Soluble Conformers of Aβ and Tau Alter Selective Proteins Governing Axonal Transport

Directionality bias underpins divergent spatiotemporal progression of Alzheimer-related tauopathy in mouse models

Mounting evidence implicates trans-synaptic connectome-based spread as a shared mechanism behind different tauopathic conditions, yet also suggests there is divergent spatiotemporal progression between them. A potential parsimonious explanation for this apparent contradiction could be that different conditions incur differential rates and directional biases in tau transmission along fiber tracts. In this meta-analysis we closely examined this hypothesis and quantitatively tested it using spatiotemporal tau pathology patterns from 11 distinct models across 4 experimental studies. For this purpose, we extended a network-based spread model by incorporating net directionality along the connectome. Our data unambiguously supports the directional transmission hypothesis. First, retrograde bias is an unambiguously better predictor of tau progression than anterograde bias. Second, while spread exhibits retrograde character, our best-fitting biophysical models incorporate the mixed effects of both retrograde- and anterograde-directed spread, with notable tau-strain-specific differences. We also found a nontrivial association between directionality bias and tau strain aggressiveness, with more virulent strains exhibiting less retrograde character. Taken together, our study implicates directional transmission bias in tau transmission along fiber tracts as a general feature of tauopathy spread and a strong candidate explanation for the diversity of spatiotemporal tau progression between conditions. This simple and parsimonious mechanism may potentially fill a critical gap in our knowledge of the spatiotemporal ramification of divergent tauopathies.

Comprehensive Overview of Alzheimer's Disease: Etiological Insights and Degradation Strategies

Alzheimer's disease (AD) is the most prevalent neurodegenerative disorder and affects millions of individuals globally. AD is associated with cognitive decline and memory loss that worsens with aging. A statistical report using U.S. data on AD estimates that approximately 6.9 million individuals suffer from AD, a number projected to surge to 13.8 million by 2060. Thus, there is a critical imperative to pinpoint and address AD and its hallmark tau protein aggregation early to prevent and manage its debilitating effects. Amyloid-β and tau proteins are primarily associated with the formation of plaques and neurofibril tangles in the brain. Current research efforts focus on degrading amyloid-β and tau or inhibiting their synthesis, particularly targeting APP processing and tau hyperphosphorylation, aiming to develop effective clinical interventions. However, navigating this intricate landscape requires ongoing studies and clinical trials to develop treatments that truly make a difference. Genome-wide association studies (GWASs) across various cohorts identified 40 loci and over 300 genes associated with AD. Despite this wealth of genetic data, much remains to be understood about the functions of these genes and their role in the disease process, prompting continued investigation. By delving deeper into these genetic associations, novel targets such as kinases, proteases, cytokines, and degradation pathways, offer new directions for drug discovery and therapeutic intervention in AD. This review delves into the intricate biological pathways disrupted in AD and identifies how genetic variations within these pathways could serve as potential targets for drug discovery and treatment strategies. Through a comprehensive understanding of the molecular underpinnings of AD, researchers aim to pave the way for more effective therapies that can alleviate the burden of this devastating disease.

Antibodies Raised Against an Aβ Oligomer Mimic Recognize Pathological Features in Alzheimer's Disease and Associated Amyloid-Disease Brain Tissue

Antibodies that target the β-amyloid peptide (Aβ) and its associated assemblies are important tools in Alzheimer's disease research and have emerged as promising Alzheimer's disease therapies. This paper reports the creation and characterization of a triangular Aβ trimer mimic composed of Aβ17-36 β-hairpins and the generation and study of polyclonal antibodies raised against the Aβ trimer mimic. The Aβ trimer mimic is covalently stabilized by three disulfide bonds at the corners of the triangular trimer to create a homogeneous oligomer. Structural, biophysical, and cell-based studies demonstrate that the Aβ trimer mimic shares characteristics with oligomers of full-length Aβ. X-ray crystallography elucidates the structure of the trimer and reveals that four copies of the trimer assemble to form a dodecamer. SDS-PAGE, size exclusion chromatography, and dynamic light scattering reveal that the trimer also forms higher-order assemblies in solution. Cell-based toxicity assays show that the trimer elicits LDH release, decreases ATP levels, and activates caspase-3/7 mediated apoptosis. Immunostaining studies on brain slices from people who lived with Alzheimer's disease and people who lived with Down syndrome reveal that the polyclonal antibodies raised against the Aβ trimer mimic recognize pathological features including different types of Aβ plaques and cerebral amyloid angiopathy.

