Detection of the membrane-retained carboxy-terminal tail containing polypeptides of the amyloid precursor protein in tissue from Alzheimer's disease brain

Biochemical assessment of precuneus and posterior cingulate gyrus in the context of brain aging and Alzheimer's disease

Defining the biochemical alterations that occur in the brain during “normal” aging is an important part of understanding the pathophysiology of neurodegenerative diseases and of distinguishing pathological conditions from aging-associated changes. Three groups were selected based on age and on having no evidence of neurological or significant neurodegenerative disease: 1) young adult individuals, average age 26 years (n = 9); 2) middle-aged subjects, average age 59 years (n = 5); 3) oldest-old individuals, average age 93 years (n = 6). Using ELISA and Western blotting methods, we quantified and compared the levels of several key molecules associated with neurodegenerative disease in the precuneus and posterior cingulate gyrus, two brain regions known to exhibit early imaging alterations during the course of Alzheimer’s disease. Our experiments revealed that the bioindicators of emerging brain pathology remained steady or decreased with advancing age. One exception was S100B, which significantly increased with age. Along the process of aging, neurofibrillary tangle deposition increased, even in the absence of amyloid deposition, suggesting the presence of amyloid plaques is not obligatory for their development and that limited tangle density is a part of normal aging. Our study complements a previous assessment of neuropathology in oldest-old subjects, and within the limitations of the small number of individuals involved in the present investigation, it adds valuable information to the molecular and structural heterogeneity observed along the course of aging and dementia. This work underscores the need to examine through direct observation how the processes of amyloid deposition unfold or change prior to the earliest phases of dementia emergence.

Reduced platelet amyloid precursor protein ratio (APP ratio) predicts conversion from mild cognitive impairment to Alzheimer's disease. J Neural Transm (Vienna). 2012;119:815-819. [PMID: 22573143 DOI: 10.1007/s00702-012-0807-x]

Studies have shown that platelet APP ratio (representing the percentage of 120-130 kDa to 110 kDa isoforms of the amyloid precursor protein) is reduced in patients with mild cognitive impairment (MCI) and Alzheimer's disease (AD). In the present study, we sought to determine if baseline APP ratio predicts the conversion from MCI to AD dementia after 4 years of longitudinal assessment. Fifty-five older adults with varying degrees of cognitive impairment (34 with MCI and 21 with AD) were assessed at baseline and after 4 years. MCI patients were re-classified according to the conversion status upon follow-up: 25 individuals retained the diagnostic status of MCI and were considered as stable cases (MCI-MCI); conversely, in nine cases the diagnosis of dementia due to AD was ascertained. The APP ratio (APPr) was determined by the Western blot method in samples of platelets collected at baseline. We found a significant reduction of APPr in MCI patients who converted to dementia upon follow-up. These individuals had baseline APPr values similar to those of demented AD patients. The overall accuracy of APPr to identify subjects with MCI who will progress to AD was 0.74 ± 0.10, p = 0.05. The cut-off of 1.12 yielded a sensitivity of 75 % and a specificity of 75 %. Platelet APPr may be a surrogate marker of the disease process in AD, with potential implications for the assessment of abnormalities in the APP metabolism in patients with and at risk for dementia. However, diagnostic accuracy was relatively low. Therefore, studies in larger samples are needed to determine whether APPr may warrant its use as a biomarker to support the early diagnosis of AD.

Abnormal cleavage of APP impairs its functions in cell adhesion and migration

Amyloid precursor protein (APP) is expressed ubiquitously but its wrong cleavage only occurs in central nervous system. In this research, overexpression of wild type human APP695 was found to stimulate the adhesion and migration of N2a cells. In the cells co-transfected by familial Alzheimer's disease (FAD)-linked Swedish mutant of APP695 gene plus big up tri, openE9 deleted presenilin1 gene (N2a/Swe. big up tri, open9), however, this stimulating function was impaired compared to that in the cells co-transfected by Swedish mutant of APP695 gene plus dominant negative mutant of presenilin1 D385A gene (N2a/Swe.385). Furthermore, it was also found that the phosphorylation of FAK Tyr-861 and GSK-3beta Ser-9 was reduced in N2a/Swe.Delta9 cells, which can be possibly taken as a reasonable explanation for the underlying mechanism. Our results suggest that impaired cell adhesion and migration induced by abnormal cleavage of APP could contribute to the pathological effects in FAD brain.

Abnormal APP processing in platelets of patients with Alzheimer's disease: correlations with membrane fluidity and cognitive decline

RATIONALE

Previous studies have implicated platelet amyloid precursor protein (APP) as a candidate biomarker for Alzheimer's disease (AD). Platelets contain more than 95% of the circulating APP and enclose the enzymatic machinery for the APP metabolism yielding both soluble APP and amyloid-beta peptides.

OBJECTIVES

The objective of this study is to compare the ratio of 130- to 110-kDa fragments of APP in platelets from patients with AD, mild cognitive impairment (MCI), and elderly controls.

MATERIALS AND METHODS

After subjects were grouped according to diagnosis, APP ratio in platelets was evaluated by means of Western blot analysis.

RESULTS

The APP ratio was significantly lower in AD patients (1.01 +/- 0.21) as compared to controls (1.24 +/- 0.21, p = 0.001) and MCI patients (1.18 +/- 0.21, p = 0.027), but no significant differences were found between MCI and controls (p = 0.904). In addition, we found positive correlations between the APP ratio and 1,6-diphenyl-1,3,5-hexatriene anisotropy (r = 0.3, p = 0.01), as well as with certain parameters of cognitive decline, namely, the mini-mental state examination score (r = 0.33, p = 0.003), the total Cambridge cognitive test (CAMCOG) score (r = 0.37, p = 0.001), and the score on the memory subscale of the CAMCOG (r = 0.38, p = 0.001).

