The cerebrospinal-fluid soluble form of Alzheimer's amyloid beta is complexed to SP-40,40 (apolipoprotein J), an inhibitor of the complement membrane-attack complex

Glycoprotein 330/megalin: probable role in receptor-mediated transport of apolipoprotein J alone and in a complex with Alzheimer disease amyloid beta at the blood-brain and blood-cerebrospinal fluid barriers

A soluble form of Alzheimer disease amyloid beta-protein (sA beta) is transported in the blood and cerebrospinal fluid mainly complexed with apolipoprotein J (apoJ). Using a well-characterized in situ perfused guinea pig brain model, we recently obtained preliminary evidence that apoJ facilitates transport of sA beta (1-40)-apoJ complexes across the blood-brain barrier and the blood-cerebrospinal fluid barrier, but the mechanisms remain poorly understood. In the present study, we examined the transport process in greater detail and investigated the possible role of glycoprotein 330 (gp330)/megalin, a receptor for multiple ligands, including apoJ. High-affinity transport systems with a Km of 0.2 and 0.5 nM were demonstrated for apoJ at the blood-brain barrier and the choroid epithelium in vivo, suggesting a specific receptor-mediated mechanism. The sA beta (1-40)-apoJ complex shared the same transport mechanism and exhibited 2.4- to 10.2-fold higher affinity than apoJ itself. Binding to microvessels, transport into brain parenchyma, and choroidal uptake of both apoJ and sA beta (1-40)-apoJ complexes were markedly inhibited (74-99%) in the presence of a monoclonal antibody to gp330/megalin and were virtually abolished by perfusion with the receptor-associated protein, which blocks binding of all known ligands to gp330. Western blot analysis of cerebral microvessels with the monoclonal antibody to gp330 revealed a protein with a mass identical to that in extracts of kidney membranes enriched with gp330/megalin, but in much lower concentration. The findings suggest that gp330/megalin mediates cellular uptake and transport of apoJ and sA beta (1-40)-apoJ complex at the cerebral vascular endothelium and choroid epithelium.

Acetylcholinesterase accelerates assembly of amyloid-beta-peptides into Alzheimer's fibrils: possible role of the peripheral site of the enzyme

Acetylcholinesterase (AChE), an important component of cholinergic synapses, colocalizes with amyloid-beta peptide (A beta) deposits of Alzheimer's brain. We report here that bovine brain AChE, as well as the human and mouse recombinant enzyme, accelerates amyloid formation from wild-type A beta and a mutant A beta peptide, which alone produces few amyloid-like fibrils. The action of AChE was independent of the subunit array of the enzyme, was not affected by edrophonium, an active site inhibitor, but it was affected by propidium, a peripheral anionic binding site ligand. Butyrylcholinesterase, an enzyme that lacks the peripheral site, did not affect amyloid formation. Furthermore, AChE is a potent amyloid-promoting factor when compared with other A beta-associated proteins. Thus, in addition to its role in cholinergic synapses, AChE may function by accelerating A beta formation and could play a role during amyloid deposition in Alzheimer's brain.

Transthyretin gene in Alzheimer's disease patients

Amyloid beta (Abeta) is known to be the main component of Alzheimer's disease (AD) senile plaques. A homologous peptide is a normal component of biological fluids and is called soluble Abeta (sAbeta). Synthetic peptides homologous to Abeta form amyloid-like fibrils in vitro. This fibril formation can be inhibited by normal human cerebrospinal fluid (CSF) [Wisniewski et al., Ann. Neurol. 34 (1993)]. Furthermore, it has been proposed that normal transthyretin (TTR), which is a component of CSF, can itself bind sAbeta, preventing amyloid fibril formation, and that variants of TTR could be associated with AD [Schwarzman et al., Proc. Natl. Acad. Sci. USA, 91 (1994)]. Because of this possible association, we screened for TTR mutations from 47 sporadic and 19 early-onset familial AD patients using single strand conformation polymorphism analysis. Our results show no correlation between TTR variants and Alzheimer's disease in this group of patients.

