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SummaryIn Alzheimer’s disease (AD), fibrillar tau pathology accumulates and spreads through the brain and synapses are lost. Evidence from mouse models indicates that tau spreads trans-synaptically from pre- to postsynapses and that oligomeric tau is synaptotoxic, but data on synaptic tau in human brain is scarce. Here we used sub-diffraction-limit microscopy to study synaptic tau accumulation in post-mortem temporal and occipital cortices of human AD and control donors. Oligomeric tau is present in both pre- and postsynaptic terminals even in areas without abundant fibrillar tau deposition. Further, there is a higher proportion of oligomeric tau compared to phosphorylated or misfolded tau found at synaptic terminals. These data suggest that accumulation of oligomeric tau in synapses is an early event in disease pathogenesis, and that tau pathology may progress through the brain via trans-synaptic spread in human disease. Thus, specifically reducing oligomeric tau at synapses may be a promising therapeutic strategy for AD.

INTRODUCTION It remains unclear why age increases risk of Alzheimer’s disease and why some people experience age-related cognitive decline in the absence of dementia. Here we test the hypothesis that resilience to molecular changes in synapses contribute to healthy cognitive ageing. METHODS We examined post-mortem brain from people in mid-life (n=15), healthy ageing with either maintained cognition (n=8) or lifetime cognitive decline (n=7), and Alzheimer’s disease (n=13). Synapses were examined with high resolution imaging, proteomics, and RNA sequencing. Stem cell-derived neurons were challenged with Alzheimer’s brain homogenate. RESULTS Synaptic pathology increased, and expression of genes involved in synaptic signalling decreased between mid-life, healthy ageing and Alzheimer’s. In contrast, brain tissue and neurons from people with maintained cognition during ageing exhibited decreases in synaptic signalling genes compared to people with cognitive decline. DISCUSSION Efficient synaptic networks without pathological protein accumulation may contribute to maintained cognition during ageing.

Synapse loss is associated with cognitive decline in Alzheimer’s disease, and owing to their plastic nature, synapses are an ideal target for therapeutic intervention. Oligomeric amyloid beta around amyloid plaques is known to contribute to synapse loss in mouse models and is associated with synapse loss in human Alzheimer’s disease brain tissue, but the mechanisms leading from Aβ to synapse loss remain unclear. Recent data suggest that the fast-activating and -inactivating voltage-gated potassium channel subtype 3.4 (Kv3.4) may play a role in Aβ-mediated neurotoxicity. Here, we tested whether this channel could also be involved in Aβ synaptotoxicity. Using adeno-associated virus and clustered regularly interspaced short palindromic repeats technology, we reduced Kv3.4 expression in neurons of the somatosensory cortex of APP/PS1 mice. These mice express human familial Alzheimer’s disease-associated mutations in amyloid precursor protein and presenilin-1 and develop amyloid plaques and plaque-associated synapse loss similar to that observed in Alzheimer’s disease brain. We observe that reducing Kv3.4 levels ameliorates dendritic spine loss and changes spine morphology compared to control virus. In support of translational relevance, Kv3.4 protein was observed in human Alzheimer’s disease and control brain and is associated with synapses in human induced pluripotent stem cell–derived cortical neurons. We also noted morphological changes in induced pluripotent stem cell neurones challenged with human Alzheimer’s disease-derived brain homogenate containing Aβ but, in this in vitro model, total mRNA levels of Kv3.4 were found to be reduced, perhaps as an early compensatory mechanism for Aβ-induced damage. Overall, our results suggest that approaches to reduce Kv3.4 expression and/or function in the Alzheimer’s disease brain could be protective against Aβ-induced synaptic alterations.

