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...

Alzheimer’s disease (AD) is the most common progressive neurodegenerative disorder causing dementia. Massive deposition of amyloid β peptide (Aβ) as senile plaques in the brain is the pathological hallmark of AD, but oligomeric, soluble forms of Aβ have been implicated as the synaptotoxic component. The apolipoprotein E epsilon 4 (apoE ε4) allele is known to be a genetic risk factor for developing AD. However it is still unknown how apoE impacts the process of Aβ oligomerization. Here, we found that the level of Aβ oligomers in APOEε4/ε4 AD patient brains is 2.7 times higher than those in APOEε3/ε3 AD patient brains, matched for total plaque burden, suggesting that apoE4 impacts the metabolism of Aβ oligomers. To test this hypothesis, we examined apoE’s effect on Aβ oligomer formation. Using both synthetic Aβ and a split-luciferase method for monitoring Aβ oligomers, we observed that apoE increased the level of Aβ oligomers in an isoform dependent manner (E2 < E3 < E4). This effect appears to be dependent on the ApoE carboxy-terminal domain. Moreover, these results were confirmed using endogenous apoE isolated from the TBS-soluble fraction of human brain, which increased the formation of Aβ oligomers. Taken together, these data show that lipidated apoE, especially apoE4, increases Aβ oligomers in the brain. Higher levels of Aβ oligomers in the brains of APOEε4/ε4 carriers compared to APOEε3/ε3 carriers may increase the loss of dendritic spines and accelerate memory impairments, leading to earlier cognitive decline in AD.

Inhibition of translation plays a role in apoptosis induced by a variety of stimuli, but the mechanism by which it promotes apoptosis has not been established. We have investigated the hypothesis that selective degradation of anti-apoptotic regulatory protein(s) is responsible for apoptosis resulting from translation inhibition. Induction of apoptosis by cycloheximide was detected within 2-4 h and blocked by proteasome inhibitors, indicating that degradation of shortlived protein (

We applied a novel application of FLIM-FRET to in situ measurement and quantification of protein interactions to explore isoform specific differences in Aβ-ApoE interaction and ApoE tertiary conformation in senile plaques in human Alzheimer brain. ApoE3 interacts more closely with Aβ than ApoE4, but a greater proportion of Aβ molecules within plaques are decorated with ApoE4 than ApoE3, lending strong support to the hypothesis that isoform specific differences in ApoE are linked with Aβ deposition. We found an increased number of ApoE N-terminal fragments in ApoE4 plaques, consistent with the observation that ApoE4 is more easily cleaved than ApoE3. In addition, we measured a small but significant isoform specific difference in ApoE domain interaction. Based on our in situ data, supported by traditional biochemical data, we propose a pathway by which isoform specific conformational differences increase the level of cleavage at the hinge region of ApoE4, leading to a loss of ApoE function to mediate clearance of Aβ and thereby increase the risk of AD for carriers of the APOEε4 allele.