Alzheimer's disease (AD) is characterized by the deposition of senile plaques (SPs) and neurofibrillary tangles (NFTs) in vulnerable brain regions. SPs are composed of aggregated beta-amyloid (Abeta) 40/42(43) peptides. Evidence implicates a central role for Abeta in the pathophysiology of AD. Mutations in betaAPP and presenilin 1 (PS1) lead to elevated secretion of Abeta, especially the more amyloidogenic Abeta42. Immunohistochemical studies have also emphasized the importance of Abeta42 in initiating plaque pathology. Cell biological studies have demonstrated that Abeta is generated intracellularly. Recently, endogenous Abeta42 staining was demonstrated within cultured neurons by confocal immunofluorescence microscopy and within neurons of PS1 mutant transgenic mice. A central question about the role of Abeta in disease concerns whether extracellular Abeta deposition or intracellular Abeta accumulation initiates the disease process. Here we report that human neurons in AD-vulnerable brain regions specifically accumulate gamma-cleaved Abeta42 and suggest that this intraneuronal Abeta42 immunoreactivity appears to precede both NFT and Abeta plaque deposition. This study suggests that intracellular Abeta42 accumulation is an early event in neuronal dysfunction and that preventing intraneuronal Abeta42 aggregation may be an important therapeutic direction for the treatment of AD.

The excessive generation and accumulation of 40-and 42-aa ␤-amyloid peptides (A␤ 40 ͞A␤ 42 ) in selectively vulnerable brain regions is a major neuropathological feature of Alzheimer's disease. A␤, derived by proteolytic cleavage from the ␤-amyloid precursor protein (␤APP), is normally secreted. However, recent evidence suggests that significant levels of A␤ also may remain inside cells. Here, we have investigated the subcellular compartments within which distinct amyloid species are generated and the compartments from which they are secreted. Three experimental approaches were used: (i) immunof luorescence performed in intact cortical neurons; (ii) sucrose gradient fractionation performed with mouse neuroblastoma cells stably expressing wild-type Alzheimer's disease (AD), the most common form of dementia in the elderly, is characterized clinically by the insidious onset and inexorable progression of dementia, and pathologically by the abnormal accumulation of amyloid plaques and neurofibrillary tangles in vulnerable brain regions. Plaques are composed of variously sized ␤-amyloid peptides (A␤) derived through proteolytic processing of the ␤-amyloid precursor protein (␤APP). Mutations within ␤APP were discovered to cause autosomal dominant familial AD (FAD) (1, 2), implicating ␤APP in the etiology of this disease. In addition, early-onset FAD also segregates with two other genes, the presenilin 1 (PS1) gene (3) and the presenilin 2 (PS2) gene (4), which appear to cause FAD by increasing the ratio of A␤ ending with amino acid 42 (A␤ 42 ) versus A␤ ending with amino acid 40 (A␤ 40 ) (5, 6). A␤ 42 is more highly amyloidogenic than A␤ 40 (7) and is believed to form the core of the amyloid plaques, despite being produced far less abundantly than A␤ 40 . Any one of several diverse molecular anomalies of ␤APP metabolism may lead to AD.␤APP, an integral membrane glycoprotein, matures through the secretory pathway and is metabolically processed by at least two distinct pathways. Cleavage of ␤APP by an enzyme, ␣-secretase, in a late secretory compartment, or at the cell surface, generates a large ␤APP fragment (s␤APP ␣ ). This cleavage, within the A␤ coding region, precludes formation of A␤. Alternatively, cleavage by two enzymes, ␤-and ␥-secretase, is believed to generate the majority of ␤-amyloid variants (8). ␤APP is initially synthesized and cotranslationally inserted into the endoplasmic reticulum (ER). Recently, evidence has been obtained for the presence of A␤ 42 within the ER (9-12), but it was not determined whether the peptides found in the ER were packaged into vesicles for trafficking through the late secretory pathway. In addition, finding A␤ species in the ER does not demonstrate that they were generated there; retrograde transport of proteins from the Golgi complex and trans-Golgi Network (TGN) to the ER has been documented (13,14). Finally, reports of A␤ 42 in the ER have been based primarily on ELISA assays using C-terminal specific antibodies. These assays cannot distinguish between the N termin...

Alzheimer's disease (AD) is characterized by the accumulation of cerebral plaques composed of 40- and 42-amino acid beta-amyloid (Abeta) peptides, and autosomal dominant forms of AD appear to cause disease by promoting brain Abeta accumulation. Recent studies indicate that postmenopausal estrogen replacement therapy may prevent or delay the onset of AD. Here we present evidence that physiological levels of 17beta-estradiol reduce the generation of Abeta by neuroblastoma cells and by primary cultures of rat, mouse and human embryonic cerebrocortical neurons. These results suggest a mechanism by which estrogen replacement therapy can delay or prevent AD.