Neuronal tau pathology worsens late-phase white matter degeneration after traumatic brain injury in transgenic mice

Traumatic brain injury (TBI) causes diffuse axonal injury which can produce chronic white matter pathology and subsequent post-traumatic neurodegeneration with poor patient outcomes. Tau modulates axon cytoskeletal functions and undergoes phosphorylation and mis-localization in neurodegenerative disorders. The effects of tau pathology on neurodegeneration after TBI are unclear. We used mice with neuronal expression of human mutant tau to examine effects of pathological tau on white matter pathology after TBI. Adult male and female hTau.P301S (Tg2541) transgenic and wild-type (Wt) mice received either moderate single TBI (s-TBI) or repetitive mild TBI (r-mTBI; once daily × 5), or sham procedures. Acutely, s-TBI produced more extensive axon damage in the corpus callosum (CC) as compared to r-mTBI. After s-TBI, significant CC thinning was present at 6 weeks and 4 months post-injury in Wt and transgenic mice, with homozygous tau expression producing additional pathology of late demyelination. In contrast, r-mTBI did not produce significant CC thinning except at the chronic time point of 4 months in homozygous mice, which exhibited significant CC atrophy (- 29.7%) with increased microgliosis. Serum neurofilament light quantification detected traumatic axonal injury at 1 day post-TBI in Wt and homozygous mice. At 4 months, high tau and neurofilament in homozygous mice implicated tau in chronic axon pathology. These findings did not have sex differences detected. Conclusions: Neuronal tau pathology differentially exacerbated CC pathology based on injury severity and chronicity. Ongoing CC atrophy from s-TBI became accompanied by late demyelination. Pathological tau significantly worsened CC atrophy during the chronic phase after r-mTBI.

Proteomics of the astrocyte secretome reveals changes in their response to soluble oligomeric Aβ

Astrocytes associate with amyloid plaques in Alzheimer's disease (AD). Astrocytes react to changes in the brain environment, including increasing concentrations of amyloid-β (Aβ). However, the precise response of astrocytes to soluble small Aβ oligomers at concentrations similar to those present in the human brain has not been addressed. In this study, we exposed astrocytes to media from neurons that express the human amyloid precursor protein (APP) transgene with the double Swedish mutation (APPSwe), and which contains APP-derived fragments, including soluble human Aβ oligomers. We then used proteomics to investigate changes in the astrocyte secretome. Our data show dysregulated secretion of astrocytic proteins involved in the extracellular matrix and cytoskeletal organization and increase secretion of proteins involved in oxidative stress responses and those with chaperone activity. Several of these proteins have been identified in previous transcriptomic and proteomic studies using brain tissue from human AD and cerebrospinal fluid (CSF). Our work highlights the relevance of studying astrocyte secretion to understand the brain response to AD pathology and the potential use of these proteins as biomarkers for the disease.

Probing differences among Aβ oligomers with two triangular trimers derived from Aβ

