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Background: Brain perfusion and blood brain barrier (BBB) integrity are reduced in Alzheimer's disease (AD). We hypothesized that b‐amyloid (Aβ) induces dysfunction of pathways in vascular cells that regulate angiogenesis. Method: We performed single nucleus RNA sequencing (snRNAseq) of vascular cells isolated from post mortem samples from multiple cortical regions from AD patients (Braak V‐VI) and non‐diseased controls (NDC, Braak 0‐II). We developed a meta‐analytic approach to integrate these data with related publicly available datasets. We performed differential gene expression (DGE) analyses as a categorical comparison of AD vs NDC and as a confound‐controlled regression of transcriptomic alterations by regional total Aβ. We also performed cell‐specific gene co‐expression analyses identify modules differentially expressed in AD relative to NDC. Result: Our results describe differential expression of genes central to pathways sustaining angiogenesis, proliferation and migration of endothelial cells and the integrity of the BBB with either AD or greater pathological protein load. HIF1A and FGF2, proangiogenic molecules which both induce VEGF, are upregulated in EC, consistent the parenchymal hypoxic response shown repeatedly in prior studies. However, components of signalling pathways downstream of VEGFR2 are downregulated, suggesting a pathological block to angiogenesis with greater Aβ load. While the proangiogenic stimuli promote the expression of genes such as PPP3CA and ITPR1, coding for calcineurin, which contribute to VEGF signalling through the PLCγ‐Ca2+ signalling pathway are upregulated in AD, expression of RAC1 and RHOJ, which enable cellular migration and adhesion, is downregulated. While ANGPT2 expression is also upregulated, the angiopoietin receptor (TIE2, coded by TEK) is downregulated. Downregulation of ADAM10 in AD and upregulation of other NOTCH pathway genes with greater Aβ load suggests altered NOTCH signalling. Conclusion: Overall, a hypoxic tissue environment in AD is accompanied by dysfunctional angiogenesis related to Aβ load. Evidence for multiple sites of dysfunction were identified, involving impaired angiopoietin responses, vascular cell migration and NOTCH signaling. [ABSTRACT FROM AUTHOR]

Background: Coding variants in the microglial TREM2 ectodomain differentially (R47H> R62H) increase the risk of Alzheimer's disease (AD). To define mechanisms responsible in glia and neurons, we aimed to characterise neuropathology and transcriptomic responses in heterozygotes for these TREM2 variant alleles and for common allele homozygotes (CV) in non‐diseased (n = 16) and AD (n = 42) brain cortical tissue from 58 donors. Method: Using immunohistochemistry (IHC) and imaging mass cytometry (IMC), we characterised neocortical b‐amyloid and AT8/PHF1 pTau pathology with CV (n = 30) and the R47H (n = 11) or R62H (n = 17) TREM2 carriers. We performed single nuclear RNA sequencing to test for the differential gene expression (DGE) in TREM2 and CV cortical tissues with greater 4G8+ b‐amyloid‐ or PHF1+ pTau‐immunostaining. Result: There was a two‐fold increase in 4G8+, a three‐fold reduction in AT8+ but no change in PHF1+ immunostaining with AD in both variants versus CV. We found a reduced expression of genes associated with TREM2 function in R47H carriers relative to CV however enhanced microglial activation in R62H carriers. This was associated with an up‐regulation of pathways such as 'neuroinflammation signalling pathway' in R62H not seen in R47H. CV astrocytes had little DGE with increasing pathology however both variants (R47H>R62H) had an increased proportion of differentially expressed genes suggestive of a heightened activation state. R47H and R62H astrocytes impacted pathways had opposing actions such as 'synaptogenesis signalling pathway' and 'PDGF signalling' pathways downregulated in R47H but upregulated in R62H. Neuronal gene expression changes were evident in all variants, however greatly enhanced in R47H in response to 4G8+ b‐amyloid. With increasing PHF1+ pTau gene expression changes were mainly seen in R47H. Excitatory and inhibitory neurons both exhibited DGE in R47H carriers and impacted pathways centred around those involved in 'synaptogenesis signalling pathway', 'SNARE signalling pathway' and 'synaptic long term depression'. Conclusion: This analysis identified novel disease‐associated transcriptomic differences in the glial response to TREM2 and the secondary neuronal responses that can explain differences in the genetic AD risk conferred by these two TREM2 risk variants. [ABSTRACT FROM AUTHOR]

