Stephen Barrie Wharton

Cellular senescence may affect the post-mitotic cells of the brain. We examined the expression of senescence markers, including p16, p21, γH2Ax and H3K9me3, in the frontal cortex of brain donations from the Cognitive Function and Ageing Study to assess their relationship to Alzheimer’s disease neuropathological change (ADNC) and dementia. p21, γH2Ax and H3K9me3 were expressed in pyramidal neurons and glia, whilst p16 was confined to glial cells. p21 and γH2Ax were correlated in neurons, and with p16 in glia. They did not increase with ADNC, tending to be higher at early Braak neurofibrillary tangle stages. Transcriptomic profiling of pyramidal neuron-enriched samples at low Braak stages showed that higher neuronal p21 expression was associated with altered pathways for neuronal function, neurodegeneration, protein homeostasis, mitochondrial dysfunction and synaptic signalling. In conclusion, the different expression profile of senescence markers in neurons and glia suggest possible differences in senescence-related mechanisms. Expression at lower ADNC stages suggests senescence may be important at earlier stages of Alzheimer’s pathogenesis, whilst transcriptomic changes suggest an impact on neuronal function. The lack of association of senescence markers with dementia status indicates that more work is needed to determine the value of senescence as a therapeutic target for dementia.

Introduction: Vascular disease is associated with late‐life dementia and other morbidities during life and commonly coexists with neurodegenerative pathologies. A complex variety of lesions can be documented, which has made development of standardised operationalised criteria difficult. A semiquantitative approach, vascular cognitive impairment neuropathology guidelines (VCING), has been developed that provides a score based on assessment of occipital white matter arteriolosclerosis, occipital leptomeningeal cerebral amyloid angiopathy (CAA) and the presence of any macroscopic infarct. Methods: We applied VCING assessment to an ageing population‐derived neuropathology cohort derived from two subcentres of the cognitive function and ageing study ( n = 158, 63.7% female and 36.3% male). Results: Vascular pathology using this method was common in this UK ageing population sample with 75% of individuals having a score of 1 or more. VCING score was associated with the presence of dementia at death, even accounting for Alzheimer's disease neuropathological change (ADNC). Of the VCING components, white matter arteriolosclerosis, but not CAA or infarcts, was associated with dementia (after adjustment for ADNC). VCING score was not associated with vascular risk factors during life, but males were at greater risk of a higher VCING score than females. VCING and arteriolosclerosis were associated with white matter pallor and perivascular space widening as markers of white matter damage. Venous collagenosis was also associated with white matter pallor. There was an association between VCING score and ADNC in regression analysis, and the association was particularly strong for the CAA component. Conclusions: VCING is a straightforward method to assess vascular pathology in neuropathology cohorts and retains its association with dementia. The arteriolosclerosis component, in particular, is related to dementia and to white matter damage, whereas venous‐side effects may also be important for the latter. The strong association of the CAA component with ADNC measures, however, raises the question of whether the VCING approach may have some confounding effects with ADNC.

Age-associated deep-subcortical white matter lesions (DSCLs) are an independent risk factor for dementia, displaying high levels of CD68⁺ microglia. This study aimed to characterize the transcriptomic profile of microglia in DSCLs and surrounding radiologically normal-appearing white matter (NAWM) compared to non-lesional control white matter. CD68⁺ microglia were isolated from white matter groups (n = 4 cases per group) from the Cognitive Function and Ageing Study neuropathology cohort using immuno-laser capture microdissection. Microarray gene expression profiling, but not RNA-sequencing, was found to be compatible with immuno-LCM-ed post-mortem material in the CFAS cohort and identified significantly differentially expressed genes (DEGs). Functional grouping and pathway analysis were assessed using the Database for Annotation Visualization and Integrated Discovery (DAVID) software, and immunohistochemistry was performed to validate gene expression changes at the protein level. Transcriptomic profiling of microglia in DSCLs compared to non-lesional control white matter identified 181 significant DEGs (93 upregulated and 88 downregulated). Functional clustering analysis in DAVID revealed dysregulation of haptoglobin–haemoglobin binding (Enrichment score 2.5, p = 0.017), confirmed using CD163 immunostaining, suggesting a neuroprotective microglial response to blood–brain barrier dysfunction in DSCLs. In NAWM versus control white matter, microglia exhibited 347 DEGs (209 upregulated, 138 downregulated), with significant dysregulation of protein de-ubiquitination (Enrichment score 5.14, p < 0.001), implying an inability to maintain protein homeostasis in NAWM that may contribute to lesion spread. These findings enhance understanding of microglial transcriptomic changes in ageing white matter pathology, highlighting a neuroprotective adaptation in DSCLs microglia and a potentially lesion-promoting phenotype in NAWM microglia.

