We discovered that CAD cells are prone to accumulation of large amounts of intracellular Aβ at the terminals of their processes (fig. ), similar to what may occur in brain neurons, during the initial phases of AD [7
]. Using carboxy-terminal-end antibodies to Aβ species, we showed that these Aβ accumulations contain both the Aβ40
]. Cross-reactivity of the accumulations with an antioligomer antibody that preferentially detects species larger than the octamer (A11) [9
] indicated that the accumulations include large-molecular-size Aβ oligomers [7
Fig. 1. CAD cells immunolabeled with antibody 6E10 (Signet, Dedham, Mass., USA), showing Aβ accumulations at neurite endings (A). B An enlargement of a process, showing localization of Aβ to large particles resembling late endosomes and autophagosomes. (more ...)
The neuritic accumulation of Aβ in CAD cells is restricted to a small population of cells that show redistribution of β-secretase (BACE1) to the processes, where it colocalizes with Aβ and markers of late endosomes and autophagic vacuoles [7
]. These findings suggest that the Aβ accumulations could be generated through endocytosis or macroautophagy, two processes previously implicated in the formation of the neuritically localized Aβ [10
]. Importantly, unlike the LC-derived CAD cells, cultured cortical and hippocampal neurons do not show detectable Aβ accumulations at their neuritic terminals (data not shown).
Here, we hypothesize that in AD brains, accumulations of Aβ similar to those observed in CAD cells (fig. ) could form at the projection terminals of brainstem neurons (which send projections throughout the central nervous system, including the cerebral cortex and hippocampus), and serve as seeds for further Aβ aggregation. This process leads to plaque formation at locations remote from the brainstem, such as the cerebral cortex and hippocampus (fig. ). The LC is largely affected by cell death early in AD. However, since this brain region mostly lacks neuritic plaques, the prevailing view was that neuronal loss in the LC is caused by the cortical Aβ, which ‘poisons’ the projections of brainstem cells [13
]. By contrast, our results suggest that the neuropathology of AD may actually begin in the subcortical regions, and then spread to the cortex and hippocampus. This is in line with earlier reports suggesting that, in AD, pathologic alterations in the LC may spread to cortical and hippocampal brain regions [14
Fig. 2. Diagram showing plaques () at the terminals of projections of brainstem (BS) neurons. The drawing was modified from Aston-Jones and Cohen , and used with permission from the Annual Review of Neuroscience, vol. 28, 2005, by Annual Reviews (more ...)
How could the intracellular Aβ accumulations (fig. ) become extracellular and serve as seeds for Aβ deposition? The mechanism could be the fusion of the Aβ-containing autophagosomes and late endosomes with the plasma membrane, or alternatively, neurite degeneration. Indeed, we occasionally found neuritic debris containing Aβ, in the areas enriched in CAD cells having Aβ accumulations (fig. ). A further question raised by this result is what could trigger neurite degeneration in these cells? We found that, while the anterograde transport of small vesicles is not significantly affected [7
], the transport and the neuritic localization of mitochondria is evidently perturbed in CAD cells that contain Aβ accumulations (fig. ). This could result in diminished supply of ATP in the neurites, which could cause their observed degeneration.
Aβ accumulations in the debris of CAD cell processes (arrows). A CAD cells immunolabeled with antibody 6E10. B Corresponding phase contrast micrograph.
Fig. 4. Mitochondrial transport into neurites that contain Aβ deposits is disrupted. Fewer mitochondria are present throughout the neurite of a CAD cell that contains Aβ deposits (short arrows). By contrast, neurites that lack Aβ deposits (more ...)