The amyloid-ß oligomer Aß*56 induces specific alterations in neuronal signaling that lead to tau phosphorylation and aggregation.

Oligomeric forms of amyloid-forming proteins are believed to be the principal initiating bioactive species in many neurodegenerative disorders, including Alzheimer's disease (AD). Amyloid-ß (Aß) oligomers are implicated in AD-associated phosphorylation and aggregation of the microtubule-associated protein tau. To investigate the specific molecular pathways activated by different assemblies, we isolated various forms of Aß from Tg2576 mice, which are a model for AD. We found that Aß*56, a 56-kDa oligomer that is detected before patients develop overt signs of AD, induced specific changes in neuronal signaling. In primary cortical neurons, Aß*56 interacted with N-methyl-d-aspartate receptors (NMDARs), increased NMDAR-dependent Ca2+ influx, and consequently increased intracellular calcium concentrations and the activation of Ca2+-dependent calmodulin kinase IIα (CaMKIIα). In cultured neurons and in the brains of Tg2576 mice, activated CaMKIIα was associated with increased site-specific phosphorylation and missorting of tau, both of which are associated with AD pathology. In contrast, exposure of cultured primary cortical neurons to other oligomeric Aß forms (dimers and trimers) did not trigger these effects. Our results indicate that distinct Aß assemblies activate neuronal signaling pathways in a selective manner and that dissecting the molecular events caused by each oligomer may inform more effective therapeutic strategies.

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.

Polyploidization in Heuchera cylindrica (Saxifragaceae) did not result in a shift in climatic requirements.

PREMISE OF THE STUDY: Polyploidization is a key factor involved in the diversification of plants. Although polyploids are commonly found, there remains controversy on the mechanisms that lead to their successful establishment. One major problem that has been identified is that newly formed polyploids lack mates of the appropriate ploidy level and may experience severely reduced fertility due to nonproductive intercytotype crosses. Niche differentiation has been proposed as a primary mechanism that can alleviate this reproductive disadvantage and facilitate polyploid establishment. Here we test whether the establishment of tetraploid cytotypes of Heuchera cylindrica (Saxifragaceae) is consistent with climatic niche differentiation. • METHODS: We use a combination of field surveys, flow cytometry and species distribution models to: (1) examine the distribution of diploid and tetraploid cytotypes; and (2) determine whether tetraploid Heuchera cylindrica occupy climates that differ from those of its diploid progenitors. • KEY RESULTS: The geographic distributions of diploid and tetraploid cytotypes are largely allopatric as an extensive survey of 636 plants from 43 locations failed to detect any populations with both cytotypes. Although diploids and tetraploids occur in different geographic areas, polyploid Heuchera cylindrica occur almost exclusively in environments that are predicted to be suitable to diploid populations. • CONCLUSIONS: Climatic niche differentiation does not explain the geographic distribution of tetraploid Heuchera cylindrica. We propose instead that tetraploid lineages were able to establish by taking advantage of glacial retreat and expanding into previously unoccupied sites.

The complex PrP(c)-Fyn couples human oligomeric Aß with pathological tau changes in Alzheimer's disease.

Amid controversy, the cellular form of the prion protein PrP(c) has been proposed to mediate oligomeric amyloid-ß (Aß)-induced deficits. In contrast, there is consistent evidence that the Src kinase Fyn is activated by Aß oligomers and leads to synaptic and cognitive impairment in transgenic animals. However, the molecular mechanism by which soluble Aß activates Fyn remains unknown. Combining the use of human and transgenic mouse brain tissue as well as primary cortical neurons, we demonstrate that soluble Aß binds to PrP(c) at neuronal dendritic spines in vivo and in vitro where it forms a complex with Fyn, resulting in the activation of the kinase. Using the antibody 6D11 to prevent oligomeric Aß from binding to PrP(c), we abolished Fyn activation and Fyn-dependent tau hyperphosphorylation induced by endogenous oligomeric Aß in vitro. Finally, we showed that gene dosage of Prnp regulates Aß-induced Fyn/tau alterations. Together, our findings identify a complete signaling cascade linking one specific endogenous Aß oligomer, Fyn alteration, and tau hyperphosphorylation in cellular and animal models modeling aspects of the molecular pathogenesis of Alzheimer's disease.

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.