Background and purpose Synapse degeneration in Alzheimer's disease (AD) correlates strongly with cognitive decline. There is well-established excitatory synapse loss in AD with known contributions of pathological amyloid beta (Aβ) to excitatory synapse dysfunction and loss. Despite clear changes in circuit excitability in AD and model systems, relatively little is known about pathology in inhibitory synapses. Methods Here human postmortem brain samples (n = 5 control, 10 AD cases) from temporal and occipital cortices were examined to investigate whether inhibitory synapses and neurons are lost in AD and whether Aβ may contribute to inhibitory synapse degeneration. Inhibitory neurons were counted in all six cortical layers using stereology software, and array tomography was used to examine synapse density and the accumulation of Aβ in synaptic terminals. Results Differing inhibitory neuron densities were observed in the different cortical layers. The highest inhibitory neuron density was observed in layer 4 in both brain regions and the visual cortex had a higher inhibitory neuron density than the temporal cortex. There was significantly lower inhibitory neuron density in AD than in control cases in all six cortical layers. High-resolution array tomography imaging revealed plaque-associated loss of inhibitory synapses and accumulation of Aβ in a small subset of inhibitory presynaptic terminals with the most accumulation near amyloid plaques. Conclusions Inhibitory neuron and synapse loss in AD may contribute to disrupted excitatory/inhibitory balance and cognitive decline. Future work is warranted to determine whether targeting inhibitory synapse loss could be a useful therapeutic strategy.

Synapse loss correlates with cognitive decline in Alzheimer’s disease (AD), and soluble amyloid beta (Aβ) is implicated in synaptic dysfunction and loss. An important knowledge gap is the lack of understanding of how synaptic accumulation of Aβ leads to synapse degeneration. In particular, there has been difficulty in determining whether there is a synaptic receptor that binds Aβ and mediates toxicity. While many candidate synaptic binding partners have been observed in model systems, their relevance to human AD brain remains unknown. This is in part due to methodological limitations preventing visualization of Aβ binding at individual synapses. To overcome this limitation, we combined two high resolution microscopy techniques: array tomography and Förster resonance energy transfer (FRET) to image over 1 million individual synaptic terminals in temporal cortex from AD (n=9) and age matched control cases (n=6). Within postsynaptic densities, Aβ generates a FRET signal with transmembrane protein 97 (TMEM97), recently discovered to be the Sigma-2 receptor, cellular prion protein, and postsynaptic density 95 (PSD95). TMEM97 is also present in a higher proportion of postsynapses in AD brain compared to control. Further, we inhibited Aβ-TMEM97 interaction in the APP/PS1+Tau mouse model of AD by treating with the Sigma-2 receptor complex allosteric antagonist CT1812 (n=20) or vehicle (n=20). CT1812 drug concentration correlated negatively with synaptic FRET signal between TMEM97 and Aβ. These data support a role for TMEM97 in the synaptic binding of Aβ in human Alzheimer’s disease brain where it may mediate synaptotoxicity.One Sentence SummaryIn Alzheimer’s disease, TMEM97 was present in a higher proportion of synapses and close enough to amyloid beta to be a potential synaptic binding partner.

One of the key knowledge gaps in the field of Alzheimer's disease research is the lack of understanding of how amyloid beta and tau cooperate to cause neurodegeneration. We recently generated a mouse model (APP/PS1 + Tau) that develops amyloid plaque pathology and expresses human tau in the absence of endogenous murine tau. These mice exhibit an age-related behavioural hyperactivity phenotype and transcriptional deficits which are ameliorated by tau transgene suppression. We hypothesized that these mice would also display memory and hippocampal synaptic plasticity deficits as has been reported for many plaque bearing mouse models which express endogenous mouse tau. We observed that our APP/PS1 + Tau model does not exhibit novel object memory or robust long-term potentiation deficits with age, whereas the parent APP/PS1 line with mouse tau did develop the expected deficits. These data are important as they highlight potential functional differences between mouse and human tau and the need to use multiple models to fully understand Alzheimer's disease pathogenesis and develop effective therapeutic strategies.