CONCLUSIONS

The pattern of platelet APP fragments was altered in patients with AD but not in patients with MCI. The alteration of APP fragments was correlated with membrane fluidity and the cognitive decline.

The role of the carboxyl-terminal fragments of amyloid precursor protein in Alzheimer's disease

Two major pathological hallmarks of Alzheimer's disease (AD) are the senile plaques that are primarily composed of amyloid beta-peptide (Abeta) and neurofibrillary tangles consisting of tau aggregates. Abeta is generated proteolytically from a family of Abeta-containing precursor proteins (APP; 695-770 amino acid) by secretase enzymes to different specific carboxyl-terminal fragments (CTFs). Herein we examined APP and its products in autopsied brain sections from 10 AD and 10 non-AD control subjects immunochemically using an antibody that was raised against APP751-770 residue (O443). The O443 antibody was initially characterized by Western blot analysis and immunoprecipitation. In this study, we used this antibody for immunohistochemical analysis to determine the distribution of APP and its CTF species. In 10 brain regions showing different levels of plaques and tangles, antibody O443 stained the perinuclear region of the nucleus, plaques, and neurites. Tangle-bearing neurons also appeared to stain with the antibody, suggesting that these dysfunctional neurons continue to synthesize APP/CTF. Alternatively, the normally short-lived APP/CTF can be stabilized and persist in these neurons. Taken together, these results suggest that, in addition to the widely believed role of Abeta, CTFs may play a key role in the pathogenesis of AD. Studying their localization and biogenesis may reveal the biological activities of CTFs of APP. The present study may pave the way for possible antiamyloidogenic therapy in the treatment of AD.

Estrogen attenuates cell death induced by carboxy-terminal fragment of amyloid precursor protein in PC12 through a receptor-dependent pathway

In the present study, we investigated effects of estrogen on cell death induced by carboxy-terminal fragment of amyloid precursor protein (CT), a candidate causative substance in the pathogenesis of Alzheimer's disease. 17 beta-Estradiol attenuated CT-induced cell death in PC12 cells, whereas 17 alpha-estradiol, nonestrogenic stereoisomer, did not exert any significant protective effect on CT-induced cell death. These results suggest that protective effects of estrogen may be mediated by estrogen receptor (ER) in PC12 cells. To confirm the results, we determined the effects of tamoxifen, an estrogen receptor antagonist. Tamoxifen blocked the protective effects of 17 beta-estradiol, although it did not affect those of 17 alpha-estradiol. Overall, it might be thought that the protective effect of estradiol on CT-induced cell death is achieved by hormonal properties mediated through the estrogen receptor rather than the structural properties as a reducing agent.

Examination of the interactions of the amyloid precursor protein carboxyl terminus to intracellular protein: possible role in apoptosis

1. The carboxyl terminus of the amyloid precursor protein (APP) has several identified regions that may potentially contribute to the pathogenic effects of Alzheimer's disease (AD). To examine these effects, the authors iodinated a short synthetic peptide corresponding to amino acids 679-687 of APP695. They also produced a [35S]-Methionine labeled peptide corresponding to the entire carboxyl 100 amino acids of APP695 via a reticulocyte lysate coupled in vitro transcription/translation system. 2. Human neuroblastoma cells (SK-N-SH) and non-neuronal epithelial cells (RK-13) were cultured, harvested, and lysed. The S1 cell extract fractions were combined with either of the labeled peptides and incubated at different temperatures to allow for interaction and binding of cellular proteins with the peptides. These interactions were identified as gel mobility shift patterns on native PAGE gels. The presence of distinct bands on the gels indicate that the APP C-terminus interacts with several intracellular proteins, some of which may be detrimental to the cell. The authors have tested the possibility that the accumulation of C-terminal proteins may result in apoptosis. 3. Apoptosis in neural cells is one detrimental effect that has been attributed to APP. The authors examined hippocampal tissue sections from Alzheimer's disease (AD) and age-matched normal control patients for a difference in the number of apoptotic nuclei present using an in situ apoptosis detection kit that labels the numerous free DNA ends present in apoptotic nuclei. The number of apoptotic nuclei found in AD neuronal tissue was significantly higher than in normal tissue.

Pro-inflammatory complement activation by the A beta peptide of Alzheimer's disease is biologically significant and can be blocked by vaccinia virus complement control protein

The amyloid plaque is the hallmark of Alzheimer's disease (AD). The transmembrane domain and a portion of the C-terminus (A beta) of the amyloid precursor protein, are known to form the nucleus of the amyloid plaque. It has been demonstrated recently, using in vitro assays, that the A beta peptide can activate both the classical (antibody-independent) and alternate pathways of complement activation. The proposed complement activation is due to the binding of A beta to the complement components C1q and C3, respectively, which initiate formation of the proinflammatory C5a and C5b-9 membrane attack complex. In this report, we have investigated the in vitro findings for the likely complement-dependent proinflammatory properties of the Alzheimer's disease A beta peptide. We have performed experiments using congenic C5-deficient and C5-sufficient mice injected with synthetic A beta and recombinant polypeptide (C-100) containing A beta. Injection of C-100 into C5-sufficient mice induced a clear increase in the number of polymorphonuclear cells (neutrophils) at the site of injection due to complement activation and the subsequent release of proinflammatory chemtoactic factors. In sharp contrast, the C5-deficient mice did not show any increase in cellular influx. The vaccinia virus complement control protein, an inhibitor of both the classical and alternate pathway can down-regulate the biologically significant activation of complement by A beta, as demonstrated by an in vitro immunassay. The therapeutic down-regulation of A beta-caused complement activation could greatly alleviate the progression of some of the chronic neurodegeneration characteristic of Alzheimer's disease.