Apolipoproteins E3 and E4 induce, and transthyretin prevents accumulation of the Alzheimer's beta-amyloid peptide in cultured vascular smooth muscle cells

Cultured brain vascular smooth muscle cells (SMCs) accumulate beta-peptide in intracytoplasmic granules [9,10,33]. We show here that apoE3 and E4 induces the intracytoplasmic beta-peptide accumulation in cultured human and canine SMCs. The induction is dose-dependent and the accumulated granules also contain apoE and some were thioflavine S-positive. The deposits induced with apoE3 were more abundant though less stable than with apoE4. Transthyretin at physiological concentrations blocked the effects of apoE3/E4. Thus, accumulation of beta-peptide appears to be regulated by beta-peptide carrier proteins.

The toxicity in vitro of beta-amyloid protein

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NACP, the precursor protein of the non-amyloid beta/A4 protein (A beta) component of Alzheimer disease amyloid, binds A beta and stimulates A beta aggregation

NACP, a 140-amino acid presynaptic protein, is the precursor of NAC [the non-amyloid beta/A4 protein (A beta) component of Alzheimer disease (AD) amyloid], a peptide isolated from and immunologically localized to brain amyloid of patients afflicted with AD. NACP produced in Escherichia coli bound to A beta peptides, the major component of AD amyloid. NACP bound to A beta 1-38 and A beta 25-35 immobilized on nitrocellulose but did not bind to A beta 1-28 on the filter under the same conditions. NACP binding to A beta 1-38 was abolished by addition of A beta 25-35 but not by A beta 1-28, suggesting that the hydrophobic region of the A beta peptide is critical to this binding. NACP-112, a shorter splice variant of NACP containing the NAC sequence, bound to A beta, but NACP delta, a deletion mutant of NACP lacking the NAC domain, did not bind A beta 1-38. Furthermore, binding between NACP-112 and A beta 1-38 was decreased by addition of peptide Y, a peptide that covers the last 15 residues of NAC. In an aqueous solution, A beta 1-38 aggregation was observed when NACP was also present in an incubation mixture at a ratio of 1:125 (NACP/A beta), whereas A beta 1-38 alone or NACP alone did not aggregate under the same conditions, suggesting that the formation of a complex between A beta and NACP may promote aggregation of A beta. Thus, NACP can bind A beta peptides through the specific sequence and can promote A beta aggregation, raising the possibility that NACP may play a role in the development of AD amyloid.

Apolipoprotein E increases the fibrillogenic potential of synthetic peptides derived from Alzheimer's, gelsolin and AA amyloids

Apolipoprotein E (apoE) has been found in association with several different types of systemic and cerebral amyloid deposits and the presence of the epsilon 4 allele constitutes a risk factor for Alzheimer's disease. It has been shown that apoE binds and promotes the fibrillogenesis in vitro of Alzheimer's amyloid beta-peptide, suggesting an important role for apoE in the modulation of amyloidogenesis. Due to the co-localization of apoE with several biochemically distinct amyloid deposits, it has been proposed that apoE plays a general role modulating and/or participating in amyloidosis. In the present study, we show for the first time that apoE, isolated from human plasma, increases fibril formation of synthetic peptides comprising the amyloidogenic sequences of gelsolin amyloid related to familial amyloidosis Finnish type, and amyloid A found in secondary amyloidosis and familial Mediterranean fever. Our results suggest that apoE acts as a general pathological chaperone in various amyloidoses by enhancing the transition from soluble peptides into amyloid-forming, pathological molecules.

Distribution of beta amyloid associated proteins in plaques in Alzheimer's disease and in the non-demented elderly

Recent studies have shown that cerebral beta amyloid (A beta) protein deposition is a necessary, but not sufficient, factor to develop the pathology of Alzheimer's disease (AD). In the present immunohistochemical study, we have investigated in AD the distribution of A beta associated proteins in the cerebral neocortex, in the cerebellar cortex where A beta plaques are mainly of the diffuse type, and also in the cerebral neocortex of non-demented patients with A beta plaques. Results show that immunolabeling for C1q, C4c, C3d, alpha 1-ACT and Apolipoprotein E (ApoE) occurs in the great majority of A beta plaques in all groups. ApoJ is present in A beta plaques of the cerebral neocortex in AD and in non-demented elderly, but is almost absent from those of the AD cerebellar cortex. C4Bp and P-component, in contrast to AD, rarely occurs in A beta plaques of the cerebral neocortex in the non-demented elderly. Heparan sulphate proteoglycan (HSPG) core protein and intercellular adhesion molecule-1 (ICAM-1) are absent in the diffuse A beta plaques in the AD cerebellum. These differences in distribution and expression of A beta associated proteins may be determined by brain region specific factors (cerebral cortex versus cerebellar cortex) and clinical state (demented versus non-demented cases). We suggest that, besides A beta peptide, certain A beta associated proteins are required for both amyloid plaque formation and for the induction of neurofibrillary changes.

