and deep portions of the stratum moleculare, and superficial to the granule cell layer. AChE activity was low in the intervening parts of the stratum moleculare. In contrast to the rat, three sublaminae could be discerned in the hilus of the mouse at most septotemporal levels: a limiting subzone, a hilar plexiform layer, and a deep hilar cell mass. Deep to the granule cell layer, AChE activity was high in the limiting subzone and, septally, in the hilar plexiform layer. The deep hilar cell mass stained lightly towards the septal pole of the region but darker at more temporal levels. Numerous AChE-stained cells were seen in the hilus, with the exception of the most temporal levels. A comparative analysis of the AChE pattern of the area dentata reveals that 1) AChE-intense supra- and infragranular bands are found in all mammals, whereas 2) considerable difference between various strains of mice and between species are seen in the stratum moleculare. The functional significance of the AChE pattern is discussed in relation to species differences and connectivity and also with respect to possible activities of the enzyme other than hydrolysis of ACh, which may be involved in growth-related functions and in the plastic and degenerative processes observed in Alzheimer's disease.

Protease inhibitors and indoleamines selectively inhibit cholinesterases in the histopathologic structures of Alzheimer disease

Neurofibrillary tangles and amyloid plaques express acetylcholinesterase and butyrylcholinesterase activity in Alzheimer disease. We previously reported that traditional acetylcholinesterase inhibitors such as BW284C51, tacrine, and physostigmine were more potent inhibitors of the acetylcholinesterase in normal axons and cell bodies than of the acetylcholinesterase in plaques and tangles. We now report that the reverse pattern is seen with indoleamines (such as serotonin and its precursor 5-hydroxytryptophan), carboxypeptidase inhibitor, and the nonspecific protease inhibitor bacitracin. These substances are more potent inhibitors of the cholinesterases in plaques and tangles than of those in normal axons and cell bodies. These results show that the enzymatic properties of plaque and tangle-associated cholinesterases diverge from those of normal axons and cell bodies. The selective susceptibility to bacitracin and carboxypeptidase inhibitor indicates that the catalytic sites of plaque and tangle-bound cholinesterases are more closely associated with peptidase or protease-like properties than the catalytic sites of cholinesterases in normal axons and cell bodies. This shift in enzymatic affinity may lead to the abnormal protein processing that is thought to play a major role in the pathogenesis of Alzheimer disease. The availability of pharmacological and dietary means for altering brain indoleamines raises therapeutic possibilities for inhibiting the abnormal cholinesterase activity associated with Alzheimer disease.

Colocalization of cholinesterases with beta amyloid protein in aged and Alzheimer's brains

The colocalization of beta amyloid protein with the enzymes acetyl- and butyrylcholinesterase was assessed using immunocytochemistry for beta amyloid protein and a sensitive histochemical technique for cholinesterases. In non-demented aged and Alzheimer's disease brains, double-stained sections for cholinesterases and thioflavin-S showed that all thioflavin-S-positive plaques were also positive for cholinesterases, indicating the presence of these enzymes in all plaques with beta-pleated amyloid protein. When amyloid angiopathy was present, cholinesterases were also observed in amyloid-laden vessels walls. Comparison of series of adjacent sections alternatively stained for acetylcholinesterase, beta amyloid protein and butyrylcholinesterase, as well as by double histo-immunocytochemical staining, showed either cholinesterase in a proportion of the preamyloid diffuse plaques. These data indicate that cholinesterases are associated with the amyloid protein from very early stages, when the beta-pleated structure is being formed. Novel functions attributed to acetyl- and butyrylcholinesterase, such us their proteolytic activity either by themselves or in association with heparan sulfate proteoglycans, may play a role in the aggregation or the consolidation processes taking place at the early stages of diffuse plaque formation.

CSF cholinesterase in early-onset and late-onset Alzheimer's disease and multi-infarct dementia of Chinese patients

Using Ellman spectrophotometric method we measured the total cholinesterase (ChE) activity in lumbar cerebrospinal fluid (CSF) of 13 persons without neurological disorder, 10 non-demented patients with cerebral infarcts, 17 patients with dementia of Alzheimer's type (DAT) (11 presenile, 6 senile cases), 10 patients with multi-infarct dementia (MID), 1 patient with Parkinson's disease associated with dementia. The ChE activity in CSF was significantly lower in the DAT group compared with age-matched control subjects (p < 0.001). This paper also analyses the possibility of using CSF ChE activity as a marker of DAT, and the relationships between its level of activity and the age of the patient at onset, stage of illness and severity of dementia as well as discrepancies in the data published so far. Previous work has shown that ChE activity in the brain tissue and CSF of MID is normal: therefore, if low ChE activity is found in the CSF of MID patients, as was obtained in 8 out of 10 cases in our series, the diagnosis of mixed dementia should be considered.

