Enhanced cytotoxicity of amyloid beta-peptide by a complement dependent mechanism

Plaque complement activation and cognitive loss in Alzheimer's disease

BACKGROUND

Complement activation is increased in Alzheimer's disease (AD), but its significance is unclear. The objective of this study was to determine the relationship between complement activation and cognition during the development of AD.

METHODS

iC3b, C9, Bielschowsky, and Gallyas staining was performed on aged normal (n = 17), mild cognitively impaired (n = 12), and AD (n = 17-18) inferior temporal gyrus specimens. Plaques were counted in 10x fields with high numbers of Bielschowsky-stained plaques. One-way ANOVA was used to determine between-group differences for plaque counts and measures of cognitive function, and linear regression was used to evaluate global cognition as a function of Bielschowsky-stained plaques. Terms for iC3b- and C9-stained plaques were then added sequentially as additional predictors in a "mediation analysis" model.

RESULTS

Complement was detected on plaques in all groups, and on neurofibrillary tangles only in AD specimens. iC3b, C9, and Bielschowsky-stained plaque counts increased 2.5- to 3-fold in AD vs. other groups (all p < or = 0.01). C9 staining was present on some diffuse plaques, as well as on neuritic plaques. Bielschowsky-stained and complement-stained plaque counts were highly correlated, and were negatively correlated with cognitive measures. When the Bielschowsky plaque count was used as a predictor, its correlations with cognitive measures were statistically significant, but when iC3b and C9 plaque counts were added as additional predictors, these correlations were no longer significant. This loss of significance was attributed to multicollinearity, i.e., high correlations between Bielschowsky-stained and complement-stained plaque counts.

CONCLUSION

Both early-stage (iC3b) and late-stage (C9) complement activation occurs on neocortical plaques in subjects across the cognitive spectrum; contrary to previous reports, C9 is present on some diffuse plaques. Because of high correlations between complement-stained and Bielschowsky-stained plaque counts, quantitative assessment of the extent to which complement activation may mediate the relationship between plaques and cognitive function could not be performed. Additional studies with animal models of AD (if late-stage complement activation can be demonstrated), or possibly a trial in AD patients with an inhibitor of late-stage complement activation, may be necessary to determine the significance of this process in AD.

Using animal models to determine the significance of complement activation in Alzheimer's disease

Complement inflammation is a major inflammatory mechanism whose function is to promote the removal of microorganisms and the processing of immune complexes. Numerous studies have provided evidence for an increase in this process in areas of pathology in the Alzheimer's disease (AD) brain. Because complement activation proteins have been demonstrated in vitro to exert both neuroprotective and neurotoxic effects, the significance of this process in the development and progression of AD is unclear. Studies in animal models of AD, in which brain complement activation can be experimentally altered, should be of value for clarifying this issue. However, surprisingly little is known about complement activation in the transgenic animal models that are popular for studying this disorder. An optimal animal model for studying the significance of complement activation on Alzheimer's – related neuropathology should have complete complement activation associated with senile plaques, neurofibrillary tangles (if present), and dystrophic neurites. Other desirable features include both classical and alternative pathway activation, increased neuronal synthesis of native complement proteins, and evidence for an increase in complement activation prior to the development of extensive pathology. In order to determine the suitability of different animal models for studying the role of complement activation in AD, the extent of complement activation and its association with neuropathology in these models must be understood.

Early complement activation increases in the brain in some aged normal subjects

Complement activation is increased in Alzheimer's disease (AD) and may contribute to the development and progression of this disorder. To compare early complement activation between normal and AD brain specimens, C4d and iC3b concentrations were measured in hippocampus, entorhinal cortex, temporal cortex, parietal cortex, and cerebellum from aged normal and AD subjects n=10-14 for both), and in hippocampus and entorhinal cortex from younger normal subjects (n=5-6). C4d and iC3b levels increased 2.3- to 4.6-fold in AD versus aged normal specimens (all P <0.05), with lowest concentrations of these activation proteins generally in cerebellum. No significant differences were present between aged and younger normal C4d and iC3b levels in hippocampus or entorhinal cortex. However, the concentrations of these proteins were markedly increased in several aged normal specimens. Normal subject age was moderately associated with both C4d (r=0.49) and iC3b (r=0.53) concentrations in the hippocampus. Increased brain complement activation in some elderly individuals may promote the subsequent development of AD.