The assembly of the β-amyloid peptide (Aβ) to form oligomers and fibrils is closely associated with the pathogenesis and progression of Alzheimer's disease. Aβ is a shape-shifting peptide capable of adopting many conformations and folds within the multitude of oligomers and fibrils the peptide forms. These properties have precluded detailed structural elucidation and biological characterization of homogeneous, well-defined Aβ oligomers. In this paper, we compare the structural, biophysical, and biological characteristics of two different covalently stabilized isomorphic trimers derived from the central and C-terminal regions Aβ. X-ray crystallography reveals the structures of the trimers and shows that each trimer forms a ball-shaped dodecamer. Solution-phase and cell-based studies demonstrate that the two trimers exhibit markedly different assembly and biological properties. One trimer forms small soluble oligomers that enter cells through endocytosis and activate capase-3/7-mediated apoptosis, while the other trimer forms large insoluble aggregates that accumulate on the outer plasma membrane and elicit cellular toxicity through an apoptosis-independent mechanism. The two trimers also exhibit different effects on the aggregation, toxicity, and cellular interaction of full-length Aβ, with one trimer showing a greater propensity to interact with Aβ than the other. The studies described in this paper indicate that the two trimers share structural, biophysical, and biological characteristics with oligomers of full-length Aβ. The varying structural, assembly, and biological characteristics of the two trimers provide a working model for how different Aβ trimers can assemble and lead to different biological effects, which may help shed light on the differences among Aβ oligomers.

Connectome-based biophysics models of Alzheimer's disease diagnosis and prognosis

With the increasing prevalence of Alzheimer's disease (AD) among aging populations and the limited therapeutic options available to slow or reverse its progression, the need has never been greater for improved diagnostic tools for identifying patients in the preclinical and prodomal phases of AD. Biophysics models of the connectome-based spread of amyloid-beta (Aβ) and microtubule-associated protein tau (τ) have enjoyed recent success as tools for predicting the time course of AD-related pathological changes. However, given the complex etiology of AD, which involves not only connectome-based spread of protein pathology but also the interactions of many molecular and cellular players over multiple spatiotemporal scales, more robust, complete biophysics models are needed to better understand AD pathophysiology and ultimately provide accurate patient-specific diagnoses and prognoses. Here we discuss several areas of active research in AD whose insights can be used to enhance the mathematical modeling of AD pathology as well as recent attempts at developing improved connectome-based biophysics models. These efforts toward a comprehensive yet parsimonious mathematical description of AD hold great promise for improving both the diagnosis of patients at risk for AD and our mechanistic understanding of how AD progresses.

Axonal degeneration in the anterior insular cortex is associated with Alzheimer's co-pathology in Parkinson's disease and dementia with Lewy bodies

BACKGROUND

Axons, crucial for impulse transmission and cellular trafficking, are thought to be primary targets of neurodegeneration in Parkinson's disease (PD) and dementia with Lewy bodies (DLB). Axonal degeneration occurs early, preceeding and exceeding neuronal loss, and contributes to the spread of pathology, yet is poorly described outside the nigrostriatal circuitry. The insula, a cortical brain hub, was recently discovered to be highly vulnerable to pathology and plays a role in cognitive deficits in PD and DLB. The aim of this study was to evaluate morphological features as well as burden of proteinopathy and axonal degeneration in the anterior insular sub-regions in PD, PD with dementia (PDD), and DLB.

METHODS

α-Synuclein, phosphorylated (p-)tau, and amyloid-β pathology load were evaluated in the anterior insular (agranular and dysgranular) subregions of post-mortem human brains (n = 27). Axonal loss was evaluated using modified Bielschowsky silver staining and quantified using stereology. Cytoskeletal damage was comprehensively studied using immunofluorescent multi-labelling and 3D confocal laser-scanning microscopy.

RESULTS

Compared to PD and PDD, DLB showed significantly higher α-synuclein and p-tau pathology load, argyrophilic grains, and  more severe axonal loss, particularly in the anterior agranular insula. Alternatively, the dysgranular insula showed a significantly higher load of amyloid-β pathology and its axonal density correlated with cognitive performance. p-Tau contributed most to axonal loss in the DLB group, was highest in the anterior agranular insula and significantly correlated with CDR global scores for dementia. Neurofilament and myelin showed degenerative changes including swellings, demyelination, and detachment of the axon-myelin unit.

CONCLUSIONS

Our results highlight the selective vulnerability of the anterior insular sub-regions to various converging pathologies, leading to impaired axonal integrity in PD, PDD and DLB, disrupting their functional properties and potentially contributing to cognitive, emotional, and autonomic deficits.

