Megan Larson

Soluble Aβ oligomer production and toxicity

J. Neurochem. (2012) 120 (Suppl. 1), 125–139.

The Complex PrPc-Fyn Couples Human Oligomeric Aβ with Pathological Tau Changes in Alzheimer's Disease

Amid controversy, the cellular form of the prion protein PrPc has been proposed to mediate oligomeric amyloid-β (Aβ)-induced deficits. In contrast, there is consistent evidence that the Src kinase Fyn is activated by Aβ oligomers and leads to synaptic and cognitive impairment in transgenic animals. However, the molecular mechanism by which soluble Aβ activates Fyn remains unknown. Combining the use of human and transgenic mouse brain tissue as well as primary cortical neurons, we demonstrate that soluble Aβ binds to PrPc 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 Aβ from binding to PrPc, we abolished Fyn activation and Fyn-dependent tau hyperphosphorylation induced by endogenous oligomeric Aβ in vitro. Finally, we showed that gene dosage of Prnp regulates Aβ-induced Fyn/tau alterations. Together, our findings identify a complete signaling cascade linking one specific endogenous Aβ oligomer, Fyn alteration, and tau hyperphosphorylation in cellular and animal models modeling aspects of the molecular pathogenesis of Alzheimers disease.

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

Recent evidence has emphasized soluble species of amyloid-β (Aβ) and tau as pathogenic effectors in Alzheimers disease (AD). Despite the fact that Aβ, tau, and α-synuclein (αSyn) can promote each others 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 Aβ 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 Aβ/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.

Genetic Modulation of Soluble Aβ Rescues Cognitive and Synaptic Impairment in a Mouse Model of Alzheimer's Disease

An unresolved debate in Alzheimers disease (AD) is whether amyloid plaques are pathogenic, causing overt physical disruption of neural circuits, or protective, sequestering soluble forms of amyloid-β (Aβ) 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 Aβ 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 Aβ from that of deposited amyloid on cognitive function and synaptic structure. Rapid inhibition of transgenic APP modulated the production of Aβ 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 Aβ with a γ-secretase inhibitor provided similar improvement, suggesting that transgene suppression restored cognition, at least in part by lowering Aβ. Cognitive improvement coincided with reduced levels of synaptotoxic Aβ oligomers, greater synaptic density surrounding amyloid plaques, and increased expression of presynaptic and postsynaptic markers. Together these findings indicate that transient Aβ 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 Conformers of Aβ and Tau Alter Selective Proteins Governing Axonal Transport

Despite the demonstration that amyloid-β (Aβ) can trigger increased tau phosphorylation and neurofibrillary tangle (NFT) formation in vivo, the molecular link associating Aβ and tau pathologies remains ill defined. Here, we observed that exposure of cultured primary neurons to Aβ trimers isolated from brain tissue of subjects with Alzheimers 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 Aβ trimers in vivo, we created a novel bigenic Tg-Aβ+Tau mouse line by crossing Tg2576 (Tg-Aβ) and rTg4510 (Tg-Tau) mice. Before neurodegeneration and amyloidosis, apparent Aβ trimers were increased by ∼2-fold in 3-month-old Tg-Aβ and Tg-Aβ+Tau mice compared with younger mice, whereas soluble monomeric Aβ levels were unchanged. Under these conditions, the expression of soluble Alz50-tau conformers rose by ∼2.2-fold in the forebrains of Tg-Aβ+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 Aβ 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 Aβ and tau cooperatively disrupt axonal transport independently from plaques and tangles. Finally, these results suggest that not all endogenous Aβ oligomers trigger the same deleterious changes and that the role of each assembly should be considered separately. SIGNIFICANCE STATEMENT The mechanistic link between amyloid-β (Aβ) and tau, the two major proteins composing the neuropathological lesions detected in brain tissue of Alzheimers disease subjects, remains unclear. Here, we report that the trimeric Aβ species induce a pathological modification of tau in cultured neurons and in bigenic mice expressing Aβ and human tau. This linkage was also observed in postmortem brain tissue from subjects with mild cognitive impairment, when Aβ trimers are abundant. Further, this modification of tau was associated with the intracellular accumulation of the precursor protein of Aβ, APP, as a result of the selective decrease in kinesin light chain 1 expression. Our findings suggest that Aβ trimers might cause axonal transport deficits in AD.