Background: Ageing is the greatest risk factor for Alzheimer's disease (AD). While accumulation of senescent cells is one of the major hallmarks of ageing, the effect of the senescence burden is yet to be explored in AD. The aims of this study were to i) quantify senescent microglia, oligodendrocyte and astrocyte in human brain in AD, ii) determine the driver of cell senescence in AD, and iii) AD pathology associated senescence. Method: We multiplexed Image Mass Cytometry (IMC) to co‐localise cell type markers (IBA1, OLIG2, GFAP), senescence markers (GLB1 and p16) and β‐amyloid (4G8) to detect senescence in tissues from Middle Temporal Gyrus (MTG) of 10 AD and 10 non‐disease control (NDC). We also generated snRNA‐seq from three different brain regions of the same subject namely Entorhinal (EC), MTG (Mid‐temporal Cortex) and Somatosensory Cortex (SSC) of 9 AD and 9 NDC. We performed geneset enrichment analysis using a set of curated senescence associated genesets in snRNA‐seq using AUcell and dream (variancePartition). We further performed trajectory analysis using Monocle3 to characterise β‐amyloid associated senescence signatures in the snRNAseq data. Result: More than 25% microglia, oligodendrocyte and astrocyte were positive for GLB1 in AD brains while only around 5% glia showed GLB1 expression in NDC. Around 25% peri‐plaque microglia within 10 µm around β‐amyloid plaques in AD showed expression of GLB1 while only 3% of microglia >10 µm had associated senescence markers. Senescence genes were significantly upregulated in AD compared to NDC and were positively correlated with increase amyloid densities in all three glial cells (not in neurons). Trajectories of the glial nuclei described increased expression of pathways related to replicative senescence, cellular activation and stress response in microglia and oxidative stress in oligodendroglia, and astrocyte. Senescence gene signatures were positively associated with both β‐amyloid and pTau. Conclusion: Our results highlight the high burden of senescent glia and provide evidence for distinct mechanisms of senescence in microglia (both replicative and stress‐induced) relative to oligodendroglia and astrocyte (stress‐induced) in AD. Both β‐amyloid and pTau associated increase in senescence gene signatures suggest that senescence is an important feature in the late stage of AD. [ABSTRACT FROM AUTHOR]

Background: Autosomal dominant familial Alzheimer's disease (ADAD), caused by APP, PSEN1 and PSEN2 mutations, is clinically and pathologically similar to sporadic AD, with both typically presenting with memory impairment. However as in sporadic AD, atypical phenotypes and neuropathological heterogeneity occur, including variable vascular Aβ deposition as cerebral amyloid angiopathy (CAA). Investigating relationships between clinical, genetic and neuropathological heterogeneity in ADAD may provide insights into pathophysiological pathways relevant to AD in general. Methods: 256 symptomatic individuals from ADAD families seen at our Centre were included in a survival analysis, with detailed clinical phenotype data available for 138 (101 PSEN1, 37 APP). 33 of these individuals underwent brain donation (24 PSEN1, 9 APP), all demonstrating end‐stage AD pathology. Frontal and occipital cortex sections were stained immunohistochemically for Aβ and vessel counts used to determine the proportion of cortical and leptomeningeal blood vessels affected by CAA. Results: Age at onset was, on average, seven years younger for PSEN1 than APP mutations and was influenced by mutation much more than survival was. Atypical (non‐amnestic) cognitive presentations with initial behavioral, language, dyscalculic or dysexecutive symptoms, and atypical clinical features (pyramidal, extrapyramidal and cerebellar signs) were more common in PSEN1 than APP cases, particularly those with post‐codon 200 mutations. Atypical cognitive presentations were associated with longer survival. There was some indication that APOE4 carriers had longer survival and that this effect may be restricted to PSEN1. In the post‐mortem cohort, atypical cognitive presentations occurred in 15%. Additional neurological features comprised myoclonus (45%), seizures (21%), pyramidal signs (21%) and extrapyramidal signs (15%), one individual had cerebellar signs. Increased frontal cortical and leptomeningeal CAA occurred in individuals with pyramidal and extrapyramidal signs, and in PSEN1 post‐codon 200 compared to pre‐codon 200 mutations. Atypical cognitive presentations were associated with increased frontal cortical CAA. Conclusions: Phenotypic heterogeneity in ADAD is influenced by causative gene and mutation type, and atypical clinical presentations are associated with increased CAA. Exploring mechanisms by which certain mutations drive Aβ deposition towards the vasculature, and how these may be impacted by other genetic factors and by therapies targeting Aβ, is an important direction for future research. [ABSTRACT FROM AUTHOR]

Background: Familial Alzheimer's disease (fAD) is caused by mutations inPSEN1, PSEN2and APP. PSEN1forms the catalytic core of g‐secretase, a membrane protease which cleaves numerous substrates including APP and Notch. We have previously shown that our panel of fAD patient‐derived iPSCs show altered APP processing and Abgeneration. Here we investigated whether mutations led to altered Notch cleavage in neural precursors. We hypothesised that reduced Notch cleavage in fAD lines would promote premature differentiation during iPSC neuronal differentiation and during adult hippocampal neurogenesis. Methods: We generated 2D cortical neurons and 3D cerebral organoids from a panel of 14 donor‐derived iPSC lines (5 controls, 2 APPmutation lines and 7 PSEN1mutation lines). Neurogenesis was quantified via the proportion of neural precursors versus the number of post‐mitotic neurons at timepoints of neural commitment. In addition, we quantified newly generated neurons in the post‐mortem hippocampus of 3 control brains and 7 brains derived from fAD mutation carriers. Results: We describe a small, yet reproducible premature differentiation phenotype in lines containing mutations in PSEN1during in vitro neurogenesis. This is, at least in part, due to reduced Notch signalling; which drives terminal differentiation. These findings are corroborated in independent orthogonal iPSC systems, namely 2D neurogenesis and 3D organoid differentiation. Using immunohistochemistry against b‐III‐tubulin, we describe robust, quantifiable neurogenesis in post‐mortem tissue. Results are suggestive of mutation‐specific effects and also a premature ageing phenotype in fAD tissue. Conclusion: These findings reveal insights into the functional outcomes of fAD mutations. Mutations in PSEN1, but not APP, alter Notch signalling in neural precursors, which in turn drives premature terminal differentiation. Premature differentiation and mutation‐specific differences both necessitate careful consideration for drug design and patient stratification. [ABSTRACT FROM AUTHOR]