Cholinotrophic basal forebrain connectome dysfunction in Down syndrome with and without dementia

Abstract

Cholinotrophic basal forebrain (CTBF) neurons depend upon the complex interaction of both upstream and downstream nerve growth factor (NGF) signaling pathways for survival and function. Although dysfunction of the NGF system occurs in both Down syndrome (DS), not all individuals with DS develop dementia. Whether NGF system dysregulation differs between demented individuals with DS (DSD+) versus those without dementia (DSD-) is unknown. Here, we report a significant reduction in neurons positive for the p75^NTR within the nucleus basalis of Meynert in DSD+, but not DSD-, compared to aged-matched controls (AMC). ChAT positive cells were significantly lower in DSD+ compared to DSD- and AMC cases. FC levels of p75^NTR and proNGF were increased, while ChAT levels decreased in DSD+ compared to AMC. A greater number of AT8 tau positive neuropil threads and Thioflavin-S labeled neurofibrillary tangles were found in DSD+. These findings suggest a greater role for p75^NTR/proNGF in demented in individuals with DS compared to those with dementia. The factors that underlie cognitive resilience in DSD- remains to be determined.

Keywords: Cell biology; Neuroscience.

Graphical abstract

Figure 1

Images and quantitation of cholinotrophic basal forebrain neuron pathology within the nucleus basalis of Meynert in DSD+, DSD- and AMC cases (A–N) Photomicrographs show p75^NTR (brown, A-C), ChAT (purple, D-F) positive cells in AMC, DSD-, and DSD+ cases. Note the decrease in the number and intensity of both p75^NTR and ChAT labeled neurons in DSD- cases compared to an even greater reduction in DSD+ and globose shaped cells (black arrows) in DSD+ cases (C, F). Arrows in (A–F) mark cells shown at a higher magnification in the boxed areas located at the lower right corner of each panel. Dark brown AT8 (G, I) and TauC3 (H, J) bearing neurofibrillary tangles (NFTs) were observed only in the DSD- and DSD+ cases. Black arrows indicate globose shaped NFTs (G, H, and I) that are shown at a higher magnification in boxed areas adjacent to the lower magnification images. Note that not all neurons within the nbM (thin black arrows) contained tau pathology (G, H) in DSD- cases. Moreover, TauC3 staining revealed two NFT phenotypes that displayed either peripherally located or intense labeling that filled the entire structure (see boxed images adjacent to (H) and (J)). Scale bar in F = 50 μm and inset = 20 μm applies to panels A-E. Scale bar in J = 20 μm and inset = 20 μm applies to (G–J). Histograms show a significant reduction in both p75^NTR (K) and ChAT (L) positive cells in DSD+ compared to AMC. Although no significant were found in number of AT8 or TauC3 NFT positive cells (M), there was an increase in NTs in DSD+ compared to DSD- (N). ACM n = 5, DSD- n = 5, DSD+ n = 10. Data shown are presented as mean ± SEM. Statistical significance was determined using the Kruskal–Wallis’s test followed by Dunn’s test for comparisons across three clinical groups, and the Mann–Whitney test for comparisons between two groups (DSD- vs. DSD+). Significance levels (∗) were set at: ∗p < 0.05, ∗∗p < 0.01, ∗∗∗p < 0.001.

Figure 2

Immunofluorescent images and quantification of p75^NTR neurons with and without AT8 and MAP2, as well as Thioflavin S single and AT8/Thioflavin S dual-labeled nbM cells in DSD+ and DSD-cases. (A–U) Immunofluorescent neurons labeled with p75^NTR (A, E, green), MAP2 (B, F, red), AT8 (C, G, Cyan), ThS (blue) (I, L) and merge images (D, H, K, N, R) in DSD- and DSD+ cases. In the DSD-cases there were greater p75^NTR cells AT8 negative neurons and p75^NTR MAP2 dual labeled neurons (A-D) compared to the DSD+ cases (E–H). Note that not all p75^NTR cells colocalize with AT8 or MAP2 in both DS groups (D, H). To determine the stage of a tangle tissue was stained for ThS, a marker of advanced pathology and the early stage AT8 phosphorylation antibody. Note that there are only a few ThS-labeled tangles that displayed AT8 in both DS groups (yellow arrows), compared to single ThS tangles, which were greater in DSD+ than in DSD- (white arrows) (I-L). Note that ThS positive NFTs that do not contain AT8 also do not colocalize with MAP2 (O-R). Scale bar in F = 25 μm applies to panels A-G, N = 25 μm applies to panels I-M and R = 10 μm and applies to O-Q. Graph showing a significant reduction in both p75^NTR AT8 immuno-negative, and p75^NTR MAP2 dual labeled neurons in DSD+ compared to DSD- (S). ThS-positive cells were greater (T), while the percentage of double-labeled cells with AT8 and ThS decreased (U) in DSD+ compared to individuals without dementia. DSD- n = 5, DSD+ n = 5. Data are presented as mean ± SEM. Statistical significance was determined using Mann–Whitney test for comparisons between DSD- and DSD+. Significance levels (∗) were set at: ∗p < 0.05, ∗∗p < 0.01.

Figure 3

FC cholinotrophic protein levels in AMC, DSD- and DSD+ (A–D) Representative immunoblots, and bar graphs show a significant upregulation of (A) proNGF and (B) p75^NTR, while (C) ChAT protein was downregulated between AMC and DSD+. (D) TrkA protein levels were stable across the groups analyzed. ACM n = 5, DSD- n = 5, DSD+ n = 13. Data are presented as mean ± SEM. Statistical significance was determined using the Kruskal–Wallis’s test followed by Dunn’s test for comparisons across three clinical groups. Significance levels (∗) were set at: ∗p < 0.05.

Figure 4

Summary of changes in the cholinotrophic basal forebrain connectome in DS Diagrammatic sagittal view of the human brain (A) and a modified stacked bar graph (B) illustrating differences in the pathobiology of the cholinotrophic projection system between non-trisomy age-matched control (AMC), DS without dementia (DSD-) and DS with dementia (DSD+) individuals with DS. Frontal cortex protein levels for ChAT (pink), proNGF (green), p75^NTR (purple) and TrkA (light blue). Nucleus basalis NFTs of ThS (blue), AT8 (orange) and TauC3 (black) and counts of p75^NTR and ChAT (red) neurons. Created with BioRender.com.