Seeding neuritic plaques from the distance: a possible role for brainstem neurons in the development of Alzheimer's disease pathology

Abstract

Background and objective: Our goal is to obtain insight into the causes of the pathological lesions in Alzheimer's disease (AD). It is thought that the beta-amyloid (Abeta) deposits within the cerebral cortex and hippocampus of AD brains are initiated by a 'bad seed' of oligomeric Abeta. The origin of this seed is unknown. Here, we focused on the events that might trigger the formation of neuritic plaques, aiming to explain how these plaques form in cortical and hippocampal regions.

Methods and results: Using immunocytochemical and biochemical methods, we showed that brainstem-derived, neuronal cells (CAD)--but not cortical or hippocampal neurons--show large amounts of Abeta accumulated at the terminals of their processes. This is similar to what is believed to occur in brain neurons, in the early phases of AD. CAD cells that contain Abeta accumulations also concentrate beta-secretase at process terminals. We show that, while the anterograde transport of small vesicles is not significantly affected, the mitochondrial transport is perturbed in CAD cells that contain Abeta accumulations. We further show that intracellular, neuritic Abeta accumulations may become extracellular upon neurite degeneration, thus providing the initial 'bad seed' of Abeta oligomers that triggers further aggregation of extracellular proteins.

Conclusion: We propose that brainstem neurons, known to send projections throughout the brain, could provide the 'bad seed' of Abeta that nucleates plaques in the cerebral cortex and hippocampus of AD brains.

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.

Fig. 2.

Diagram showing plaques (∗) at the terminals of projections of brainstem (BS) neurons. The drawing was modified from Aston-Jones and Cohen [12], and used with permission from the Annual Review of Neuroscience, vol. 28, 2005, by Annual Reviews (www.annualreviews.org).

Fig. 3.

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 have numerous mitochondria that accumulate at the terminal (long arrows). Mitochondria were detected with an antibody to lipoic acid (A) and Aβ was detected with antibody 6E10 (B).