Phases of Aβ-deposition in the human brain and its relevance for the development of AD

TLDR: Aβ-deposition in the entire brain follows a distinct sequence in which the regions are hierarchically involved and expands anterogradely into regions that receive neuronal projections from regions already exhibiting Aβ.

Abstract: Background: The deposition of the amyloid β protein (Aβ) is a histopathologic hallmark of AD. The regions of the medial temporal lobe (MTL) are hierarchically involved in Aβ-deposition. Objective: To clarify whether there is a hierarchical involvement of the regions of the entire brain as well and whether there are differences in the expansion of Aβ-pathology between clinically proven AD cases and nondemented cases with AD-related pathology, the authors investigated 47 brains from demented and nondemented patients with AD-related pathology covering all phases of β-amyloidosis in the MTL (AβMTL phases) and four control brains without any AD-related pathology. Methods: Aβ deposits were detected by the use of the Campbell-Switzer silver technique and by immunohistochemistry in sections covering all brain regions and brainstem nuclei. It was analyzed how often distinct regions exhibited Aβ deposits. Results: In the first of five phases in the evolution of β-amyloidosis Aβ deposits are found exclusively in the neocortex. The second phase is characterized by the additional involvement of allocortical brain regions. In phase 3, diencephalic nuclei, the striatum, and the cholinergic nuclei of the basal forebrain exhibit Aβ deposits as well. Several brainstem nuclei become additionally involved in phase 4. Phase 5, finally, is characterized by cerebellar Aβ-deposition. The 17 clinically proven AD cases exhibit Aβ-phases 3, 4, or 5. The nine nondemented cases with AD-related Aβ pathology show Aβ-phases 1, 2, or 3. Conclusions: Aβ-deposition in the entire brain follows a distinct sequence in which the regions are hierarchically involved. Aβ-deposition, thereby, expands anterogradely into regions that receive neuronal projections from regions already exhibiting Aβ. There are also indications that clinically proven AD cases with full-blown β-amyloidosis may be preceded in early stages by nondemented cases exhibiting AD-related Aβ pathology.