Soluble multimeric Alzheimer beta(1-40) pre-amyloid complexes in dilute solution

Aqueous solutions of beta(1-40) peptide spontaneously associate to form pentameric/hexameric complexes that can be demonstrated by SDS-PAGE following treatment with glutaraldehyde and borohydride reduction. Under amyloidogenic conditions of pH and high peptide concentration these aggregates can further associate to form pentameric/hexameric complexes that can be demonstrated by SDS-PAGE following treatment with glutaraldehyde and borohydride reduction. Under amyloidogenic conditions of pH and high peptide concentration these aggregates can further associate to form sedimentable and filterable structures with beta-sheet amyloid characteristics of Thioflavine T fluorescence. The presence of such preamyloid structures at low peptide concentration suggests a mechanism by which amyloid plaques can accrete additional material by a cooperative rather than monomeric growth. The existence of a monomer<==>multimer equilibrium may partly explain the divergence of biological consequences with respect to neurotoxicity.

Conformational mimicry in Alzheimer's disease. Role of apolipoproteins in amyloidogenesis

Several apolipoproteins are known to be closely associated with amyloid fibrillogenesis. Serum amyloid A, apolipoprotein (apo) AII and apo A1 are each deposited as biochemically distinct forms of amyloid. Late-onset Alzheimer's disease is linked to one isotype of apo E, apo E4. Apo E and apo E4 in particular have been shown to modulate amyloid fibril formation by amyloid-beta peptides in vitro. Furthermore, the carboxy terminus of apo E has been shown to be a constituent of plaque amyloid. We show immunohistochemically and electron microscopically the presence of apo A1 in senile plaques. The intact apo A1 can itself form amyloid-like fibrils in vitro that are Congo Red positive. We propose that some proteins when misfolded can propagate this misfolding to identical units, either autocatalytically or to other proteins that are induced to fold into the same abnormal conformation. This conformational mimicry may initiate and/or augment fibrillogenesis in Alzheimer's disease.

Apolipoprotein E carboxyl-terminal fragments are complexed to amyloids A and L. Implications for amyloidogenesis and Alzheimer's disease

Apolipoprotein E (ApoE) immunoreactivity is consistently present in the senile plaques and neurofibrillary tangles of Alzheimer's disease (AD) brain. In vitro, apoE, and in particular its apoE4 isoform, can bind to and promote fibrillogenesis of the amyloid A beta peptide, the main constituent of senile plaques. These findings, together with the strong genetic association between late onset AD and the E4 allele of apoE, have strengthened the hypothesis that apoE may have a central role in the pathogenesis of AD by modulating A beta cerebral accumulation. However, apoE immunoreactivity is present in all cerebral and systemic amyloidoses tested, and tryptic apoE fragments have been identified in association with amyloid A (AA). In order to further elucidate the interaction between apoE and amyloids, we purified AA and amyloid L (AL) fibrils from patients with familial Mediterranean fever and primary amyloidosis, respectively, and studied the association of apoE with AA and AL proteins. In each case, apoE fragments, detected by Western blot, co-purified with the amyloid fibrils. Microsequencing analysis identified COOH-terminal fragments of apoE, similar to the 10-kDa fragment produced by thrombin digestion that contains the purported binding region to A beta. In vitro co-incubation of AA with purified human apoE resulted in the formation of an SDS-resistant AA.apoE complex and a higher degree of polymerization of the AA peptide. These findings and similar results obtained from AD senile plaques suggest that 1) the carboxyl-terminal fragment of apoE is complexed to amyloid fibrils and resists proteolysis in vivo and 2) apoE may promote amyloidogenesis through a conformation-dependent interaction regardless of the primary structure of the amyloid precursors.