The epidemiological neuropathology perspective of population and community‐based studies allows unbiased assessment of the prevalence of various pathologies and their relationships to late‐life dementia. In addition, this approach provides complementary insights to conventional case–control studies, which tend to be more representative of a younger clinical cohort. The Cognitive Function and Ageing Study (CFAS) is a longitudinal study of cognitive impairment and frailty in the general United Kingdom population. In this review, we provide an overview of the major findings from CFAS, alongside other studies, which have demonstrated a high prevalence of pathology in the ageing brain, particularly Alzheimer's disease neuropathological change and vascular pathology. Increasing burdens of these pathologies are the major correlates of dementia, especially neurofibrillary tangles, but there is substantial overlap in pathology between those with and without dementia, particularly at intermediate burdens of pathology and also at the oldest ages. Furthermore, additional pathologies such as limbic‐predominant age‐related TDP‐43 encephalopathy, ageing‐related tau astrogliopathy and primary age‐related tauopathies contribute to late‐life dementia. Findings from ageing population‐representative studies have implications for the understanding of dementia pathology in the community. The high prevalence of pathology and variable relationship to dementia status has implications for disease definition and indicate a role for modulating factors on cognitive outcome. The complexity of late‐life dementia, with mixed pathologies, indicates a need for a better understanding of these processes across the life‐course to direct the best research for reducing risk in later life of avoidable clinical dementia syndromes.

(1) Background: Systemic infection is associated with increased neuroinflammation and accelerated cognitive decline in AD patients. Activated neutrophils produce neutrophil-derived microvesicles (NMV), which are internalised by human brain microvascular endothelial cells and increase their permeability in vitro, suggesting that NMV play a role in blood–brain barrier (BBB) integrity during infection. The current study investigated whether microRNA content of NMV from AD patients is significantly different compared to healthy controls and could impact cerebrovascular integrity. (2) Methods: Neutrophils isolated from peripheral blood samples of five AD and five healthy control donors without systemic infection were stimulated to produce NMV. MicroRNAs isolated from NMV were analysed by RNA-Seq, and online bioinformatic tools were used to identify significantly differentially expressed microRNAs in the NMV. Target and pathway analyses were performed to predict the impact of the candidate microRNAs on vascular integrity. (3) Results: There was no significant difference in either the number of neutrophils (p = 0.309) or the number of NMV (p = 0.3434) isolated from AD donors compared to control. However, 158 microRNAs were significantly dysregulated in AD NMV compared to controls, some of which were associated with BBB dysfunction, including miR-210, miR-20b-5p and miR-126-5p. Pathway analysis revealed numerous significantly affected pathways involved in regulating vascular integrity, including the TGFβ and PDGFB pathways, as well as Hippo, IL-2 and DNA damage signalling. (4) Conclusions: NMV from AD patients contain miRNAs that may alter the integrity of the BBB and represent a novel neutrophil-mediated mechanism for BBB dysfunction in AD and the accelerated cognitive decline seen as a result of a systemic infection.

Although a variety of brain lesions may contribute to the pathological diagnosis of dementia, the relationship of these lesions to dementia, how they interact and how to quantify them remain uncertain. Systematically assessing neuropathological measures in relation to the cognitive and functional definitions of dementia may enable the development of better diagnostic systems and treatment targets. The objective of this study is to apply machine learning approaches for feature selection to identify key features of Alzheimer-related pathologies associated with dementia. We applied machine learning techniques for feature ranking and classification as an unbiased comparison of neuropathological features and assessment of their diagnostic performance using a cohort (n=186) from the Cognitive Function and Ageing Study (CFAS). Seven feature ranking methods using different information criteria consistently ranked 22 out of the 34 neuropathology features for importance to dementia classification. Braak neurofibrillary tangle stage, Beta-amyloid and cerebral amyloid angiopathy features were the most highly ranked, although were highly correlated with each other. The best performing dementia classifier using the top eight ranked neuropathology features achieved 79% sensitivity, 69% specificity, and 75% precision. A substantial proportion (40.4%) of dementia cases was consistently misclassified by all seven algorithms and any combination of the 22 ranked features. These results highlight the potential of using machine learning to identify key indices of plaque, tangle and cerebral amyloid angiopathy burdens that may be useful for the classification of dementia.

Oxidative DNA damage induces changes in the neuronal cell cycle and activates a DNA damage response to promote repair, but these processes may be altered under a chronic oxidative environment, leading to the accumulation of unrepaired DNA damage and continued activation of a DNA damage response. Failure to repair DNA damage can lead to apoptosis or senescence, which is characterized by a permanent cell-cycle arrest. Increased oxidative stress and accumulation of oxidative DNA damage are features of brain ageing and neurodegeneration but the effects of persistent DNA damage in neurons are not well-characterized. We developed a model of persistent oxidative DNA damage in immortalized post-mitotic neurons in vitro by exposing them to a sub-lethal concentration of hydrogen peroxide following a “double stress” protocol, and performed a detailed characterization of the neuronal transcriptome using microarray analysis. Persistent DNA damage significantly altered the expression of genes involved in cell cycle regulation, DNA damage response and repair mechanisms, and mitochondrial function, suggesting an active DDR response to replication stress and alterations in mitochondrial electron transport chain. qPCR and functional validation experiments confirmed hyperactivation of mitochondrial Complex I in response to persistent DNA damage. These changes in response to persistent oxidative DNA damage may lead to further oxidative stress, contributing to neuronal dysfunction and ultimately neurodegeneration.