Soluble amyloid beta peptide concentration as a predictor of synaptic change in Alzheimer's disease

Abstract

We have characterized amyloid beta peptide (Abeta) concentration, Abeta deposition, paired helical filament formation, cerebrovascular amyloid angiopathy, apolipoprotein E (ApoE) allotype, and synaptophysin concentration in entorhinal cortex and superior frontal gyrus of normal elderly control (ND) patients, Alzheimer's disease (AD) patients, and high pathology control (HPC) patients who meet pathological criteria for AD but show no synapse loss or overt antemortem symptoms of dementia. The measures of Abeta deposition, Abeta-immunoreactive plaques with and without cores, thioflavin histofluorescent plaques, and concentrations of insoluble Abeta, failed to distinguish HPC from AD patients and were poor correlates of synaptic change. By contrast, concentrations of soluble Abeta clearly distinguished HPC from AD patients and were a strong inverse correlate of synapse loss. Further investigation revealed that Abeta40, whether in soluble or insoluble form, was a particularly useful measure for classifying ND, HPC, and AD patients compared with Abeta42. Abeta40 is known to be elevated in cerebrovascular amyloid deposits, and Abeta40 (but not Abeta42) levels, cerebrovascular amyloid angiopathy, and ApoE4 allele frequency were all highly correlated with each other. Although paired helical filaments in the form of neurofibrillary tangles or a penumbra of neurites surrounding amyloid cores also distinguished HPC from AD patients, they were less robust predictors of synapse change compared with soluble Abeta, particularly soluble Abeta40. Previous experiments attempting to relate Abeta deposition to the neurodegeneration that underlies AD dementia may have failed because they assayed the classical, visible forms of the molecule, insoluble neuropil plaques, rather than the soluble, unseen forms of the molecule.

Figure 1.

Representative examples in AD patients (left panels), HPC patients (middle panels), and ND patients (right panels) of thioflavin histofluorescent plaques (A) and tangles (B), Aβ immunoreactive plaques with and without cores (C), paired helical filament occurrence around plaques (D), and cerebrovascular amyloid angiopathy (E).

Figure 2.

Soluble and insoluble Aβ40 and Aβ42 concentrations in superior frontal gyrus and entorhinal cortex of ND, HPC, and AD patients.

Figure 3.

Synaptic density estimates from synaptophysin Western blot analysis (synaptic density OD) as a function of entorhinal cortex and superior frontal gyrus soluble Aβ concentrations. Upper left panel: Sum of soluble Aβ40 plus Aβ42 versus synaptophysin optical density (OD) in all patients. Upper right panel: Sum of soluble Aβ40 plus Aβ42 in HPC and AD patients only. Correlations within individual structures or for soluble Aβ40 and Aβ42 alone were also highly significant and gave trends similar to those illustrated here. For example, the bottom panels show the data for soluble Aβ40 in superior frontal gyrus of all patients (left) or AD patients alone (right).

Figure 4.

Synaptic density estimates from synaptophysin Western blot analysis as a function of entorhinal cortex and superior frontal gyrus paired helical filament immunoreactive (PHF⁺) plaques/mm (left panel) or thioflavin histofluorescent tangles/mm (right panel). Data points for ND, HPC, and AD patients are included.