Deposition of apolipoproteins E and J in senile plaques is topographically determined in both Alzheimer's disease and Down's syndrome brain

The link between the immunolocalization of apolipoproteins E (apo E) and J (apo J) and the different severity of beta-amyloid deposition in various areas of Alzheimer's disease (AD) and Down's syndrome (DS) brain was analyzed. Both apolipoproteins were found in all types of senile plaques (SPs) in the cerebral cortex, which is early and severely involved in beta-amyloidosis, but apo E was seen more often than apo J in diffuse A beta deposits, especially in young DS cases and nondemented elderly persons. In the striatum and cerebellum, which show predominance of diffuse A beta deposits throughout the lifespan, apo J was absent, except for few compact deposits, whereas apo E was more widely distributed, apart from diffuse plaques in the striatum. By immunoelectron microscopy, A beta fibrils were disclosed in diffuse plaques in all brain regions studied, but not all of these early fibrillar deposits, even in the neocortex of young DS cases, showed apo E and apo J labeling. Thus, our data indicate that the immunoreactivity to apo E and J within A beta deposits is topographically determined in both AD and DS brain. Moreover, although it appears that neither of apolipoproteins studied is necessary to initiate A beta fibrillogenesis, disclosed topographic dissimilarities of their distribution within parenchymal A beta deposits suggest that they may be involved in different ways in the pathogenesis of beta-amyloidosis.

Clusterin: physiologic and pathophysiologic considerations

Clusterin is a heterodimeric glycoprotein produced by a wide array of tissues and found in most biologic fluids. A number of physiologic functions have been proposed for clusterin based on its distribution and in vitro properties. These include complement regulation, lipid transport, sperm maturation, initiation of apoptosis, endocrine secretion, membrane protection, and promotion of cell interactions. A prominent and defining feature of clusterin is its induction in such disease states as glomerulonephritis, polycystic kidney disease, renal tubular injury, neurodegenerative conditions including Alzheimer's disease, atherosclerosis, and myocardial infarction. The expression of clusterin in these states is puzzling, from the specific molecular species and cellular pathways eliciting such expression, to the roles subserved by clusterin once induced. This review will discuss these physiologic and pathophysiologic aspects of clusterin and speculate on its role in disease.

Identification of glycoprotein 330 as an endocytic receptor for apolipoprotein J/clusterin

Glycoprotein 330 (gp330) is a member of a family of endocytic receptors related to the low density lipoprotein receptor. gp330 has previously been shown to bind a number of ligands in common with its family member, the low density lipoprotein receptor-related protein (LRP). To identify ligands specific for gp330 and relevant to its localization on epithelia such as in the mammary gland, gp330-Sepharose affinity chromatography was performed. As a result, a 70-kDa protein was selected from human milk and identified by protein sequencing to be apolipoprotein J/clusterin (apoJ). Solid-phase binding assays confirmed that gp330 bound to apoJ with high affinity (Kd = 14.2 nM). Similarly, gp330 bound to apoJ transferred to nitrocellulose after SDS-polyacrylamide gel electrophoresis. LRP, however, showed no binding to apoJ in either type of assay. The binding of gp330 to apoJ could be competitively inhibited with excess apoJ as well as with the gp330 ligands apolipoprotein E, lipoprotein lipase, and the receptor-associated protein, a 39-kDa protein that acts to antagonize binding of all known ligands for gp330 and LRP. Several cultured cell lines that express gp330 and ones that do not express the receptor were examined for their ability to bind and internalize 125I-apoJ. Only cells that expressed gp330 endocytosed and degraded radiolabeled apoJ. Furthermore, F9 cells treated with retinoic acid and dibutyryl cyclic AMP to increase expression levels of gp330 displayed an increased capacity to internalize and degrade apoJ. Cellular internalization and degradation of radiolabeled apoJ could be inhibited with unlabeled apoJ, receptor-associated protein, and gp330 antibodies. The results indicate that gp330 but not LRP can bind to apoJ in vitro and that gp330 expressed by cells can mediate apoJ endocytosis leading to lysosomal degradation.

Apolipoprotein E and Alzheimer disease

Inheritance of specific apolipoprotein E (apoE) alleles determines, in large part, the risk and mean age of onset of late-onset familial and sporadic Alzheimer disease. The mechanism by which the apoE isoforms differentially contribute to disease expression is, however, unknown. Isoform-specific differences have been identified in the binding of apoE to the microtubule-associated protein tau, which forms the paired helical filament and neurofibrillary tangles, and to amyloid beta peptide, a major component of the neuritic plaque. These and other isoform-specific interactions of apoE give rise to testable hypotheses for the mechanism(s) of pathogenesis of Alzheimer disease. An unresolved issue of increasing importance is the relationship between the structural pathological lesions and the cellular pathogenesis responsible for the clinical disease phenotype, progressive dementia. The identification of apoE in the cytoplasm of human neurons and the characterization of isoform-specific binding of apoE to the microtubule-associated proteins tau and MAP-2 present the possibility that apoE may affect microtubule function in the Alzheimer brain.