Molecular signatures underlying neurofibrillary tangle susceptibility in Alzheimer's disease

Tau aggregation in neurofibrillary tangles (NFTs) is closely associated with neurodegeneration and cognitive decline in Alzheimer's disease (AD). However, the molecular signatures that distinguish between aggregation-prone and aggregation-resistant cell states are unknown. We developed methods for the high-throughput isolation and transcriptome profiling of single somas with NFTs from the human AD brain, quantified the susceptibility of 20 neocortical subtypes for NFT formation and death, and identified both shared and cell-type-specific signatures. NFT-bearing neurons shared a marked upregulation of synaptic transmission-related genes, including a core set of 63 genes enriched for synaptic vesicle cycling. Oxidative phosphorylation and mitochondrial dysfunction were highly cell-type dependent. Apoptosis was only modestly enriched, and the susceptibilities of NFT-bearing and NFT-free neurons for death were highly similar. Our analysis suggests that NFTs represent cell-type-specific responses to stress and synaptic dysfunction. We provide a resource for biomarker discovery and the investigation of tau-dependent and tau-independent mechanisms of neurodegeneration.

Amyloid Beta Dynamics in Biological Fluids-Therapeutic Impact

Despite the significant impact of Alzheimer's disease (AD) at individual and socioeconomic levels and the numerous research studies carried out on this topic over the last decades, the treatments available in daily clinical practice remain less than satisfactory. Among the accepted etiopathogenic hypotheses, the amyloidogenic pathway theory, although intensively studied and even sometimes controversial, is still providing relevant theoretical elements for understanding the etiology of AD and for the further development of possible therapeutic tools. In this sense, this review aims to offer new insights related to beta amyloid (Aβ), an essential biomarker in AD. First the structure and function of Aβ in normal and pathological conditions are presented in detail, followed by a discussion on the dynamics of Aβ at the level of different biological compartments. There is focus on Aβ elimination modalities at central nervous system (CNS) level, and clearance via the blood-brain barrier seems to play a crucial/dominant role. Finally, different theoretical and already-applied therapeutic approaches for CNS Aβ elimination are presented, including the recent "peripheral sink therapeutic strategy" and "cerebrospinal fluid sinks therapeutic strategy". These data outline the need for a multidisciplinary approach designed to deliver a solution to stimulate Aβ clearance in more direct ways, including from the cerebrospinal fluid level.

Association between Sleep, Alzheimer's, and Parkinson's Disease

The majority of neurodegenerative diseases are pathologically associated with protein misfolding and aggregation. Alzheimer's disease (AD) is a type of dementia that slowly affects memory and cognitive function, and is characterized by the aggregation of the β-amyloid protein and tau neurofibrillary tangles in the brain. Parkinson's disease (PD) is a movement disorder typically resulting in rigidity and tremor, which is pathologically linked to the aggregation of α-synuclein, particularly in dopaminergic neurons in the midbrain. Sleep disorders commonly occur in AD and PD patients, and it can precede the onset of these diseases. For example, cognitively normal older individuals who have highly fragmented sleep had a 1.5-fold increased risk of subsequently developing AD. This suggests that sleep abnormalities may be a potential biomarker of these diseases. In this review, we describe the alterations of sleep in AD and PD, and discuss their potential in the early diagnosis of these diseases. We further discuss whether sleep disturbance could be a target for the treatment of these diseases.

Protective effects of a small molecule inhibitor ligand against hyperphosphorylated tau-induced mitochondrial and synaptic toxicities in Alzheimer disease

