The accumulation of lysosomes and their hydrolases within neurons is a well-established neuropathologic feature of Alzheimer disease (AD). Here we show that lysosomal pathology in AD brain involves extensive alterations of macroautophagy, an inducible pathway for the turnover of intracellular constituents, including organelles. Using immunogold labeling with compartmental markers and electron microscopy on neocortical biopsies from AD brain, we unequivocally identified autophagosomes and other prelysosomal autophagic vacuoles (AVs), which were morphologically and biochemically similar to AVs highly purified from mouse liver. AVs were uncommon in brains devoid of AD pathology but were abundant in AD brains particularly, within neuritic processes, including synaptic terminals. In dystrophic neurites, autophagosomes, multivesicular bodies, multilamellar bodies, and cathepsin-containing autophagolysosomes were the predominant organelles and accumulated in large numbers. These compartments were distinguishable from lysosomes and lysosomal dense bodies, previously shown also to be abundant in dystrophic neurites. Autophagy was evident in the perikarya of affected neurons, particularly in those with neurofibrillary pathology where it was associated with a relative depletion of mitochondria and other organelles. These observations provide the first evidence that macroautophagy is extensively involved in the neurodegenerative/regenerative process in AD. The striking accumulations of immature AV forms in dystrophic neurites suggest that the transport of AVs and their maturation to lysosomes may be impaired, thereby impeding the suspected neuroprotective functions of autophagy.

Macroautophagy, which is a lysosomal pathway for the turnover of organelles and long-lived proteins, is a key determinant of cell survival and longevity. In this study, we show that neuronal macroautophagy is induced early in Alzheimer's disease (AD) and before β-amyloid (Aβ) deposits extracellularly in the presenilin (PS) 1/Aβ precursor protein (APP) mouse model of β-amyloidosis. Subsequently, autophagosomes and late autophagic vacuoles (AVs) accumulate markedly in dystrophic dendrites, implying an impaired maturation of AVs to lysosomes. Immunolabeling identifies AVs in the brain as a major reservoir of intracellular Aβ. Purified AVs contain APP and β-cleaved APP and are highly enriched in PS1, nicastrin, and PS-dependent γ-secretase activity. Inducing or inhibiting macroautophagy in neuronal and nonneuronal cells by modulating mammalian target of rapamycin kinase elicits parallel changes in AV proliferation and Aβ production. Our results, therefore, link β-amyloidogenic and cell survival pathways through macroautophagy, which is activated and is abnormal in AD.

Endocytosis is critical to the function and fate of molecules important to Alzheimer's disease (AD) etiology, including the beta protein precursor (betaPP), amyloid beta (Abeta) peptide, and apolipoprotein E (ApoE). Early endosomes, a major site of Abeta peptide generation, are markedly enlarged within neurons in the Alzheimer brain, suggesting altered endocytic pathway (EP) activity. Here, we show that neuronal EP activation is a specific and very early response in AD. To evaluate endocytic activation, we used markers of internalization (rab5, rabaptin 5) and recycling (rab4), and found that enlargement of rab5-positive early endosomes in the AD brain was associated with elevated levels of rab4 immunoreactive protein and translocation of rabaptin 5 to endosomes, implying that both endocytic uptake and recycling are activated. These abnormalities were evident in pyramidal neurons of the neocortex at preclinical stages of disease when Alzheimer-like neuropathology, such as Abeta deposition, was restricted to the entorhinal region. In Down syndrome, early endosomes were significantly enlarged in some pyramidal neurons as early as 28 weeks of gestation, decades before classical AD neuropathology develops. Markers of EP activity were only minimally influenced by normal aging and other neurodegenerative diseases studied. Inheritance of the epsilon4 allele of APOE, however, accentuated early endosome enlargement at preclinical stages of AD. By contrast, endosomes were normal in size at advanced stages of familial AD caused by mutations of presenilin 1 or 2, indicating that altered endocytosis is not a consequence of Abeta deposition. These results identify EP activation as the earliest known intraneuronal change to occur in sporadic AD, the most common form of AD. Given the important role of the EP in Abeta peptide generation and ApoE function, early endosomal abnormalities provide a mechanistic link between EP alterations, genetic susceptibility factors, and Abeta generation and suggest differences that may be involved in Abeta generation and beta amyloidogenesis in subtypes of AD.