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

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

Purification of apolipoprotein E attenuates isoform-specific binding to beta-amyloid

Apolipoprotein E (apoE), particularly the e4 allele, is genetically linked to the incidence of Alzheimer's disease. In vitro, apoE has been shown to bind beta-amyloid (A beta), an amyloidogenic peptide that aggregates to form the primary component of senile plaques. In previous work, we demonstrated that apoE3 from tissue culture medium binds to A beta with greater avidity than apoE4 (LaDu, M. J., Falduto, M. T., Manelli, A. M., Reardon, C. A., Getz, G. S., and Frail, D. E. (1994) J. Biol. Chem. 269, 23403-23406). This is in contrast to data using purified apoE isoforms as substrate for A beta (Strittmatter, W. J., Weisgraber, K. H., Huang, D. Y., Dong, L.-M., Salvesen, G. S., Pericak-Vance, M., Schmechel, D., Saunders, A. M., Goldgaber, D., and Roses, A. D. (1993) Proc. Natl. Acad. Sci. U. S. A. 90, 8098-8102). Here we resolve this apparent discrepancy by demonstrating that the preferential binding of A beta to apoE3 is attenuated and even abolished with purification, a process that includes delipidation and denaturation. We compared the A beta binding capacity of unpurified apoE isoforms from both tissue culture medium and intact human very low density lipoproteins with that of apoE purified from these two sources. The interaction of human A beta-(1-40)-peptide and apoE was analyzed by nonreducing SDS-polyacrylamide gel electrophoresis followed by Western immunoblotting for either A beta or apoE immunoreactivity. While the level of the apoE3.A beta complex was approximately 20-fold greater compared with the apoE4.A beta complex in unpurified conditioned medium, apoE3 and apoE4 purified from this medium bound to A beta with comparable avidity. Moreover, using endogenous apoE on very low density lipoproteins from plasma of apoE3/3 and apoE4/4 homozygotes, apoE3 was again a better substrate for A beta than apoE4. However, apoE purified from these plasma lipoproteins exhibited little isoform specificity in binding to A beta. These results suggest that native preparations of apoE may be a more physiologically relevant substrate for A beta binding than purified apoE and further underscore the importance of subtle differences in apoE conformation to its biological activity.

Secretion and accumulation of Alzheimer's beta-protein by cultured vascular smooth muscle cells from old and young dogs

Cultured smooth muscle cells isolated from beta-amyloid-affected blood vessels from old dogs accumulate beta-protein at early passages [5,24]. Now, we show that smooth muscle cells derived from amyloid-free brain blood vessels and peripheral arteries from old and young animals are induced by culture conditions to deposit intracellularly fibrillar and non-fibrillar beta-protein. Accumulation of beta-protein is associated with a higher secretion of beta-protein, but not with a higher secretion of beta-amyloid precursor protein (beta APP) or higher cellular content of beta APP. Gradual cessation of proliferative activity was observed in cultures that accumulate beta-protein.

Characterization of apolipoprotein J-Alzheimer's A beta interaction

The main component of Alzheimer's amyloid deposits, A beta, has been found also as a soluble (sA beta) normal constituent of biological fluids and cell culture supernatants. Whether or not sA beta is the immediate precursor of A beta, it is clear that peptides with the same amino acid sequence can have both fibrillar and non-fibrillar conformations. The interconversion mechanism from one form to another is presently under intensive investigation. We have previously described that (i) a synthetic peptide A beta 1-40 immobilized on affinity matrices was able to retrieve apolipoprotein J (apoJ) from plasma and cerebrospinal fluid; and (ii) the interaction of sA beta with apoJ occurs in vivo, as demonstrated by the ability of anti-apoJ to co-precipitate sA beta from normal cerebrospinal fluid. We have characterized the binding between A beta 1-40 and apoJ and found that the interaction is saturable, specific, and reversible. The dissociation constant of 2 x 10(-9) M is indicative of high affinity binding. The stoichiometry of the reaction is 1:1; apoJ has five times more affinity for fresh A beta 1-40 than for the aggregated peptide. Competitive inhibition studies carried out with apolipoprotein E (isoforms E2, E3, and E4), transthyretin, vitronectin, and alpha 1-antichymotrypsin indicate that the complex apoJ.A beta 1-40 cannot be dissociated by any of these competitors at physiologic concentrations. The data strongly suggest that apoJ plays an important role as a carrier protein for sA beta.