The purpose of our study is to understand the protective effects of small molecule ligands for phosphorylated tau (p-tau) in Alzheimer's disease (ad) progression. Many reports show evidence that p-tau is reported to be an important contributor to the formation of paired helical filaments (PHFs) and neurofibrillary tangles (NFTs) in ad neurons. In ad, glycogen synthase kinase-3 beta (GSK3β), cyclin-dependent kinase- 5 (CDK5) and dual specificity tyrosine-phosphorylation-regulated kinase 1A (DYRK1A), are the three important kinases responsible for tau hyperphosphorylation. Currently, there are no drugs and/or small molecules that reduce the toxicity of p-tau in ad. In the present study, we rationally selected and validated small molecule ligands that binds to the phosphorylated tau at SER23 (Ser 285). We also assessed the molecular dynamics and validated molecular docking sites for the three best ligands. Based on the best docking scores -8.09, -7.9 and - 7.8 kcal/mol, we found that ligand 1 binds to key hyperphosphorylation residues of p-tau that inhibit abnormal PHF-tau, DYRK1A, and GKS3β that reduce p-tau levels in ad. Using biochemical, molecular, immunoblotting, immunofluorescence, and transmission electron microscopy analyses, we studied the ligand 1 inhibition as well as mitochondrial and synaptic protective effects in immortalized primary hippocampal neuronal (HT22) cells. We found interactions between NAT10-262501 (ligand 1) and p-tau at key phosphorylation sites and these ligand-based inhibitions decreased PHF-tau, DYRK1A and GSK3β levels. We also found increased mitochondrial biogenesis, mitochondrial fusion and synaptic activities and reduced mitochondrial fission in ligand 1-treated mutant tau HT22 cells. Based on these results, we cautiously conclude that p-tau NAT10-262501 (ligand 1) reduces hyperphosphorylation of tau based GKS3β and CDK5 kinase regulation in ad, and aids in the maintenance of neuronal structure, mitochondrial dynamics, and biogenesis with a possible therapeutic drug target for ad.

Soluble endogenous oligomeric α-synuclein species in neurodegenerative diseases: Expression, spreading, and cross-talk

There is growing recognition in the field of neurodegenerative diseases that mixed proteinopathies are occurring at greater frequency than originally thought. This is particularly true for three amyloid proteins defining most of these neurological disorders, amyloid-beta (Aβ), tau, and alpha-synuclein (αSyn). The co-existence and often co-localization of aggregated forms of these proteins has led to the emergence of concepts positing molecular interactions and cross-seeding between Aβ, tau, and αSyn aggregates. Amongst this trio, αSyn has received particular attention in this context during recent years due to its ability to modulate Aβ and tau aggregation in vivo, to interact at a molecular level with Aβ and tau in vivo and to cross-seed tau in mice. Here we provide a comprehensive, critical, and accessible review about the expression, role and nature of endogenous soluble αSyn oligomers because of recent developments in the understanding of αSyn multimerization, misfolding, aggregation, cross-talk, spreading and cross-seeding in neurodegenerative disorders, including Parkinson's disease, dementia with Lewy bodies, multiple system atrophy, Alzheimer's disease, and Huntington's disease. We will also discuss our current understanding about the relative toxicity of endogenous αSyn oligomers in vivo and in vitro, and introduce potential opportunities to counter their deleterious effects.

Synthesis and structure-activity relationships of 3,4,5-trisubstituted-1,2,4-triazoles: high affinity and selective somatostatin receptor-4 agonists for Alzheimer's disease treatment

Somatostatin receptor-4 (SST₄) is highly expressed in brain regions affiliated with learning and memory. SST₄ agonist treatment may act to mitigate Alzheimer's disease (AD) pathology. An integrated approach to SST₄ agonist lead optimization is presented herein. High affinity and selective agonists with biological efficacy were identified through iterative cycles of a structure-based design strategy encompassing computational methods, chemistry, and preclinical pharmacology. 1,2,4-Triazole derivatives of our previously reported hit (4) showed enhanced SST₄ binding affinity, activity, and selectivity. Thirty-five compounds showed low nanomolar range SST₄ binding affinity, 12 having a K _i < 1 nM. These compounds showed >500-fold affinity for SST₄ as compared to SST_2A. SST₄ activities were consistent with the respective SST₄ binding affinities (EC₅₀ < 10 nM for 34 compounds). Compound 208 (SST₄ K _i = 0.7 nM; EC₅₀ = 2.5 nM; >600-fold selectivity over SST_2A) display a favorable physiochemical profile, and was advanced to learning and memory behavior evaluations in the senescence accelerated mouse-prone 8 model of AD-related cognitive decline. Chronic administration enhanced learning with i.p. dosing (1 mg kg^-1) compared to vehicle. Chronic administration enhanced memory with both i.p. (0.01, 0.1, 1 mg kg^-1) and oral (0.01, 10 mg kg^-1) dosing compared to vehicle. This study identified a novel series of SST₄ agonists with high affinity, selectivity, and biological activity that may be useful in the treatment of AD.

