Neuronal and glial beta-secretase (BACE) protein expression in transgenic Tg2576 mice with amyloid plaque pathology

We measured tissue distribution and expression pattern of the beta-site amyloid precursor protein (APP)-cleaving enzyme (BACE) in the brains of transgenic Tg2576 mice that show amyloid pathology. BACE protein was expressed at high levels in brain; at lower levels in heart and liver; and at very low levels in pancreas, kidney, and thymus and was almost absent in spleen and lung when assayed by Western blot analysis. We observed strictly neuronal expression of BACE protein in the brains of nontransgenic control mice, with the most robust immunocytochemical labeling present in the cerebral cortex, hippocampal formation, thalamus, and cholinergic basal forebrain nuclei. BACE protein levels did not differ significantly between control and transgenic mice or as a result of aging. However, in the aged, 17-month-old Tg2576 mice there was robust amyloid plaque formation, and BACE protein was also present in reactive astrocytes present near amyloid plaques, as shown by double immunofluorescent labeling and confocal laser scanning microscopy. The lack of astrocytic BACE immunoreactivity in young transgenic Tg2576 mice suggests that it is not the APP overexpression but rather the amyloid plaque formation that stimulates astrocytic BACE expression in Tg2576 mice. Our data also suggest that the neuronal overexpression of APP does not induce the overexpression of its metabolizing enzyme in neurons. Alternatively, the age-dependent accumulation of amyloid plaques in the Tg2576 mice does not require increased neuronal expression of BACE. Our data support the hypothesis that neurons are the primary source of beta-amyloid peptides in brain and that astrocytic beta-amyloid generation may contribute to amyloid plaque formation at later stages or under conditions when astrocytes are activated.     

Presenilin 1 is involved in the maturation of beta-site amyloid precursor protein-cleaving enzyme 1 (BACE1)

One of the pathologic hallmarks of Alzheimer's disease is the excessive deposition of beta-amyloid peptides (Abeta) in senile plaques. Abeta is generated when beta-amyloid precursor protein (APP) is cleaved sequentially by beta-secretase, identified as beta-site APP-cleaving enzyme 1 (BACE1), and gamma-secretase, a putative enzymatic complex containing presenilin 1 (PS1). However, functional interaction between PS1 and BACE1 has never been known. In addition to this classical role in the generation of Abeta peptides, it has also been proposed that PS1 affects the intracellular trafficking and maturation of selected membrane proteins. We show that the levels of exogenous and endogenous mature BACE1 expressed in presenilin-deficient mouse embryonic fibroblasts (PS-/-MEFs) were reduced significantly compared to those in wild-type MEFs. Moreover, the levels of mature BACE1 were increased in human neuroblastoma cell line, SH-SY5Y, stably expressing wild-type PS1, compared to native cells. Conversely, the maturation of BACE1 was compromised under the stable expression of dominant-negative mutant PS1 overexpression. Immunoprecipitation assay showed that PS1 preferably interacts with proBACE1 rather than mature BACE1, indicating that PS1 can be directly involved in the maturation process of BACE1. Further, endogenous PS1 was immunoprecipitated with endogenous BACE1 in SH-SY5Y cells and mouse brain tissue. We conclude that PS1 is directly involved in the maturation of BACE1, thus possibly functioning as a regulator of both beta- and gamma-secretase in Abeta generation.     

Cysteine proteases are the major beta-secretase in the regulated secretory pathway that provides most of the beta-amyloid in Alzheimer's disease: role of BACE 1 in the constitutive secretory pathway

This article focuses on beta-amyloid (Abeta) peptide production and secretion in the regulated secretory pathway and how this process relates to accumulation of toxic Abeta in Alzheimer's disease. New findings are presented demonstrating that most of the Abeta is produced and secreted, in an activity-dependent manner, through the regulated secretory pathway in neurons. Only a minor portion of cellular Abeta is secreted via the basal, constitutive secretory pathway. Therefore, regulated secretory vesicles contain the primary beta-secretases that are responsible for producing the majority of secreted Abeta. Investigation of beta-secretase activity in regulated secretory vesicles of neuronal chromaffin cells demonstrated that cysteine proteases account for the majority of the beta-secretase activity. BACE 1 is present in regulated secretory vesicles but provides only a small percentage of the beta-secretase activity. Moreover, the cysteine protease activities prefer to cleave the wild-type beta-secretase site, which is relevant to the majority of AD cases. In contrast, BACE 1 prefers to cleave the Swedish mutant beta-secretase site that is expressed in a minor percentage of the AD population. These new findings lead to a unifying hypothesis in which cysteine proteases are the major beta-secretases for the production of Abeta in the major regulated secretory pathway and BACE 1 is the beta-secretase responsible for Abeta production in the minor constitutive secretory pathway. These results indicate that inhibition of multiple proteases may be needed to decrease Abeta production as a therapeutic strategy for Alzheimer's disease.     

Cytoplasmic domain of the beta-amyloid protein precursor of Alzheimer's disease: function, regulation of proteolysis, and implications for drug development

The beta-amyloid protein precursor (APP) has been extensively studied for its role in amyloid production and the pathogenesis of Alzheimer's disease (AD). However, little is known about the normal function of APP and its biological interactions. In this Mini-Review, the role of the cytoplasmic domain of APP in APP trafficking and proteolysis is described. These studies suggest that proteins that bind to the cytoplasmic domain may be important targets for drug development in AD.     

The diagnostic value of tau protein, beta-amyloid (1-42) and their ratio for the discrimination of alcohol-related cognitive disorders from Alzheimer's disease in the early stages

BACKGROUND: Chronic and heavy alcohol abuse or dependence may result in impaired cognition and dementia. The increased risk of Alzheimer's disease (AD) in older individuals interferes with the differential diagnosis, especially when dealing with elderly patients with a long history of alcohol abuse. The aim of the present study was to evaluate the diagnostic value of the putative cerebrospinal fluid (CSF) biomarkers tau, beta-amyloid 1-42 (Abeta42) and their ratio in differentiating alcohol related cognitive disorder (ARCD) from AD. METHODS: Double-sandwich ELISA (Innotest htau antigen and beta-Amyloid (1-42), Innogenetics) were used to quantify the above markers in a total of 20 patients with ARCD, 33 AD patients with mild to moderate dementia and 50 mentally intact subjects. RESULTS: Tau protein successfully differentiated AD from normal ageing with 96% specificity and 93.9% sensitivity and from ARCD with 95% specificity, and 87.9% sensitivity. Abeta42 alone had a specificity of 88% and a sensitivity of 69.7% in differentiating AD from normal ageing, while the corresponding values for differentiating AD from ARCD were 80% and 84.8% respectively. The tau/Abeta42 ratio was better than tau alone for differentiating AD from normal ageing (specificity 94%, sensitivity 97%) and better than any of the candidate markers alone, for differentiating AD from ARCD (specificity 100%, sensitivity 97%). CONCLUSIONS: The combined use of CSF tau and Abeta42 may be a useful tool in the differential diagnosis of ARCD from AD, especially in the early stages, where diagnostic uncertainty is greater.