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Background: Synapse degeneration is one of the earliest changes in Alzheimer's disease (AD) and is the major neuropathologic correlate of cognitive impairment. The aim of this study was to characterize microRNA (miRNA) dysregulation at AD synapses post‐mortem brain tissue and explore the specificity for AD. Method: We prepared synaptosomes (SYN) by serial ultracentrifugation of 10 AD (mean age = 77.4±13.3) and 10 control (CN; mean age = 72.3±18.8) temporal cortex samples over a sucrose gradient and saved the homogenates (H). We extracted and quantified miRNA from SYN and H fractions by TaqMan Advanced miRNA Assay‐Openarray system. We calculated deltaCt and performed linear regression to identify differentially expressed (DE) miRNA across AD and CN SYN, adjusting for multiple testing using Benjamini‐Hochberg method (α<0.05). We identified gene targets of the DE miRNA using two predictive databases (TargetScan and miRDB) and performed pathway analysis using the Panther database. We selected miRNA candidates that are DE in AD synapses and performed a follow‐up study in 4 different brain regions of 10 CN (mean age = 80.7±12.9), 10 AD (mean age = 77.4±13.3) and 10 non‐AD tauopathy (mean age = 70.6±6.4) cases to determine the regional distribution and the AD specificity. Result: Synapse fractions were isolated demonstrating a 3.2‐fold enrichment in the synaptic protein VAMP‐2 (p<0.001) compared to homogenates. We detected 411 miRNA in H and 432 in SYN of the total in the panel with 89% overlap between AD and CN in SYN. Three miRNA from the same family (miR‐132‐3p, miR‐132‐5p and miR‐212‐3p) were under‐expressed and 2 miRNA (miR‐181a‐3p and miR‐1260a) were over‐expressed in AD compared to CN SYN. The gene targets of these miRNA were overrepresented in pathways regulating synaptic and mitochondrial function. In addition, targets of miR‐132 also were overrepresented in pathways related to tau pathology, Aβ production, interleukin production, apoptosis and oxidative stress. We will describe the regional distribution in AD brains post‐mortem and the specificity of the changes to AD by comparing to non‐pathological controls and non‐AD tauopathies. Conclusion: miRNA that are dysregulated in AD synapses target genes that are involved in pathways related to AD pathogenesis. Future studies will focus on the use of these miRNA as therapeutic targets. [ABSTRACT FROM AUTHOR]

Background: Synaptic degeneration is a primary neuropathological factor associated with cognitive decline in Alzheimer's disease (AD). In 2021, we generated a synaptic Polygenic Risk Score (PRS) that comprised only 8 variants within 6 synaptic genes (APOE, PICALM, BIN1, PTK2B, DLG2 and MINK1) that predicted AD with 72% accuracy in two neuropathological cohorts. This supports the hypothesis that genetic variants that regulate an individual's vulnerability to AD‐related synapse degeneration could be used to identify individuals at‐risk for AD prior to the appearance of clinical symptoms. The aim of this study was to determine whether the constituent variants of the linkage disequilibrium (LD) blocks represented by the PRS have regulatory activity in vitro. Method: We cloned an oligonucleotide library of 137 putative regulatory variants each represented by 5 barcodes per allele into pMPRA1 vector. Then we transfected the plasmids into HEK293 cells (n = 5). We extracted DNA and RNA from the cells and sequenced on an Illumina MiSeq. Using the mpra package in R, we normalized the tag counts to a common size of 10 million reads and computed paired log ratios of RNA/DNA counts for each barcode. We used weighted linear models to test for differential activity of the minor versus the major allele of each SNP using the mpralm function, adjusting for multiple testing using the Benjamini‐Hochberg method. Result: We acquired approximately 15 million reads of DNA and RNA from 5 independent experiments. We found that 35 out of the 137 SNPs tested had differential activity between alleles (adj. p <0.05, Figure 1A). Three of the SNPs that showed regulatory activity were included in the PRS (BIN1: rs17014923 and rs35114168; DLG2: rs286043) and the remaining 32 are captured on LD blocks within the synaptic PRS. Conclusion: All LD captured by the synaptic PRS contain SNPs that impact on regulatory activity, thus supporting a potential mechanism by which changes in the expression of these specific loci that encode synaptic proteins could lead to a modified cumulative risk for AD. Further studies to determine the precise mechanism involved in this regulatory activity at the synapse could guide future therapeutic strategies for AD. [ABSTRACT FROM AUTHOR]

Aims: Amyloid precursor protein (APP) 훽‐C‐terminal fragment (훽CTF) may have a neurotoxic role in Alzheimer's disease (AD). 훽CTF accumulates in the brains of patients with sporadic (SAD) and genetic forms of AD. Synapses degenerate early during the pathogenesis of AD. We studied whether the 훽CTF accumulates in synapses in SAD, autosomal dominant AD (ADAD) and Down syndrome (DS). Methods: We used array tomography to determine APP at synapses in human AD tissue. We measured 훽CTF, A훽40, A훽42 and phosphorylated tau181 (p‐tau181) concentrations in brain homogenates and synaptosomes of frontal and temporal cortex of SAD, ADAD, DS and controls. Results: APP colocalised with pre‐ and post‐synaptic markers in human AD brains. APP 훽CTF was enriched in AD synaptosomes. Conclusions: We demonstrate that 훽CTF accumulates in synapses in SAD, ADAD and DS. This finding might suggest a role for 훽CTF in synapse degeneration. Therapies aimed at mitigating 훽CTF accumulation could be potentially beneficial in AD.Using array tomography in post‐mortem brain tissue from the temporal and frontal cortex, we found that APP is located in synaptic compartments in different forms of AD. We also describe that APP βCTF accumulates in synaptosomal fractions in different forms of AD and that this accumulation correlates with Aβ and tau concentrations. [ABSTRACT FROM AUTHOR]