Selective lowering of synapsins induced by oligomeric α-synuclein exacerbates memory deficits.

Mounting evidence indicates that soluble oligomeric forms of amyloid proteins linked to neurodegenerative disorders, such as amyloid-ß (Aß), tau, or α-synuclein (αSyn) might be the major deleterious species for neuronal function in these diseases. Here, we found an abnormal accumulation of oligomeric αSyn species in AD brains by custom ELISA, size-exclusion chromatography, and nondenaturing/denaturing immunoblotting techniques. Importantly, the abundance of αSyn oligomers in human brain tissue correlated with cognitive impairment and reductions in synapsin expression. By overexpressing WT human αSyn in an AD mouse model, we artificially enhanced αSyn oligomerization. These bigenic mice displayed exacerbated Aß-induced cognitive deficits and a selective decrease in synapsins. Following isolation of various soluble αSyn assemblies from transgenic mice, we found that in vitro delivery of exogenous oligomeric αSyn but not monomeric αSyn was causing a lowering in synapsin-I/II protein abundance. For a particular αSyn oligomer, these changes were either dependent or independent on endogenous αSyn expression. Finally, at a molecular level, the expression of synapsin genes SYN1 and SYN2 was down-regulated in vivo and in vitro by αSyn oligomers, which decreased two transcription factors, cAMP response element binding and Nurr1, controlling synapsin gene promoter activity. Overall, our results demonstrate that endogenous αSyn oligomers can impair memory by selectively lowering synapsin expression.

Soluble Conformers of Aß and Tau Alter Selective Proteins Governing Axonal Transport.

UNLABELLED: Despite the demonstration that amyloid-ß (Aß) can trigger increased tau phosphorylation and neurofibrillary tangle (NFT) formation in vivo, the molecular link associating Aß and tau pathologies remains ill defined. Here, we observed that exposure of cultured primary neurons to Aß trimers isolated from brain tissue of subjects with Alzheimer's disease led to a specific conformational change of tau detected by the antibody Alz50. A similar association was supported by postmortem human brain analyses. To study the role of Aß trimers in vivo, we created a novel bigenic Tg-Aß+Tau mouse line by crossing Tg2576 (Tg-Aß) and rTg4510 (Tg-Tau) mice. Before neurodegeneration and amyloidosis, apparent Aß trimers were increased by ∼2-fold in 3-month-old Tg-Aß and Tg-Aß+Tau mice compared with younger mice, whereas soluble monomeric Aß levels were unchanged. Under these conditions, the expression of soluble Alz50-tau conformers rose by ∼2.2-fold in the forebrains of Tg-Aß+Tau mice compared with nontransgenic littermates. In parallel, APP accumulated intracellularly, suggestive of a putative dysfunction of anterograde axonal transport. We found that the protein abundance of the kinesin-1 light chain (KLC1) was reduced selectively in vivo and in vitro when soluble Aß trimers/Alz50-tau were present. Importantly, the reduction in KLC1 was prevented by the intraneuronal delivery of Alz50 antibodies. Collectively, our findings reveal that specific soluble conformers of Aß and tau cooperatively disrupt axonal transport independently from plaques and tangles. Finally, these results suggest that not all endogenous Aß oligomers trigger the same deleterious changes and that the role of each assembly should be considered separately. SIGNIFICANCE STATEMENT: The mechanistic link between amyloid-ß (Aß) and tau, the two major proteins composing the neuropathological lesions detected in brain tissue of Alzheimer's disease subjects, remains unclear. Here, we report that the trimeric Aß species induce a pathological modification of tau in cultured neurons and in bigenic mice expressing Aß and human tau. This linkage was also observed in postmortem brain tissue from subjects with mild cognitive impairment, when Aß trimers are abundant. Further, this modification of tau was associated with the intracellular accumulation of the precursor protein of Aß, APP, as a result of the selective decrease in kinesin light chain 1 expression. Our findings suggest that Aß trimers might cause axonal transport deficits in AD.

Genetic modulation of soluble Aß rescues cognitive and synaptic impairment in a mouse model of Alzheimer's disease.

An unresolved debate in Alzheimer's disease (AD) is whether amyloid plaques are pathogenic, causing overt physical disruption of neural circuits, or protective, sequestering soluble forms of amyloid-ß (Aß) that initiate synaptic damage and cognitive decline. Few animal models of AD have been capable of isolating the relative contribution made by soluble and insoluble forms of Aß to the behavioral symptoms and biochemical consequences of the disease. Here we use a controllable transgenic mouse model expressing a mutant form of amyloid precursor protein (APP) to distinguish the impact of soluble Aß from that of deposited amyloid on cognitive function and synaptic structure. Rapid inhibition of transgenic APP modulated the production of Aß without affecting pre-existing amyloid deposits and restored cognitive performance to the level of healthy controls in Morris water maze, radial arm water maze, and fear conditioning. Selective reduction of Aß with a γ-secretase inhibitor provided similar improvement, suggesting that transgene suppression restored cognition, at least in part by lowering Aß. Cognitive improvement coincided with reduced levels of synaptotoxic Aß oligomers, greater synaptic density surrounding amyloid plaques, and increased expression of presynaptic and postsynaptic markers. Together these findings indicate that transient Aß species underlie much of the cognitive and synaptic deficits observed in this model and demonstrate that significant functional and structural recovery can be attained without removing deposited amyloid.

Soluble α-synuclein is a novel modulator of Alzheimer's disease pathophysiology.

Recent evidence has emphasized soluble species of amyloid-ß (Aß) and tau as pathogenic effectors in Alzheimer's disease (AD). Despite the fact that Aß, tau, and α-synuclein (αSyn) can promote each other's aggregation, the potential contribution of soluble αSyn to AD pathogenesis is unknown. Here, we found an approximate twofold increase over controls in soluble αSyn levels in AD brains in the absence of Lewy body cytopathology. Importantly, soluble αSyn levels were a quantitatively stronger correlate of cognitive impairment than soluble Aß and tau levels. To examine a putative role for αSyn in modulating cognitive function, we used the Barnes circular maze to assess spatial reference memory in transgenic mice overexpressing human wild-type αSyn. The results revealed that an approximate threefold elevation of αSyn in vivo induced memory deficits similar to those observed in AD mouse models. The neurobiological changes associated with this elevation of soluble αSyn included decreases in selected synaptic vesicle proteins and an alteration of the protein composition of synaptic vesicles. Finally, a synergism between Aß/APP and human tau seems to be responsible for the abnormal elevation of soluble αSyn in transgenic mice. Altogether, our data reveal an unexpected role for soluble, intraneuronal αSyn in AD pathophysiology.

Soluble Aß oligomer production and toxicity.

For nearly 100 years following the first description of this neurological disorder by Dr Alois Alzheimer, amyloid plaques and neurofibrillary tangles have been hypothesized to cause neuronal loss. With evidence that the extent of insoluble, deposited amyloid poorly correlated with cognitive impairment, research efforts focused on soluble forms of Aß, also referred as Aß oligomers. Following a decade of studies, soluble oligomeric forms of Aß are now believed to induce the deleterious cascade(s) involved in the pathophysiology of Alzheimer's disease. In this review, we will discuss our current understanding about endogenous oligomeric Aß production, their relative toxicity in vivo and in vitro, and explore the potential future directions needed for the field.