Complement activation in chromosome 13 dementias. Similarities with Alzheimer's disease

Chromosome 13 dementias, familial British dementia (FBD) and familial Danish dementia (FDD), are associated with neurodegeneration and cerebrovascular amyloidosis, with striking neuropathological similarities to Alzheimer's disease (AD). Despite the structural differences among the amyloid subunits (ABri in FBD, ADan in FDD, and Abeta in AD), these disorders are all characterized by the presence of neurofibrillary tangles and parenchymal and vascular amyloid deposits co-localizing with markers of glial activation, suggestive of local inflammation. Proteins of the complement system and their pro-inflammatory activation products are among the inflammation markers associated with AD lesions. Immunohistochemistry of FBD and FDD brain sections demonstrated the presence of complement activation components of the classical and alternative pathways as well as the neo-epitope of the membrane attack complex. Hemolytic experiments and enzyme-linked immunosorbent assays specific for the activation products iC3b, C4d, Bb, and C5b-9 indicated that ABri and ADan are able to fully activate the complement cascade at levels comparable to those generated by Abeta1-42. ABri and ADan specifically bound C1q with high affinity and formed stable complexes in physiological conditions. Activation proceeds approximately 70-75% through the classical pathway while only approximately 25-30% seems to occur through the alternative pathway. The data suggest that the chronic inflammatory response generated by the amyloid peptides in vivo might be a contributing factor for the pathogenesis of FBD and FDD and, in more general terms, to other neurodegenerative conditions.

Alzheimer disease: current therapy and future therapeutic strategies

Antidementia drugs can be defined as drugs that significantly improve the decreased cognitive functions and/or inhibit the progression of dementia, compared with placebo. The main target of antidementia drugs is Alzheimer disease (AD), and the advent of such drugs is ardently desired. Antidementia drugs that are currently in use or undergoing trial are briefly reviewed. To date, only a few acetylcholine esterase inhibitors have been licensed as antidementia drugs for AD, but more beneficial drugs are being actively sought by many different approaches. The development of additional drugs requires greater basic research on the pathogenesis of AD. Future therapeutic strategies for AD on the basis of recent findings concerning the pathogenesis of AD are also reviewed.

Detection of complement alternative pathway mRNA and proteins in the Alzheimer's disease brain

Previous research on complement activation in the Alzheimer's disease (AD) brain has focused almost exclusively on the classical complement pathway. The alternative pathway represents another important arm for complement activation, converging with the classical cascade at the C5 cleavage step. Here, we show that mRNA for a critical alternative pathway component, factor B, is present in AD frontal cortex and that the factor D cleaved split products of factor B, Bb and Ba, are significantly increased, indicating alternative pathway activation. By contrast, the two major inhibitors of alternative pathway activation, factor H and factor I, are present at the level of mRNA and protein but are not significantly upregulated. Immunohistochemical analysis reveals significant positive staining in AD sections for all three components. Taken together with previous reports demonstrating alternative pathway activation by amyloid beta peptide, these findings suggest that conditions conducive to chronic alternative pathway activation may exist in the AD brain.

Complement activation in acquired and hereditary amyloid neuropathy

The pathogenesis of the axonal degeneration in acquired or hereditary amyloidosis is unknown. In this immunohistochemistry study, we examined 20 sural nerve biopsies from individuals with amyloid neuropathy (14 acquired and 6 hereditary) for evidence of complement activation. Complement activation products were detected on and around amyloid deposits within peripheral nerves. We found no difference in the extent, location or pattern of complement activation products between the 2 forms of amyloidosis. The presence of early classical pathway activation markers in the absence of antibody in hereditary cases suggests an antibody-independent activation of the classical pathway through binding of C1q. The lack of Factor Bb-suggested alternative pathway activation was not significant in these cases. The detection of C5b-9 neoantigen on amyloid deposits demonstrated that the full complement cascade was activated. Complement activation on amyloid deposits and the generation of C5b-9 in vivo may contribute to bystander injury of axons in the vicinity of amyloid deposits.

The role of complement in Alzheimer's disease pathology

Complement proteins are integral components of amyloid plaques and cerebral vascular amyloid in Alzheimer brains. They can be found at the earliest stages of amyloid deposition and their activation coincides with the clinical expression of Alzheimer's dementia. This review will examine the origins of complement in the brain and the role of beta-amyloid peptide (Abeta) in complement activation in Alzheimer's disease, an event that might serve as a nidus of chronic inflammation. Pharmacology therapies that may serve to inhibit Abeta-mediated complement activation will also be discussed.

Induced expression of neuronal membrane attack complex and cell death by Alzheimer's beta-amyloid peptide

beta-amyloid peptide (A beta) and complement-derived membrane attack complex (MAC) are co-localized in senile plaques of brains from Alzheimer's disease (AD) patients. But the relationship between A beta and complement activation is unclear. We have used human neurotypic cells, differentiated SH-SY5Y, as a model system to examine regulation of neuronal MAC expression and cell death by A beta. We demonstrated that mRNAs (C1q, C2, C3, C4, C5, C6, C7, C8 and C9) and proteins (C1q, C3 and C9) for the major components of the classical complement cascade are present in the SH-SY5Y neurotypic cells, indicating that neuronal cells can synthesize the necessary proteins required for MAC formation. Furthermore, immunocytochemical studies showed the A beta-induced neuronal MAC expression on the SH-SY5Y cells after CD59 was removed by PIPLC or blocked by anti-CD59 antibody. Meanwhile, increased A beta-induced neuronal cell death was observed following treatment with anti-CD59. Taken together, these results suggest that A beta activates neuronal complement cascade to induce MAC, and a deficiency of endogenous complement regulatory proteins, e.g., CD59, may increase the vulnerability of neurons to complement-mediated cytotoxicity.