beta-Amyloid polypeptide increases calcium-uptake in PC12 cells: a possible mechanism for its cellular toxicity in Alzheimer's disease

Calcium-uptake into PC12 cells was measured by incubation with 45Ca after the cells were exposed for 24 h to beta-amyloid peptide(1-40) at concentrations between 0 and 46 microM. The rate of influx of 45Ca into PC12 cells was constant for the first 10 min. For 46 microM beta-amyloid peptide(1-40), the rate of influx was about 1,300 ions/s/microns 2 and the number of cells decreased significantly. There was no significant decrease in cell number when cells were exposed to beta-amyloid in calcium-free medium. These results indicate that beta-amyloid increases calcium uptake into PC12 cells, and suggest that the increased uptake is responsible for the toxicity of beta-amyloid in PC12 cells.

Amyloid beta protein levels in cerebrospinal fluid are elevated in early-onset Alzheimer's disease

The 4-kd amyloid beta protein (A beta) deposited as amyloid in Alzheimer's disease (AD) is produced and released by normal proteolytic processing of the amyloid beta protein precursor (beta APP) and is readily detected in cerebrospinal fluid (CSF). Here, we present the levels of A beta in CSF from a total of 95 subjects, including 38 patients with AD, 14 with early-onset AD and 24 with late-onset AD, 25 normal control subjects, and 32 patients with other neurological diseases. The level of A beta decreased with normal aging, and there was a significant elevation in the level of A beta in the CSF of early-onset AD patients (4.14 +/- 1.37 pmol/ml, p < 0.01). Neither Mini-Mental State nor Functional Assessment Staging were correlated with the amount of A beta in the CSF. The A beta/secreted form of beta APP ratio was elevated, but the level of alpha 1-antichymotrypsin in the CSF did not correlate with the level of CSF A beta in early-onset AD patients. Thus, the level of A beta in the CSF is elevated in early-onset AD patients and is suggested to be correlated with the pathology in the brain that characterizes AD.

Potential role of apolipoprotein-E in fibrillogenesis

Immunohistochemical and biochemical studies have demonstrated several different proteins in amyloid deposits that are not intrinsic components of the fibril itself but may play a role in their deposition and fibril formation. We compared the distribution of several amyloid-associated proteins, ie, amyloid P component, apolipoprotein-E, apolipoprotein-J, and vitronectin, in the deposits of several different amyloids, in particular light chain amyloid, with those in the deposits of nonamyloid monoclonal immunoglobulin, which may be considered a form of preamyloid disease. Although 100% of amyloid specimens (7 amyloid A, 15 immunoglobulin light chain, and 1 transthyretin) had amyloid P component and 100% had apolipoprotein-E (2 amyloid A, 10 immunoglobulin light chain, and 1 transthyretin) co-localized with the primary amyloid protein, none of the monoclonal nonamyloid cases (14 light chain deposition disease and 6 light and heavy chain deposition disease) had amyloid P component and only 1 of 11 had apolipoprotein-E. On the other hand, staining for apolipoprotein-J and vitronectin was positive in 100% of cases of amyloid and nonamyloid monoclonal deposits. The association between the presence of apolipoprotein-E and amyloid P component in the fibrillar form of monoclonal light chain deposits and their absence in the nonfibrillar form of deposits suggest a role for these proteins in the process of fibrillogenesis. This lends support for the previously proposed concept that apolipoprotein-E functions as a pathological chaperone by altering the conformation of amyloidogenic proteins.

Transthyretin sequesters amyloid beta protein and prevents amyloid formation

The cardinal pathological features of Alzheimer disease are depositions of aggregated amyloid beta protein (A beta) in the brain and cerebrovasculature. However, the A beta is found in a soluble form in cerebrospinal fluid in healthy individuals and patients with Alzheimer disease. We postulate that sequestration of A beta precludes amyloid formation. Failure to sequester A beta in Alzheimer disease may result in amyloidosis. When we added A beta to cerebrospinal fluid of patients and controls it was rapidly sequestered into stable complexes with transthyretin. Complexes with apolipoprotein E, which has been shown to bind A beta in vitro, were not observed in cerebrospinal fluid. Additional in vitro studies showed that both purified transthyretin and apolipoprotein E prevent amyloid formation.

