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.

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.

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