Amyloid-beta peptide is elevated in the brains of patients with Alzheimer disease and is believed to be causative in the disease process. Amyloid-beta reduces glutamatergic transmission and inhibits synaptic plasticity, although the underlying mechanisms are unknown. We found that application of amyloid-beta promoted endocytosis of NMDA receptors in cortical neurons. In addition, neurons from a genetic mouse model of Alzheimer disease expressed reduced amounts of surface NMDA receptors. Reducing amyloid-beta by treating neurons with a gamma-secretase inhibitor restored surface expression of NMDA receptors. Consistent with these data, amyloid-beta application produced a rapid and persistent depression of NMDA-evoked currents in cortical neurons. Amyloid-beta-dependent endocytosis of NMDA receptors required the alpha-7 nicotinic receptor, protein phosphatase 2B (PP2B) and the tyrosine phosphatase STEP. Dephosphorylation of the NMDA receptor subunit NR2B at Tyr1472 correlated with receptor endocytosis. These data indicate a new mechanism by which amyloid-beta can cause synaptic dysfunction and contribute to Alzheimer disease pathology.

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.

A central question in Alzheimer's disease concerns the mechanism by which beta-amyloid contributes to neuropathology, and in particular whether intracellular versus extracellular beta-amyloid plays a critical role. Alzheimer transgenic mouse studies demonstrate brain dysfunction, as beta-amyloid levels rise, months before the appearance of beta-amyloid plaques. We have now used immunoelectron microscopy to determine the subcellular site of neuronal beta-amyloid in normal and Alzheimer brains, and in brains from Alzheimer transgenic mice. We report that beta-amyloid 42 localized predominantly to multivesicular bodies of neurons in normal mouse, rat, and human brain. In transgenic mice and human Alzheimer brain, intraneuronal beta-amyloid 42 increased with aging and beta-amyloid 42 accumulated in multivesicular bodies within presynaptic and especially postsynaptic compartments. This accumulation was associated with abnormal synaptic morphology, before beta-amyloid plaque pathology, suggesting that intracellular accumulation of beta-amyloid plays a crucial role in Alzheimer's disease.

Molecular chaperones and their functions in protein folding have been implicated in several neurodegenerative diseases, includingParkinson's disease and Huntington's disease, which are characterized by accumulation of protein aggregates (e.g., ␣-synuclein and huntingtin, respectively). These aggregates have been shown in various experimental systems to respond to changes in levels of molecular chaperones suggesting the possibility of therapeutic intervention and a role for chaperones in disease pathogenesis. It remains unclear whether chaperones also play a role in Alzheimer's disease, a neurodegenerative disorder characterized by ␤-amyloid and tau protein aggregates. Here, we report an inverse relationship between aggregated tau and the levels of heat shock protein (Hsp)70͞90 in tau transgenic mouse and Alzheimer's disease brains. In various cellular models, increased levels of Hsp70 and Hsp90 promote tau solubility and tau binding to microtubules, reduce insoluble tau and cause reduced tau phosphorylation. Conversely, lowered levels of Hsp70 and Hsp90 result in the opposite effects. We have also demonstrated a direct association of the chaperones with tau proteins. Our results suggest that up-regulation of molecular chaperones may suppress formation of neurofibrillary tangles by partitioning tau into a productive folding pathway and thereby preventing tau aggregation.

Multiple lines of evidence implicate ␤-amyloid (A␤) in the pathogenesis of Alzheimer's disease (AD), but the mechanisms whereby A␤ is involved remain unclear. Addition of A␤ to the extracellular space can be neurotoxic. Intraneuronal A␤42 accumulation is also associated with neurodegeneration. We reported previously that in Tg2576 amyloid precursor protein mutant transgenic mice, brain A␤42 localized by immunoelectron microscopy to, and accumulated with aging in, the outer membranes of multivesicular bodies, especially in neuronal processes and synaptic compartments. We now demonstrate that primary neurons from Tg2576 mice recapitulate the in vivo localization and accumulation of A␤42 with time in culture. Furthermore, we demonstrate that A␤42 aggregates into oligomers within endosomal vesicles and along microtubules of neuronal processes, both in Tg2576 neurons with time in culture and in Tg2576 and human AD brain. These A␤42 oligomer accumulations are associated with pathological alterations within processes and synaptic compartments in Tg2576 mouse and human AD brains.

The aberrant accumulation of aggregated β-amyloid peptides (Aβ) as plaques is a hallmark of Alzheimer’s disease (AD) neuropathology and reduction of Aβ has become a leading direction of emerging experimental therapies for the disease. The mechanism(s) whereby Aβ is involved in the pathophysiology of the disease remain(s) poorly understood. Initially fibrils, and subsequently oligomers of extracellular Aβ have been viewed as the most important pathogenic form of Aβ in AD. More recently, the intraneuronal accumulation of Aβ has been described in the brain, although technical considerations and its relevance in AD have made this a controversial topic. Here we review the emerging evidence linking intraneuronal Aβ accumulation to the development of synaptic pathology and plaques in AD, and discuss the implications of intraneuronal β-amyloid for AD pathology, biology, diagnosis and therapy.

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...

No abstract