A beta amyloidogenesis: unique, or variation on a systemic theme?

For more than a century amyloid was considered to be an interesting, unique, but inconsequential pathologic entity that rarely caused significant clinical problems. We now recognize that amyloid is not one entity. In vivo it is a uniform organization of a disease, or process, specific protein co-deposited with a set of common structural components. Amyloid has been implicated in the pathogenesis of diseases affecting millions of patients. These range from Alzheimer's disease, adult-onset diabetes, consequences of prolonged renal dialysis, to the historically recognized systemic forms associated with inflammation and plasma cell disturbances. Strong evidence is emerging that even when deposited in local organ sites significant physiologic effects may ensue. With emphasis on A beta amyloid, we review the present definition, classification, and general in vivo pathogenetic events believed to be involved in the deposition of amyloids. This encompasses the need for an adequate amyloid precursor protein pool, whether precursor proteolysis is required prior to deposition, amyloidogenic amino acid sequences, fibrillogenic nucleating particles, and an in vivo microenvironment conducive to fibrillogenesis. The latter includes several components that seem to be part of all amyloids. The role these common components may play in amyloid accumulation, why amyloids tend to be associated with basement membranes, and how one may use these findings for anti-amyloid therapeutic strategies is also examined.

Mechanisms of C5a and C3a complement fragment-induced [Ca2+]i signaling in mouse microglia

Microglial cells are activated in response to brain insults; the mechanisms of this process are not yet understood. One of the important signaling mechanisms that might be involved in microglia activation is related to changes in the intracellular calcium concentration ([Ca2+]i). Using fluo-3 microfluorimetry, we have found that external application of the complement fragment C5a (4-10 nM) induced [Ca2+]i elevation in microglial cells in situ in corpus callosum slices. Similarly, application of complement fragments C5a (0.1-10.0 nM) or C3a (100 nM) generates biphasic [Ca2+]i transients composed of an initial peak followed by a plateau in cultured microglia. Incubation of microglial cells for 30 min with pertussis toxin (PTX; 1 microgram/ml) inhibited both C5a- and C3a-triggered [Ca2+]i responses, suggesting the involvement of PTX-sensitive G-proteins in the signal transduction chain. Removal of Ca2+ ions from the extracellular solution eliminated the plateau phase and limited the response to the initial peak. The restoration of the extracellular Ca2+ concentration within 30-60 sec after the beginning of the complement fragment-induced [Ca2+]i elevation led to the recovery of the plateau phase. Inhibition of the endoplasmic reticulum Ca2+ pumps with 500 nM thapsigargin transiently increased the [Ca2+]i and blocked the [Ca2+]i signals in response to subsequent complement fragment application. Our data suggest that complement factors induce [Ca2+]i responses by Ca2+ release from internal pools and subsequent activation of Ca2+ entry controlled by the filling state of the intracellular Ca2+ depots.

Beta-amyloid peptides initiate the complement cascade without producing a comparable effect on the terminal pathway in vitro

Activation of the classical complement cascade by beta-amyloid peptides has been hypothesized to underlie the neurodegeneration observed in Alzheimer's diseased brains. In this study, various lots of synthetic beta-amyloid peptides, A beta(1-40), A beta(1-42), and A beta(25-35), were tested for their ability to activate both early complement cascade events and formation of the membrane attack complex through terminal pathway activation. Unlike recent reports which did not assess activation of complement terminal pathway, we found that concentrations of beta-amyloid which activated early cascade events, to an extent comparable to aggregated IgG, failed to elicit formation of comparable levels of membrane attack complex.

The presence of the complement cascade does not lead to neuronal cell death in primary hippocampal cultures

To investigate the consequences of complement activation on neuronal viability, the effects of serum treatment on neuron-rich and mixed neuronal/glial cultures were evaluated. The neurotoxicity observed following treatment with either human or rat serum was variable and did not appear to be mediated through a complement-mediated mechanism. Serum lots lacking CH50 activity induced neurotoxicity, and heat treatment of toxic lots of either human or rat sera did not abolish toxicity. In cases where serum treatment did not induce cell death, treatment with PIPLC to remove endogenous membrane-bound complement inhibitors prior to serum exposure, did not result in cell death.

