Cognitive resilience and severe Alzheimer's disease neuropathology

Resilience to structural and molecular changes in excitatory synapses in the hippocampus contributes to cognitive function recovery in Tg2576 mice

JOURNAL/nrgr/04.03/01300535-202409000-00040/figure1/v/2024-01-16T170235Z/r/image-tiff Plaques of amyloid-β (Aβ) and neurofibrillary tangles are the main pathological characteristics of Alzheimer's disease (AD). However, some older adult people with AD pathological hallmarks can retain cognitive function. Unraveling the factors that lead to this cognitive resilience to AD offers promising prospects for identifying new therapeutic targets. Our hypothesis focuses on the contribution of resilience to changes in excitatory synapses at the structural and molecular levels, which may underlie healthy cognitive performance in aged AD animals. Utilizing the Morris Water Maze test, we selected resilient (asymptomatic) and cognitively impaired aged Tg2576 mice. While the enzyme-linked immunosorbent assay showed similar levels of Aβ42 in both experimental groups, western blot analysis revealed differences in tau pathology in the pre-synaptic supernatant fraction. To further investigate the density of synapses in the hippocampus of 16-18 month-old Tg2576 mice, we employed stereological and electron microscopic methods. Our findings indicated a decrease in the density of excitatory synapses in the stratum radiatum of the hippocampal CA1 in cognitively impaired Tg2576 mice compared with age-matched resilient Tg2576 and non-transgenic controls. Intriguingly, through quantitative immunoelectron microscopy in the hippocampus of impaired and resilient Tg2576 transgenic AD mice, we uncovered differences in the subcellular localization of glutamate receptors. Specifically, the density of GluA1, GluA2/3, and mGlu5 in spines and dendritic shafts of CA1 pyramidal cells in impaired Tg2576 mice was significantly reduced compared with age-matched resilient Tg2576 and non-transgenic controls. Notably, the density of GluA2/3 in resilient Tg2576 mice was significantly increased in spines but not in dendritic shafts compared with impaired Tg2576 and non-transgenic mice. These subcellular findings strongly support the hypothesis that dendritic spine plasticity and synaptic machinery in the hippocampus play crucial roles in the mechanisms of cognitive resilience in Tg2576 mice.

Neuroglia in cognitive reserve

The concept of cognitive reserve was born to account for the disjunction between the objective extent of brain damage in pathology and its clinical and intellectual outcome. The cognitive reserve comprises structural (brain reserve) and functional (brain maintenance, resilience, compensation) aspects of the nervous tissue reflecting exposome-driven life-long plasticity, which defines the ability of the brain to withstand aging and pathology. The mechanistic background of this concept was primarily focused on adaptive changes in neurones and neuronal networks. We present arguments favoring the more inclusive view, positing that neuroglia are fundamental for defining the cognitive reserve through homeostatic, neuroprotective, and neurodegenerative mechanisms. Neuroglia are critical for the life-long shaping of synaptically connected neuronal circuits as well as the brain connectome thus defining cognitive reserve. Neuroglial homeostatic and protective physiological responses define brain maintenance and resilience, while neuroglia regenerative capabilities are critical for brain compensation in pathology. Targeting neuroglia may represent an untrodden path for prolonging cognitive longevity.

Associations of brain morphology with cortical proteins of cognitive resilience

In a recent proteome-wide study, we identified several candidate proteins for drug discovery whose cortical abundance was associated with cognitive resilience to late-life brain pathologies. This study examines the extent to which these proteins are associated with the brain structures of cognitive resilience in decedents from the Religious Orders Study and Memory and Aging Project. Six proteins were associated with brain morphometric characteristics related to higher resilience (i.e., larger anterior and medial temporal lobe volumes), and five were associated with morphometric characteristics related to lower resilience (i.e., enlarged ventricles). Two synaptic proteins, RPH3A and CPLX1, remained inversely associated with the lower resilience signature, after further controlling for 10 neuropathologic indices. These findings suggest preserved brain structure in periventricular regions as a potential mechanism by which RPH3A and CPLX1 are associated with cognitive resilience. Further work is needed to elucidate other mechanisms by which targeting these proteins can circumvent the effects of pathology on individuals at risk for cognitive decline.

