Alzheimer disease (AD) is characterized by excessive deposition of the (3-amyloid peptide (QAP) in the central nervous system. Although several lines of evidence suggest that 3-AP is neurotoxic, a mechanism for (-AP toxicity in AD brain remains unclear. In this paper we provide both direct in vito evidence that S-AP can bind and activate the classical complement cytolytic pathway in the absence of antibody and indirect in situ evidence that such actions occur in the AD brain in association with areas of AD pathology.Alzheimer disease (AD) is characterized by excessive central nervous system (CNS) deposition of the f-amyloid peptide ((3-AP), a 40-to 42-amino acid peptide derived from a larger amyloid precursor protein (APP) (1-3). Although no specific mechanism of -3-AP deposition has yet been formally proven, there are several lines ofevidence (4-6) that, once generated, 3-AP causes direct or indirect toxicity to CNS neurons. Proposed mechanisms of AD neurotoxicity include membrane changes (7), alterations in Ca2+ homeostasis (6,8), excitotoxicity (5,6), and disruption of cytoskeletal or axon transport systems (9, 10). However, no single AD pathogenetic mechanism has yet achieved a wide consensus of acceptance.In addition to studies of 3-AP, over the last decade a number ofinvestigators have noted that the AD brain exhibits many of the classical markers of immune-mediated damage. These include elevated numbers of major histocompatibility complex class I-and II-immunoreactive microglia (cells believed to be an endogenous CNS form of the peripheral macrophage) (11-15) and astrocytes expressing interleukin 1 (16) and a1-antichymotrypsin (17) (both acute phase reactants). Of particular importance, complement proteins of the classical pathway have been immunohistochemically detected in the AD brain (12,13,(18)(19)(20), and we have noted that they most often appear associated with ,3-AP-containing pathological structures such as senile plaques. Proteins specific to the alternative pathway do not appear to be present (12,13,18). The first step in the classical complement pathway entails binding of the Clq component of C1, with subsequent activation of the Clr and Cls components. This is followed by a complex series of autocatalytic reactions, proceeding through C4, C2, and C3, and culminating in formation of the membrane attack complex (MAC), C5b-9. The MAC inserts a lytic plug in adjacent cell membranes, mediating cellular toxicity (21). Although Clq binding to the Fc region of immunoglobulins is the most common mechanism for initiating the classical pathway, several substratesincluding viruses, parasites, and mannan-binding proteinhave also been demonstrated to activate C1 and to do so in an antibody-independent fashion (22). In this paper we present six converging lines of evidence suggesting that S-AP activates the classical pathway complement cascade without mediation by immunoglobulin. This previously unrecognized mechanism may contribute significantly to the neurotoxicity of (3-AP as well as to the patho...

In the course of analyzing the chemical composition of Alzheimer's disease neuritic and vascular amyloid, we have purified stable dimeric and trimeric components of A␤ peptides. These peptides (molecular mass 9.0 and 13.5 kDa) were separated by size exclusion chromatography in the presence of 80% formic acid or 5 M guanidine thiocyanate, pH 7.4. The average ratio of monomers, dimers, and trimers was 55:30:15, respectively. Similar structures were produced over time upon incubation of synthetic A␤-(1-42) at pH 7.4. The stability of these oligomeric forms was also demonstrated by Western blot and mass spectrometry. Atomic force microscopy and electron microscopy rotary shadowing revealed that the monomers polymerized into 8 -10-nm filaments, whereas the dimers generated prolate ellipsoids measuring 3-4 nm in diameter. Although evidence implicates ␤-amyloid peptide (A␤) in the pathogenesis of Alzheimer's disease (AD) 1 (reviewed in Ref. 1), little is known about the nature of the A␤ mediating the pathology. Toxicity initially was attributed to aggregated A␤ in amyloid plaques (1), the morphological hallmarks of AD brains. A␤-(1-42) is the major peptide constituent of amyloid plaques (2), and increased production of the 42-amino acid peptide correlates with an earlier onset of AD (1). However, recent studies show that small quantities of A␤-(1-42) also exists as soluble peptide in the plasma, cerebrospinal fluid, and cerebral cortex of AD and normal individuals and are also secreted by cells in tissue culture (3-13). Utilizing ultracentrifugation, graded membrane filtration, and ELISA, we have recently isolated and quantitated the oligomeric water-soluble A␤ present in the brains of AD and control individuals (13). The levels of insoluble A␤ in AD brains are at least 100 times higher than those found in control brains. The amounts of water-soluble A␤ in AD brains are approximately six times higher than those detected in control brains. Interestingly, we isolated an A␤ fraction, from the A␤ water-soluble oligomeric pool, with a molecular mass of Ͻ10 kDa containing monomeric and/or dimeric forms of A␤ peptide (13). In all probability these peptides represent the initial building blocks that may ultimately aggregate into insoluble A␤ filaments. In the course of analyzing the chemical composition of AD neuritic plaque and vascular amyloid, we have purified stable dimeric and trimeric components of A␤-(1-40/42) (2, 14 -15). In the present study we report the chemical and morphological characteristics of the dimeric A␤ as elucidated by atomic force microscopy and transmission electron microscopy techniques. In addition, the potential for toxicity of the AD brain-derived A␤-(1-40/42) dimer was assessed on glial-neuronal cell cultures. MATERIALS AND METHODSPurification of A␤-(1-42) from AD Brain-Brains were obtained from eight patients who died of AD (postmortem delay 3-6 h). After separation of the leptomeninges, the right hemispheres were frozen at Ϫ70°C. Examination of the left hemispheres revealed numerous neuritic plaq...

No abstract

No abstract