Degeneration of synapses in Alzheimer’s disease (AD) strongly correlates with cognitive decline, and synaptic pathology contributes to disease pathophysiology. We recently discovered that the strongest genetic risk factor for sporadic AD, apolipoprotein E epsilon 4 (APOE4), exacerbates synapse loss and synaptic accumulation of oligomeric amyloid beta in human AD brain. To begin to understand the molecular cascades involved in synapse loss in AD and how this is mediated byAPOE, and to generate a resource of knowledge of changes in the synaptic proteome in AD, we conducted a proteomic screen and systematicin-silicoanalysis of synaptoneurosome preparations from temporal and occipital cortices of human AD and control subjects with knownAPOEgene status. Our analysis identified over 5,500 proteins in human synaptoneurosomes and highlighted disease, brain region, and APOE-associated changes in multiple molecular pathways including a decreased abundance in AD of proteins important for synaptic and mitochondrial function and an increased abundance of proteins involved in neuroimmune interactions and intracellular signaling.HighlightsProteomic analysis of synapses isolated from Alzheimer’s disease and control subject brains identifies over 5,500 proteins in human synapses.In silico analysis reveals region-specific decreases in proteins involved in synaptic and mitochondrial function and increases in proteins involved in neuroimmune signaling and intracellular signaling in AD.The apolipoprotein E4 risk gene is associated with exacerbated changes in synaptic proteins in AD.

SummarySynapse loss correlates with cognitive decline in Alzheimer’s disease (AD). Data from mouse models suggests microglia are important for synapse degeneration, but direct human evidence for any glial involvement in synapse removal in human AD remains to be established. Here we observe astrocytes and microglia from human brains contain greater amounts of synaptic protein in AD compared to non-disease controls, and that proximity to amyloid-β plaques and theAPOE4risk gene exacerbate this effect. In culture, mouse and human astrocytes and primary mouse and human microglia phagocytose AD patient-derived synapses more than synapses from controls. Inhibiting MFG-E8 function rescued the elevated engulfment of AD synapses by astrocytes and microglia without affecting control synapse uptake. Thus, AD promotes increased synapse ingestion by human glial cells via an MFG-E8 opsonophagocytic mechanism with potential for targeted therapeutic manipulation.One-Sentence SummaryGlial cells ingest synapses in Alzheimer’s disease and antibody treatment reduces this ingestion in cultured human cells.

Jackson and colleagues observe that clusterin accumulates in synapses in human Alzheimer’s disease brain tissue, and that this accumulation is increased in people carrying the Alzheimer’s disease risk gene apolipoprotein E4. These data use high-resolution imaging to link two genetic risk factors with amyloid-mediated synapse degeneration in Alzheimer’s disease., One of the major challenges in developing effective therapeutic strategies for Alzheimer’s disease is understanding how genetic risk factors contribute to neurodegeneration. The apolipoprotein epsilon 4 isoform (APOE4) and variants in the Clusterin (CLU) gene (also known as apolipoprotein J) are associated with increased risk of developing Alzheimer’s. Our previous work demonstrated that APOE4 exacerbates synapse degeneration and synaptic accumulation of toxic oligomeric amyloid beta in human Alzheimer’s and mouse models of disease. Here, we observe clusterin in synapses in human Alzheimer's disease brain. The percentage of synapses containing clusterin is higher in APOE4 carriers than APOE3 carriers. Furthermore, we observe oligomeric amyloid beta accumulation within synapses containing clusterin which is also higher in APOE4 carriers. These data link two genetic risk factors with synapse degeneration in Alzheimer’s and support a potential role for clusterin working with APOE in causing synaptic damage., Graphical Abstract Graphical Abstract

SummaryOne of the key knowledge gaps blocking development of effective therapeutics for Alzheimer’s disease (AD) is the lack of understanding of how amyloid beta (Aβ) and tau cooperate in causing disease phenotypes. Within a mouse tau deficient background, we probed the molecular, cellular and behavioural disruption triggered by wild-type human tau’s influence on human Aβ-induced pathology. We find that Aβ and tau work cooperatively to cause a hyperactivity phenotype and to cause downregulation of gene transcription including many involved in synaptic function. In both our mouse model and in human post-mortem tissue, we observe accumulation of pathological tau in synapses, supporting the potential importance of synaptic tau. Importantly, tau depletion in the mice, initiated after behavioural deficits emerge, was found to correct behavioural deficits, reduce synaptic tau levels, and substantially reverse transcriptional perturbations, suggesting that lowering tau levels, particularly at the synapse, may be beneficial in AD.Highlights- Expression of human familial Alzheimer’s associated mutant amyloid precursor protein and presenillin 1 with wild-type human tau in the absence of endogenous tau in a novel MAPT-AD mouse model results in behavioural deficits and downregulation of genes involved in synaptic function.- Tau is present in pre and postsynaptic terminals in MAPT-AD mice and human AD brain. In mice, lowering synaptic tau levels was associated with improved cognition and recovered gene expression.- These data suggest that Aβ and tau act cooperatively in impairing synaptic function and that lowering tau at synapses could be a beneficial therapeutic approach in AD.