The concept of resilience to Alzheimer's Disease: current definitions and cellular and molecular mechanisms

Some individuals are able to maintain their cognitive abilities despite the presence of significant Alzheimer's Disease (AD) neuropathological changes. This discrepancy between cognition and pathology has been labeled as resilience and has evolved into a widely debated concept. External factors such as cognitive stimulation are associated with resilience to AD, but the exact cellular and molecular underpinnings are not completely understood. In this review, we discuss the current definitions used in the field, highlight the translational approaches used to investigate resilience to AD and summarize the underlying cellular and molecular substrates of resilience that have been derived from human and animal studies, which have received more and more attention in the last few years. From these studies the picture emerges that resilient individuals are different from AD patients in terms of specific pathological species and their cellular reaction to AD pathology, which possibly helps to maintain cognition up to a certain tipping point. Studying these rare resilient individuals can be of great importance as it could pave the way to novel therapeutic avenues for AD.

Neighborhood disadvantage reduces cognitive reserve independent of neuropathologic change

INTRODUCTION

Individuals in socioeconomically disadvantaged neighborhoods exhibit increased risk for impaired cognitive function. Whether this association relates to the major dementia-related neuropathologies is unknown.

METHODS

This cross-sectional study included 469 autopsy cases from 2011 to 2023. The relationships between neighborhood disadvantage measured by Area Deprivation Index (ADI) percentiles categorized into tertiles, cognition evaluated by the last Mini-Mental State Examination (MMSE) scores before death, and 10 dementia-associated proteinopathies and cerebrovascular disease were assessed using regression analyses.

RESULTS

Higher ADI was significantly associated with lower MMSE score. This was mitigated by increasing years of education. ADI was not associated with an increase in dementia-associated neuropathologic change. Moreover, the significant association between ADI and cognition remained even after controlling for changes in major dementia-associated proteinopathies or cerebrovascular disease.

DISCUSSION

Neighborhood disadvantage appears to be associated with decreased cognitive reserve. This association is modified by education but is independent of the major dementia-associated neuropathologies.

The Role of Insulin-like Growth Factor I in Mechanisms of Resilience and Vulnerability to Sporadic Alzheimer's Disease

Despite decades of intense research, disease-modifying therapeutic approaches for Alzheimer's disease (AD) are still very much needed. Apart from the extensively analyzed tau and amyloid pathological cascades, two promising avenues of research that may eventually identify new druggable targets for AD are based on a better understanding of the mechanisms of resilience and vulnerability to this condition. We argue that insulin-like growth factor I (IGF-I) activity in the brain provides a common substrate for the mechanisms of resilience and vulnerability to AD. We postulate that preserved brain IGF-I activity contributes to resilience to AD pathology as this growth factor intervenes in all the major pathological cascades considered to be involved in AD, including metabolic impairment, altered proteostasis, and inflammation, to name the three that are considered to be the most important ones. Conversely, disturbed IGF-I activity is found in many AD risk factors, such as old age, type 2 diabetes, imbalanced diet, sedentary life, sociality, stroke, stress, and low education, whereas the Apolipoprotein (Apo) E4 genotype and traumatic brain injury may also be influenced by brain IGF-I activity. Accordingly, IGF-I activity should be taken into consideration when analyzing these processes, while its preservation will predictably help prevent the progress of AD pathology. Thus, we need to define IGF-I activity in all these conditions and develop a means to preserve it. However, defining brain IGF-I activity cannot be solely based on humoral or tissue levels of this neurotrophic factor, and new functionally based assessments need to be developed.