Quantitative proteomics identifies surfactant-resistant alpha-synuclein in cerebral cortex of Parkinsonism-dementia complex of Guam but not Alzheimer's disease or progressive supranuclear palsy

Abstract

Parkinsonism-dementia complex (PDC) remains a significant health burden to the Chamorro population. We tested the hypothesis that quantitative proteomics might provide fresh insight into this enigmatic illness by analyzing proteins resistant to surfactant extraction from patients with Alzheimer's disease (AD) or PDC and their matched controls using isobaric tags for relative and absolute quantification. In addition to the expected increase in abnormal frontal cortical Abeta peptides, tau, ubiquitin, and apolipoprotein E in AD, and tau in PDC, we identified alpha-synuclein (SNCA) as a major abnormal protein in PDC but not AD. We confirmed our isobaric tags for relative and absolute quantification findings by enzyme-linked immunosorbent assay in frontal and temporal cortices. We extended our assays to include a limited number of cases of progressive supranuclear palsy (PSP) and dementia with Lewy bodies; we observed increased abnormal tau but not SNCA in PSP, and abnormal SNCA in dementia with Lewy bodies that was quantitatively similar to PDC. Finally, soluble Abeta oligomers were selectively increased in AD but not PDC or PSP. These results show that frontal and temporal cortex in PDC is distinguished from AD and PSP by its accumulation of abnormal SNCA and suggest that PDC be considered a synucleinopathy as well as a tauopathy.

Figure 1

Data are average ± SEM protein. SI protein concentration determined by BCA assay. Neurodegenerative group for Seattle is AD and for Guam is PDC. One-way analysis of variance had P < 0.0001. Bonferroni-corrected repeated pair comparisons had P < 0.01 for PDC versus CG and P < 0.001 for AD versus CS. Two-way analysis of variance had P < 0.0001 for neurodegenerative disease versus control but not for location; there was no significant interaction between these two terms. Bonferroni-corrected post tests had P < 0.01 for both Seattle and Guam groups, but P > 0.05 for AD versus PDC or CS versus CG.

Figure 2

Antibody capture assays for SI proteins in frontal and temporal cortex. Data are presented as log scatterplots (average marked with line) from patients with the three different tauopathies and controls (combined from Seattle and Guam). All data are from antibody capture assays with absorbance read at 405 nm and were analyzed by one-way nonparametric analysis of variance (Kruskal-Wallis test) followed by Dunn’s corrected multiple comparison test. A: SI tau analysis of variance had P < 0.0001, and Dunn’s tests had P < 0.05 for AD versus PDC and AD versus PSP, and P < 0.01 for AD versus control and PDC versus control. B: SI Aβ₄₂ analysis of variance had P < 0.0001 and Dunn’s tests had P < 0.001 for AD versus PDC, AD versus PSP, and AD versus control, but P > 0.05 for all repeat comparisons among PDC, PSP, and controls. C: SI apoE analysis of variance had P < 0.0001, and Dunn’s tests had P < 0.05 for AD versus PDC, AD versus PSP, and AD versus control, but P > 0.05 for all repeat comparisons among PDC, PSP, and controls. D: SI ubiquitin analysis of variance had P < 0.0001, and Dunn’s tests had P < 0.01 for AD versus PDC, AD versus PSP, and AD versus control, but P > 0.05 for all repeat comparisons among PDC, PSP, and controls.

Figure 3

Antibody capture assay for TI SNCA in frontal and temporal cortex using Chemicon antibody. Data are average ± SEM for each group. One-way analysis of variance had P < 0.001. Bonferroni-corrected repeat paired comparisons had P < 0.01 for PDC versus control and P < 0.001 for DLB versus control, but P > 0.05 for AD or PSP versus control.

Figure 4

Luminex assay for soluble Aβ oligomers in frontal and temporal cortex. Data are presented as log scatterplot (average marked with line) from patients with three different tauopathies and controls (combined from Seattle and Guam). Note that the ordinate is a log scale and that PDC and AD groups do not overlap and PDC and controls completely overlap. One-way analysis of variance nonparametric analysis of variance (Kruskal-Wallis test) had P < 0.0001 with Dunn’s corrected repeat comparisons having P < 0.001 for AD versus all paired comparisons, but P > 0.05 for all other paired comparisons.