The amyloid-β oligomer Aβ∗56 induces specific alterations in neuronal signaling that lead to tau phosphorylation and aggregation

The 56-kDa amyloid-β oligomer triggers pathological tau aggregation through increased Ca2+ influx. A more pathological amyloid-β oligomer Amyloid-β (Aβ) is implicated in the pathology of Alzheimer’s disease (AD), and various types of Aβ oligomers emerge at different stages of AD. Aβ promotes the modification and aggregation of the microtubule-associated protein tau, which is associated with neuronal toxicity and impaired cognition in various neurodegenerative disorders. Using several transgenic mouse models of AD and cultured cortical neurons, Amar et al. found that the 56-kDa oligomer Aβ*56, but not Aβ dimers or trimers, stimulated an influx in intracellular Ca2+ that triggered phosphorylation of tau at a site that promoted its aggregation. The findings link a specific amyloid form to tau pathology and suggest that dissecting the molecular and stage-specific roles of Aβ oligomers may lead to improved therapies. 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-β (Aβ) 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 Aβ from Tg2576 mice, which are a model for AD. We found that Aβ*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, Aβ*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 Aβ forms (dimers and trimers) did not trigger these effects. Our results indicate that distinct Aβ 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.

Genetic modulation of soluble Aβ rescues cognitive and synaptic impairment in a mouse model of AD

Background There has been a longstanding debate on whether amyloid plaques are pathogenic by causing physical damage to surrounding tissue or protective by sequestering more toxic soluble forms of amyloid b (Ab) peptide. Numerous studies in mouse models have documented a complex relationship between soluble Ab, deposited aggregates, and cognitive decline, but few experimental systems have been capable of dissecting how each form of the peptide contributes to the progressive memory deficits of Alzheimer’s disease. Here we use a controllable transgenic model expressing a mutant form of amyloid precursor protein (APP) to distinguish the impact of APP and soluble Ab from that of deposited amyloid on cognitive function and synaptic structure.

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

Significance Alzheimer’s disease (AD) is the most common form of dementia affecting an estimated 5.3 million Americans based on the 2015 Report of the Alzheimer Association. Our current understanding of the pathogenesis of AD suggests that soluble, nonfibrillar forms of amyloid proteins [e.g. amyloid-β, tau, and α-synuclein (αSyn)] may be responsible for impairing cognition and have therefore been advanced to be the most bioactive species in this brain disorder. We sought to determine the potential contribution of αSyn oligomers to AD-associated cognitive decline. We found that selective αSyn oligomers are elevated in AD brains and that genetically elevating oligomeric αSyn in an AD mouse model led to a selective decrease in presynaptic proteins and cognitive performance. Mounting evidence indicates that soluble oligomeric forms of amyloid proteins linked to neurodegenerative disorders, such as amyloid-β (Aβ), 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 Aβ-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.

Differential contribution of soluble forms of α-synuclein, TDP-43 and PrPc to Alzheimer's disease-associated cognitive impairment

ROCK2 knockdown resulted in both increased aand b secretase cleavage of APP and increased As40 (p1⁄4 0.007) and As42 (p1⁄4 0.03). This data is suggestive of a role for ROCK2 phosphorylation of SorL1 in the regulation of APP metabolism. Activation of another protein kinase, PKC, resulted in increased alpha-secretase mediated shedding of both APP and SorL1, and this was paralleled by an apparent increase in the level of the phosphorylated form of SorL1.Conclusions:We propose here that SorCS1 regulates the intracellular trafficking and metabolism of APP through interaction with the core component of the retromer, Vps35. These results additionally highlight the potential importance of the Vps10 receptors, SorL1 and SorCS1 in elucidating phosphostate sensitive mechanisms in the regulation of the metabolism of APP and As by ROCK2 and PKC.We hypothesize that probing the mechanism(s) by which SorCS1 and SorL1 regulate As generationmay lead to novel insights about the underpinnings and/or therapy of AD.