Inflammation and Alzheimer's disease pathogenesis

Appreciation of the role that inflammatory mediators play in Alzheimer's disease (AD) pathogenesis continues to be hampered by two related misconceptions. The first is that to be pathogenically significant a neurodegenerative mechanism must be primary. The second is that inflammation merely occurs to clear the detritis of already existent pathology. The present review addresses these issues by showing that 1) inflammatory molecules and mechanisms are uniquely present or significantly elevated in the AD brain, 2) inflammation may be a necessary component of AD pathogenesis, 3) inflammation may be sufficient to cause AD neurodegeneration, and 4) retrospective and direct clinical trials suggest a therapeutic benefit of conventional antiinflammatory medications in slowing the progress or even delaying the onset of AD.

Inflammatory mechanisms in neurodegeneration and Alzheimer's disease: the role of the complement system

This review discusses key findings indicating potential roles of the complement (C)-system in chronic inflammation in Alzheimer's disease (AD) brain. Although there is no means to cure or prevent the disease, recent studies suggest that antiinflammatory drugs may delay the onset of AD dementia. One target of these drugs may be the (C)-system, which is best known for its roles in inflammatory processes in peripheral tissues. However, recent data show C-system expression and regulation in brain cells, and C-system protein deposition in AD plaques. It is still nuclear whether C-system activation contributes to neuropathology in the AD brain, as shown in multiple sclerosis (MS). New clinical studies with antiinflammatory agents are now under general consideration by the Alzheimer's Disease Cooperative Study program. In this review I outline research directions which address possible C-system contributions to neurodegeneration. Finally, I discuss potential pharmacological interventions designed to control segments of classical inflammatory cascades in which the C-system is highly implicated. These aspects are critical to the understanding of C-mediated responses in normal and pathologic brain.

alpha-1-antichymotrypsin interaction with A beta (1-40) inhibits fibril formation but does not affect the peptide toxicity

Recent studies have shown that senile plaque-associated or glial-derived proteins can prevent fibril formation of beta-amyloid peptide (A beta), while increasing the neurotoxicity of the latter (in the case of glutamine synthetase, apolipoprotein J or thrombin). alpha-1-Antichymotrypsin (ACT) is a glial-derived protein associated with senile plaques in the Alzheimer's brain. In this report we show that ACT, a minor protein component of beta-amyloid deposits, is able to inhibit A beta (1-40) aggregation into fibrils, but unable to modulate the toxicity of A beta (1-40) in primary rat hippocampal cell cultures. These results are discussed in terms of the potential role of glial-derived proteins on A beta aggregation and neurotoxicity.

Expression of the protooncogene bcl-2 in Alzheimer's disease brain

To investigate the role of the apopotosis-related protooncogene bcl-2 in Alzheimer's disease (AD), we compared levels of its protein product, designated Bcl-2, in AD and nondemented (ND) age-matched control neocortical samples. The 26 kD Bcl-2 protein is increased in expression by more than three-fold in AD compared to ND samples as detected by immunoblots. Immunohistochemical analyses give similar results. In AD patients Bcl-2 immunoreactivity is dense and profuse and appears to occur on reactive astrocytes, whereas Bcl-2 immunoreactivity of astrocytes in ND patients is light and sparse. Staining of both gray and white matter is observed but is most prominent in the latter. Increased expression of Bcl-2 by astrocytes may partially underlie their resistance to loss in AD. By contrast, neuronal Bcl-2 immunoreactivity is more sparse and equivocal, perhaps reflecting the vulnerability of this cell type to apoptotic mechanisms in AD. High levels of Bcl-2 in glial cells may aid in cell survival of reactive astrocytes resulting in either beneficial or deleterious effects on neuronal viability.

The toxicity in vitro of beta-amyloid protein

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Isolation of glyceraldehyde 3-phosphate dehydrogenase (Gapdh) cDNA from the distal half of mouse chromosome 16: further indication of a link between Alzheimer's disease and glycolysis

The amyloid precursor protein (APP), which is localized on both human chromosome 21 and its murine counterpart, chromosome 16 and which is involved in the formation of deposits in Alzheimer's disease, could be shown to bind effectively to a glytolytic enzyme: rat glyceraldehyde 3-phosphate dehydrogenase (Gapdh). We report here the isolation of a cDNA of murine Gapdh from mouse chromosome 16 (MMU16) originating from microclones of the distal part of MMU16 and the use of homologous genomic DNA sequences to further screen a cDNA phage library. The cDNA was sequenced, confirmed by polymerase chain reaction following reverse transcriptase (RT-PCR) and the open reading frame was expressed in vitro. The possible localization of Gapdh on MMU16--which may provide a mouse model for Down's syndrome and Alzheimer's disease--may lead to new insights into glycolysis and its role in the two syndromes.