Emergence of directional bias in tau deposition from axonal transport dynamics

Defects in axonal transport may partly underpin the differences between the observed pathophysiology of Alzheimer's disease (AD) and that of other non-amyloidogenic tauopathies. Particularly, pathological tau variants may have molecular properties that dysregulate motor proteins responsible for the anterograde-directed transport of tau in a disease-specific fashion. Here we develop the first computational model of tau-modified axonal transport that produces directional biases in the spread of tau pathology. We simulated the spatiotemporal profiles of soluble and insoluble tau species in a multicompartment, two-neuron system using biologically plausible parameters and time scales. Changes in the balance of tau transport feedback parameters can elicit anterograde and retrograde biases in the distributions of soluble and insoluble tau between compartments in the system. Aggregation and fragmentation parameters can also perturb this balance, suggesting a complex interplay between these distinct molecular processes. Critically, we show that the model faithfully recreates the characteristic network spread biases in both AD-like and non-AD-like mouse tauopathy models. Tau transport feedback may therefore help link microscopic differences in tau conformational states and the resulting variety in clinical presentations.

Normal levels of KIF5 but reduced KLC1 levels in both Alzheimer disease and Alzheimer disease in Down syndrome: evidence suggesting defects in anterograde transport

BACKGROUND

Impaired axonal transport may contribute to the pathogenesis of neurodegenerative diseases, including Alzheimer's disease (AD) and Down syndrome (DS). Axonal transport is a complex process in which specific motor proteins move cargoes to and from neuronal cell bodies and their processes. Inconsistent reports point to the changes in AD in the levels of the classical anterograde motor protein kinesin family member 5 (KIF5) and the primary neuronal KIF regulator kinesin light chain 1 (KLC1), raising the possibility that anterograde transport is compromised in AD.

METHODS AND MATERIALS

To address inconsistencies and determine if the shared pathologies in AD and elderly DS subjects with dementia (AD in DS; AD-DS) extend to the changes in KIF5 and KLC1, we measured the levels of all the three KIF5 family members and KLC1 in the AD and AD-DS frontal cortex and AD temporal cortex and cerebellum in samples taken with a short postmortem interval. To support future studies to explore the cell biological basis for any changes detected, we also examined the levels of these proteins in the brains of young and aged adult mice in the Dp (16)1Yey/+ (Dp16) mouse model of DS and J20 mouse model of AD.

RESULTS

There were no changes in comparison with controls in KIF5 family members in either the AD or AD-DS samples when normalized to either β-actin or glyceraldehyde-3-phosphate dehydrogenase (GAPDH). Interestingly, however, samples from control brains as well as from AD and AD-DS demonstrated strong positive correlations between the levels of KIF5 family members, suggesting positive co-regulated expression. Importantly, while earlier reports pointed to a negative correlation between the levels of the amyloid precursor protein (APP) and KIF5A levels, we found the opposite to be true in AD-DS; this was especially striking given triplication of the APP gene, with increased APP protein levels. AD and control samples showed positive correlations between fl-hAPP and KIF5 members, but they were less consistent. In contrast to the findings for KIF5, the levels of KLC1 were downregulated in the frontal cortex of both AD and AD-DS brains; interestingly, this change was not seen in the AD temporal cortex or cerebellum. As postmortem interval has a negative effect on the levels of KLC1, but not KIF5 members, we analyzed a subset of samples with a very short postmortem interval (PMI) (≤ 6 h), a PMI that was not significantly correlated with the levels of KLC1 in either AD or AD-DS samples; we confirmed the presence of a statistically significant reduction of KLC1 in AD and AD-DS brains as compared with control brains. Studies comparing Dp16 to its euploid control recapitulated human studies in demonstrating no change in KIF5 levels and a positive correlation between the levels of KIF5 family members. J20 mice also showed normal KIF5 levels. However, unlike the AD and AD-DS frontal cortex, KLC1 levels were not reduced in the brains of Dp16 or J20 mice.

