Synapse loss correlates with a cognitive decline in Alzheimer's disease (AD), but whether this is caused by fibrillar deposits known as senile plaques or soluble oligomeric forms of amyloid ␤ (A␤) is controversial. By using array tomography, a technique that combines ultrathin sectioning of tissue with immunofluorescence, allowing precise quantification of small structures, such as synapses, we have tested the hypothesis that oligomeric A␤ surrounding plaques contributes to synapse loss in a mouse model of AD. We find that senile plaques are surrounded by a halo of oligomeric A␤. Analysis of >14,000 synapses (represented by PSD95-stained excitatory synapses) shows that there is a 60% loss of excitatory synapses in the halo of oligomeric A␤ surrounding plaques and that the density increases to reach almost control levels in volumes further than 50 m from a plaque in an approximately linear fashion (linear regression, r 2 ‫؍‬ 0.9; P < 0.0001). Further, in transgenic cortex, microdeposits of oligomeric A␤ associate with a subset of excitatory synapses, which are significantly smaller than those not in contact with oligomeric A␤. The proportion of excitatory synapses associated with A␤ correlates with decreasing density (correlation, ؊0.588; P < 0.0001). These data show that senile plaques are a potential reservoir of oligomeric A␤, which colocalizes with the postsynaptic density and is associated with spine collapse, reconciling the apparently competing schools of thought of ''plaque'' vs. ''oligomeric A␤'' as the synaptotoxic species in the brain of AD patients.Alzheimer ͉ array tomography ͉ neurodegeneration ͉ synaptotoxicity L oss of connectivity caused by neuronal death and synapse loss is thought to underlie cognitive decline in neurodegenerative conditions, such as Alzheimer's disease (AD). Synapse loss appears to be particularly important in the pathogenesis of AD. Indeed, it is known that synapses are lost during AD and that in AD tissue, synapse loss correlates strongly with cognitive decline (1-3). There is a growing consensus, based primarily on cell-based assays, that amyloid ␤ (A␤), the main component of senile plaques, is toxic to synapses (4-6). In both AD patients and animal models of the disease, synapse loss is greatest near senile plaques, indicating a link between amyloid pathology and synaptotoxicity in vivo. Work by several groups has shown a decrease in dendritic spine density and synaptophysin-positive synapses radiating out from the surface of plaques in mouse models of AD (7-10). Whether this is caused by fibrillar plaques or soluble oligomeric A␤ is controversial. We used multiphoton imaging of the living brain to show that this spine loss is caused by impaired spine stability over time near plaques and postulated that a plaque-related diffusible bioactive molecule was responsible (11). Here, we test the hypothesis that oligomeric A␤ is directly synaptotoxic.We hypothesize that soluble oligomeric A␤ associates with the postsynaptic density and causes the loss of synapses and spines observ...

Amyloid ␤ (A␤)-containing plaques are surrounded by dystrophic neurites in the Alzheimer's disease (AD) brain, but whether and how plaques induce these neuritic abnormalities remain unknown. We tested the hypothesis that soluble oligomeric assemblies of A␤, which surround plaques, induce calcium-mediated secondary cascades that lead to dystrophic changes in local neurites. We show that soluble A␤ oligomers lead to activation of the calcium-dependent phosphatase calcineurin (CaN) (PP2B), which in turn activates the transcriptional factor nuclear factor of activated T cells (NFAT). Activation of these signaling pathways, even in the absence of A␤, is sufficient to produce a virtual phenocopy of A␤-induced dystrophic neurites, dendritic simplification, and dendritic spine loss in both neurons in culture and in the adult mouse brain. Importantly, the morphological deficits in the vicinity of A␤ deposits in a mouse model of AD are ameliorated by CaN inhibition, supporting the hypothesis that CaN-NFAT are aberrantly activated by A␤ and that CaN-NFAT activation is responsible for disruption of neuronal structure near plaques. In accord with this, we also detect increased levels of an active form of CaN and NFATc4 in the nuclear fraction from the cortex of patients with AD. Thus, A␤ appears to mediate the neurodegeneration of AD, at least in part, by activation of CaN and subsequent NFAT-mediated downstream cascades.

In Alzheimer disease (AD), deposition of neurofibrillary tangles and loss of synapses in the neocortex and limbic system each correlate strongly with cognitive impairment. Tangles are composed of misfolded hyperphosphorylated tau proteins; however, the link between tau abnormalities and synaptic dysfunction remains unclear. We examined the location of tau in control and AD cortices using biochemical and morphologic methods. We found that, in addition to its well-described axonal localization, normal tau is present at both presynaptic and postsynaptic terminals in control human brains. In AD, tau becomes hyperphosphorylated and misfolded at both presynaptic and postsynaptic terminals, and this abnormally posttranslationally modified tau is enriched in synaptoneurosomal fractions. Synaptic tau seems to be hyperphosphorylated and ubiquitinated, and forms stable oligomers resistant to SDS denaturation. The accumulation of hyperphosphorylated tau oligomers at human AD synapses is associated with increased ubiquitinated substrates and increased proteasome components, consistent with dysfunction of the ubiquitin-proteasome system. Our findings suggest that synaptic hyperphosphorylated tau oligomers may be an important mediator of the proteotoxicity that disrupts synapses in AD.