Brain and cerebrospinal fluid cholinesterases in Alzheimer's disease, Parkinson's disease and aging. A critical review of clinical and experimental studies

Acetylcholinesterase (AChE), an enzyme responsible for the break-down of acetylcholine, is found both in cholinergic and non-cholinergic neurons in the central nervous system. In addition to its role in the catabolism of acetylcholine, AChE have other functions in brain, e.g. in the processing of peptides and proteins, and in the modulation of dopaminergic neurons in the brain stem. Several clinical and experimental studies have investigated AChE in brain and cerebrospinal fluid (CSF) in aging and dementia. The results suggest that brain AChE and its molecular forms show interesting changes in dementia and aging. However, CSF-AChE activity is not a very reliable or sensitive marker of the integrity and function of cholinergic neurons in the basal forebrain complex. Additional work is needed to clarify the role of AChE abnormality in the formation of pathology changes in patients with Alzheimer's disease.

Secreted acetylcholinesterase: non-classical aspects of a classical enzyme

Recent evidence suggests that termination of cholinergic transmission is just one of the many ways in which acetylcholinesterase (AChE) could influence neuronal function. Neuronal AChE can be secreted from several brain regions, while purified AChE possesses several properties (in addition to its cholinesterase activity) that can affect neuronal function, including the abilities to influence certain membrane conductances, enhance excitatory amino acid transmission and hydrolyse peptides. Loss of AChE and its non-classical actions would have a profound effect on brain function in neurodegenerative diseases such as Alzheimer's disease where there is widespread loss of AChE-containing neurons.

Acetylcholinesterase and butyrylcholinesterase activities in cerebrospinal fluid from different levels of the neuraxis of patients with dementia of the Alzheimer type

Acetylcholinesterase (AChE) and butyrylcholinesterase (BuChE) activities of cerebrospinal fluid (CSF) collected post mortem from the lateral ventricles, cisterna magna, and lumbar regions of the spinal cord of patients with a histologically confirmed diagnosis of Alzheimer's disease were compared with those of normal, age matched control patients, patients with dementia of non-Alzheimer aetiology, and patients with non-dementing neurological disorders. The AChE activity of the ventricular CSF of patients with Alzheimer's disease was 48% lower (p < 0.005) than that of age matched controls or patients with other types of dementia, and the AChE activity of CSF sampled from the basal cistern was 40% lower (p < 0.005) in patients with Alzheimer's disease. There were no significant differences between the AChE activity in Alzheimer's disease and control patients in CSF collected from the lumbar cistern. AChE activity was lower in CSF sampled from the basal and lumbar cistern of patients with dementia of non-Alzheimer aetiology, while ventricular activity was in the normal range. BuChE activity in ventricular CSF of Alzheimer's disease patients was 41% lower than normal (p < 0.05) and in the normal range in all other samples. The secretion of AChE from forebrain and hindbrain regions is reduced in Alzheimer's disease patients, leading to decreased ventricular and cisternal levels of the enzyme. Secretion from more caudal regions of the central nervous system seems to be unaffected by the disease, resulting in AChE in the lumbar CSF of patients with Alzheimer's disease being in the control range. Such altered secretion of AChE in the brain could have profound implications not only for cholinergic transmission in these patients but also for the proposed noncholinergic modulatory actions of AChE.

Age-related changes in the proportion of amyloid precursor protein mRNAs in Alzheimer's disease and other neurological disorders

In the human brain, alternative splicing of amyloid precursor protein (APP) gene transcript generates at least three types of mRNA coding for APP770, APP751 and APP695. The former two types harbor, but the latter one lacks a domain of Kunitz-type serine protease inhibitor (KPI). We studied, by using the RNase protection technique, the expression of APP mRNAs in brains of Alzheimer's disease (AD) and other neurological disorders with special reference to aging. We found that the ratio of (APP770 mRNA+APP751 mRNA)/APP695 mRNA in the frontal cortex increased approximately 1.5-fold in AD compared with other neurodegenerative or cerebrovascular disorders. The ratio in other neurological disorders did not change significantly from control even in their affected brain regions. On the other hand, we found a positive correlation between the ratio and age; the ratio (y) increased gradually with the advance of age (x) as expressed by y = 0.005x + 0.014 (r = 0.372) for the AD group, and y = 0.004x -0.037 (r = 0.486) for the non-AD group. These correlations indicate that the AD brain reached the same ratio of KPI-harboring to lacking APP mRNAs a few decades earlier than the non-AD brain in senescence. This finding of AD-specific and age-related change led us to the idea that a relative increase in KPI-harboring APPs over a KPI-lacking APP may perturb normal degradation of APPs, thereby leading to deposition of beta A4 protein as amyloid.

Human brain beta A4 amyloid protein precursor of Alzheimer's disease: purification and partial characterization

The major component of the amyloid deposition that characterizes Alzheimer's disease is the 4-kDa beta A4 protein, which is derived from a much larger amyloid protein precursor (APP). A procedure for the complete purification of APP from human brain is described. The same amino terminal sequence of APP was found in two patients with Alzheimer's disease and one control subject. Two major forms of APP were identified in human brain with apparent molecular masses of 100-110 kDa and 120-130 kDa. Soluble and membrane fractions of brain contained nearly equal amounts of APP in both humans and rats. Immunoprecipitation with carboxyl terminus-directed antibodies indicates that the soluble forms of APP are truncated. Carboxyl terminus truncation of membrane-associated forms of human brain APP was also found to occur during postmortem autolysis. The availability of purified human brain APP will facilitate the investigation of its normal function and the events that lead to its abnormal cleavage in patients with Alzheimer's disease.