Apolipoprotein E associates with beta amyloid peptide of Alzheimer's disease to form novel monofibrils. Isoform apoE4 associates more efficiently than apoE3

Late-onset and sporadic Alzheimer's disease are associated with the apolipoprotein E (apoE) type 4 allele expressing the protein isoform apoE4. Apolipoprotein E binds avidly to beta amyloid (A beta) peptide, a major component of senile plaque of Alzheimer's disease, in an isoform-specific manner. The apoE4 isoform binds to A beta peptide more rapidly than apoE3. We observed that soluble SDS-stable complexes of apoE3 or apoE4, formed by coincubation with A beta peptide, precipitated after several days of incubation at 37 degrees C with apoE4 complexes precipitating more rapidly than apoE3 complexes. A beta(1-28) and A beta(1-40) peptides were incubated in the presence or absence of apoE3, apoE4, or bovine serum albumin for 4 d at 37 degrees C (pH 7.3). Negative stain electron microscopy revealed that the A beta peptide alone self-assembled into twisted ribbons containing two or three strands but occasionally into multistranded sheets. The apoE/A beta coincubates yielded monofibrils 7 nm in diameter. ApoE4/A beta coincubates yielded a denser matrix of monofibrils than apoE3/A beta coincubates. Unlike purely monofibrillar apoE4/A beta coincubates, apoE3/A beta coincubates also contained double- and triple-stranded structures. Both apoE isoforms were shown by immunogold labeling to be uniformly distributed along the A beta peptide monofibrils. Monofibrils appeared earlier in apoE4/A beta than in apoE3/A beta in time-course experiments. Thus apoE3 and apoE4 each interact with beta amyloid peptide to form novel monofibrillar structures, apoE4 more avidly, a finding consistent with the biochemical and genetic association between apoE4 and Alzheimer's disease.

Alzheimer's disease and soluble A beta

The discovery of soluble amyloid beta (sA beta) suggests that the role of amyloid in Alzheimer's disease (AD) is similar to the previously studied systemic amyloidoses and alters the notion that membrane damage is the initial event in AD. The disease state is characterized by the abnormal accumulation of a normal degradative peptide, which becomes resistant to further proteolysis due to a conformational change. Mutations in the beta PP gene have been found in a very small percentage of AD cases; hence other factors, both genetic and environmental, need to be identified. Priority needs to be given to detailed studies of the structural differences between sA beta and the A beta in amyloid deposits. This will help uncover the determining factors governing the aggregation of sA beta. These structural alterations may be critical for the possible toxic effects A beta and/or associated proteins (molecular chaperones, e.g., apolipoprotein E) have on brain cell function.

Unifying features of systemic and cerebral amyloidosis

Amyloidosis is a generic term for a group of clinically and biochemically diverse diseases that are characterized by the deposition of an insoluble fibrillar protein in the extracellular space. Over 16 biochemically distinct amyloids are known. Despite this diversity, all amyloids have a particular ultrastructural and tinctorial appearance, a beta-pleated sheet structure, and are codeposited with a group of amyloid-associated proteins. The most common amyloidosis is Alzheimer's disease (AD), where A beta is the main component of the amyloid. Recently it has been found that A beta exists as a normal soluble protein (sA beta) in biological fluids. This links AD more closely to some of the systemic amyloidoses, where the amyloid precursor is found in the circulation normally. Numerous mutations have been found in the A beta precursor (beta PP) gene, associated with familial AD. Many mutations are also found in some of the hereditary systemic amyloidoses. For example, over 40 mutations in the transthyretin (TTR) gene are associated with amyloid. However, both A beta and TTR related amyloid deposition can occur with no mutation. The pathogenesis of amyloid is complex, and appears to be associated with genetic and environmental risk factors that can be similar in the systemic and cerebral amyloidoses.

Cerebrospinal fluid inhibits Alzheimer beta-amyloid fibril formation in vitro

Alzheimer's disease is characterized by the deposition of beta-protein (A beta) as amyloid. Recently, it was found that A beta is a normal component of serum and cerebrospinal fluid. Synthetic peptides homologous to A beta form amyloid-like fibrils spontaneously in water or physiological solutions. Using a peptide homologous to A beta 1-40, we find that fibril formation is inhibited by the presence of cerebrospinal fluid.