Cognitive integrity in Non-Demented Individuals with Alzheimer's Neuropathology is associated with preservation and remodeling of dendritic spines

Abstract

Introduction: Individuals referred to as Non-Demented with Alzheimer's Neuropathology (NDAN) exhibit cognitive resilience despite presenting Alzheimer's disease (AD) histopathological signs. Investigating the mechanisms behind this resilience may unveil crucial insights into AD resistance.

Methods: DiI labeling technique was used to analyze dendritic spine morphology in control (CTRL), AD, and NDAN post mortem frontal cortex, particularly focusing on spine types near and far from amyloid beta (Aβ) plaques.

Results: NDAN subjects displayed a higher spine density in regions distant from Aβ plaques versus AD patients. In distal areas from the plaques, NDAN individuals exhibited more immature spines, while AD patients had a prevalence of mature spines. Additionally, our examination of levels of Peptidyl-prolyl cis-trans isomerase NIMA-interacting 1 (Pin1), a protein associated with synaptic plasticity and AD, showed significantly lower expression in AD versus NDAN and CTRL.

Discussion: These results suggest that NDAN individuals undergo synaptic remodeling, potentially facilitated by Pin1, serving as a compensatory mechanism to preserve cognitive function despite AD pathology.

Highlights: Spine density is reduced near Aβ plaques compared to the distal area in CTRL, AD, and NDAN dendrites. NDAN shows higher spine density than AD in areas far from Aβ plaques. Far from Aβ plaques, NDAN has a higher density of immature spines, AD a higher density of mature spines. AD individuals show significantly lower levels of Pin1 compared to NDAN and CTRL.

Keywords: Alzheimer's disease; Non‐Demented with Alzheimer's Neuropathology; Pin1; amyloid beta; cognitive resilience; dendritic spines; neurodegeneration; synaptic plasticity.

FIGURE 1

Illustration of DiI labeling steps of neuronal structure in post mortem human frontal cortex and analysis of dendrites and spine parameters. (A) Simplified steps of DiI application on brain tissue sections and thioflavin labeling. (B) Representative picture of dendrites (red) surrounding the Aβ plaque (blue in a small oval). The area defined as proximal to the plaque includes Aβ plaque and the surrounding 10‐μm area. The area outside the large oval without plaque deposition is considered distal area. (C, D) Image of dendrite (C) and 3D reconstructions of the different types of spine (D) after analysis using Filament tracer and Classify Spines XTension Imaris analysis software version 9.9. Dendrite length, dendrite diameter, spine number, spine density, and dendritic spine morphology in the proximal area versus distal area were analyzed. (E) Spine classification criteria according to Classify Spines XTension Imaris analysis software version 9.9. Donor information described in Table 1A.

FIGURE 2

Quantitative analysis of dendrite diameter, length, and total spine density. Proximal (A) and distal (B) dendrites and their dendritic spines shown in red. A white asterisk highlights an amyloid plaque. An arrowhead points to a bulbous dilation of a dendrite observed in the frontal cortex of an AD post mortem brain sample. The respective right columns present reconstructed 3D images of the dendritic spines and dendrites, based on the images in the left columns. Turquoise circles indicate the starting points of each quantified dendrite. Stick‐like shapes are color coded: red for stubby spines, green for mushroom spines, blue for long thin spines, magenta for filopodia. No significant differences were identified in the Aβ plaque proximity: dendrite diameter (B), length (E), or total spine density (H) among CTRL, AD, and NDAN. (C) In the distal area, AD individuals showed larger dendrite diameter compared to NDAN. The dendrite length (F) in the distal area was not different among the groups. (I) Total spine density was higher in NDAN compared to AD, in the distal area. When comparing the proximal area to the distal area, the dendrite diameter (D), length (G), and total spine density (J) in the distal area exceeded those in the proximal area for all groups, except for the dendrite length of NDAN. For statistical tests, we used a mixed‐effects ANOVA model with a random intercept for the individuals. Donor information described in Table 1A.

FIGURE 3

Quantitative analysis of spine types relative to Aβ plaques. Stubby spines (A–C), mushroom spines (D–F), long thin spines (G–I), and filopodia (J–L) were analyzed expressed as number of spines per 10 μm, in proximity and distally to the Aβ plaques. There were no significant differences in spine density of stubby (A), mushroom (D), long thin (G), or filopodial (L) between CTRL, AD, and NDAN in the proximal area. In the distal area: (B) Stubby spines did not show differences within groups; however, (E) AD subjects have significantly higher levels of mushroom spines compared to NDAN. In the distal area, (H) long thin spine density was higher in NDAN compared to CTRL and AD individuals, and filopodia (K) spines were higher in NDAN compared to AD. The densities of the four spine subtypes (C, F, I, L) were higher in distal areas than in the proximal area, except for mushroom spines in NDAN (F), long thin in AD (I), and filopodia in CTRL (L). For statistical tests, we used a mixed‐effects ANOVA model with a random intercept for the individuals. Donor information is presented in Table 1A.

FIGURE 4

Percentage of spine subtypes relative to Aβ plaques. Donut diagrams showing CTRL, AD, and NDAN spine subtype relative abundances expressed in percentages, in proximal (A) and in distal (B) areas. Spine subtypes are color coded: red for stubby spines, magenta for filopodia spines, blue for long thin spines, and green for mushroom spines. In the distal area, the densities of stubby, mushroom, long thin, and filopodia spines differ significantly between CTRL, AD, and NDAN. In contrast, in the proximal area, the spine densities of all types, except for the mushroom spines, did not show significant differences (see p values in (C). For statistical analysis, a chi‐squared test was used to determine whether the percentage spines differed between diagnoses (C). Donor information is presented in Table 1A.

FIGURE 5

Levels of Pin1 relative to Aβ plaques. (A, B) Representative images displaying Pin1 distribution in the proximal area compared to the distal area in CTRL, NDAN, and AD subjects. (C) Integrated density of Pin1 distribution showed higher levels of this protein in NDAN compared to AD and CTRL, close to the plaque. (D) Distal to plaques, AD showed significantly lower levels of Pin1. (E) NDAN and CTRL showed higher levels of Pin1 in distal regions compared to proximal; this difference was not observed in AD individuals. (F) Western blot analysis of Pin1 protein expression relative to β‐actin did not show differences within groups. For statistical analysis, one‐way ANOVA was used, followed by Tukey's multiple‐comparisons test, p < .05. Value is expressed as mean ± SD. Donor information is presented in Table 1B.