SummaryReconsolidation is a putative neuronal process in which the retrieval of a previously consolidated memory returns it to a labile state that is once again subject to stabilization. This study explored the idea that reconsolidation occurs in spatial memory when animals retrieve memory under circumstances in which new memory encoding is likely to occur. Control studies confirmed that intrahippocampal infusions of anisomycin inhibited protein synthesis locally and that the spatial training protocols we used are subject to overnight protein synthesis-dependent consolidation. We then compared the impact of anisomycin in two conditions: when memory retrieval occurred in a reference memory task after performance had reached asymptote over several days; and after a comparable extent of training of a delayed matching-to-place task in which new memory encoding was required each day. Sensitivity to intrahippocampal anisomycin was observed only in the protocol involving new memory encoding at the time of retrieval.

Two experiments were conducted to investigate the possibility of faster forgetting by PDAPP mice (a well-established model of Alzheimer’s disease as reported by Games and colleagues in an earlier paper). Experiment 1, using mice aged 13–16 mo, confirmed the presence of a deficit in a spatial reference memory task in the water maze by hemizygous PDAPP mice relative to littermate controls. However, after overtraining to a criterion of equivalent navigational performance, a series of memory retention tests revealed faster forgetting in the PDAPP group. Very limited retraining was sufficient to reinstate good memory in both groups, indicating that their faster forgetting may be due to retrieval failure rather than trace decay. In Experiment 2, 6-mo-old PDAPP and controls were required to learn each of a series of spatial locations to criterion with their memory assessed 10 min after learning each location. No memory deficit was apparent in the PDAPP mice initially, but a deficit built up through the series of locations suggestive of increased sensitivity to interference. Faster forgetting and increased interference may each reflect a difficulty in accessing memory traces. This interpretation of one aspect of the cognitive deficit in human mutant APP mice has parallels to deficits observed in patients with Alzheimer’s disease, further supporting the validity of transgenic models of the disease.

Summary A key knowledge gap blocking development of effective therapeutics for Alzheimer’s disease (AD) is the lack of understanding of how amyloid beta (Aβ) peptide and pathological forms of the tau protein cooperate in causing disease phenotypes. Within a mouse tau-deficient background, we probed the molecular, cellular, and behavioral disruption triggered by the influence of wild-type human tau on human Aβ-induced pathology. We find that Aβ and tau work cooperatively to cause a hyperactivity behavioral phenotype and to cause downregulation of transcription of genes involved in synaptic function. In both our mouse model and human postmortem tissue, we observe accumulation of pathological tau in synapses, supporting the potential importance of synaptic tau. Importantly, tau reduction in the mice initiated after behavioral deficits emerge corrects behavioral deficits, reduces synaptic tau levels, and substantially reverses transcriptional perturbations, suggesting that lowering synaptic tau levels may be beneficial in AD., Graphical Abstract, Highlights • Aβ and tau work together to cause behavioral and transcriptional deficits in mice • In mice with Aβ and tau, glial gene expression increases and synaptic genes decrease • Tau is present in synaptic terminals in APP/PS1+Tau mice and human Alzheimer brain • In mice, lowering tau levels improves cognition and restores gene expression, One of the mysteries of Alzheimer’s disease is how the two key pathological proteins, amyloid beta and tau, interact. Pickett et al. use a mouse model to show that these proteins cooperate to change behavior and gene expression and that these phenotypes recover when tau levels are lowered.