CONCLUSION

These data point to significant reductions in KLC1 in AD and AD-DS. In so doing, they raise the possibility of compromised KLC1-mediated axonal transport in these conditions, a posit that can now be pursued in model systems in which KLC1 expression is reduced.

Targeting Tau to Treat Clinical Features of Huntington's Disease

Huntington's disease (HD) is an autosomal dominant neurodegenerative disorder characterized by severe motor, cognitive and psychiatric impairments. While motor deficits often confirm diagnosis, cognitive dysfunctions usually manifest early in the disease process and are consistently ranked among the leading factors that impact the patients' quality of life. The genetic component of HD, a mutation in the huntingtin (HTT) gene, is traditionally presented as the main contributor to disease pathology. However, accumulating evidence suggests the implication of the microtubule-associated tau protein to the pathogenesis and therefore, proposes an alternative conceptual framework where tau and mutant huntingtin (mHTT) act conjointly to drive neurodegeneration and cognitive dysfunction. This perspective on disease etiology offers new avenues to design therapeutic interventions and could leverage decades of research on Alzheimer's disease (AD) and other tauopathies to rapidly advance drug discovery. In this mini review, we examine the breadth of tau-targeting treatments currently tested in the preclinical and clinical settings for AD and other tauopathies, and discuss the potential application of these strategies to HD.

Axonal Degeneration in AD: The Contribution of Aβ and Tau

Alzheimer's disease (AD) represents the most common age-related neurodegenerative disorder, affecting around 35 million people worldwide. Despite enormous efforts dedicated to AD research over decades, there is still no cure for the disease. Misfolding and accumulation of Aβ and tau proteins in the brain constitute a defining signature of AD neuropathology, and mounting evidence has documented a link between aggregation of these proteins and neuronal dysfunction. In this context, progressive axonal degeneration has been associated with early stages of AD and linked to Aβ and tau accumulation. As the axonal degeneration mechanism has been starting to be unveiled, it constitutes a promising target for neuroprotection in AD. A comprehensive understanding of the mechanism of axonal destruction in neurodegenerative conditions is therefore critical for the development of new therapies aimed to prevent axonal loss before irreversible neuronal death occurs in AD. Here, we review current evidence of the involvement of Aβ and tau pathologies in the activation of signaling cascades that can promote axonal demise.

The Toxicity and Polymorphism of β-Amyloid Oligomers

It is widely accepted that β-amyloid oligomers (Aβos) play a key role in the progression of Alzheimer's disease (AD) by inducing neuron damage and cognitive impairment, but Aβos are highly heterogeneous in their size, structure and cytotoxicity, making the corresponding studies tough to carry out. Nevertheless, a number of studies have recently made remarkable progress in the describing the characteristics and pathogenicity of Aβos. We here review the mechanisms by which Aβos exert their neuropathogenesis for AD progression, including receptor binding, cell membrane destruction, mitochondrial damage, Ca2+ homeostasis dysregulation and tau pathological induction. We also summarize the characteristics and pathogenicity such as the size, morphology and cytotoxicity of dimers, trimers, Aβ*56 and spherical oligomers, and suggest that Aβos may play a different role at different phases of AD pathogenesis, resulting in differential consequences on neuronal synaptotoxicity and survival. It is warranted to investigate the temporal sequence of Aβos in AD human brain and examine the relationship between different Aβos and cognitive impairment.

Tau is required for progressive synaptic and memory deficits in a transgenic mouse model of α-synucleinopathy

Parkinson's disease dementia (PDD) and dementia with Lewy bodies (DLB) are clinically and neuropathologically highly related α-synucleinopathies that collectively constitute the second leading cause of neurodegenerative dementias. Genetic and neuropathological studies directly implicate α-synuclein (αS) abnormalities in PDD and DLB pathogenesis. However, it is currently unknown how αS abnormalities contribute to memory loss, particularly since forebrain neuronal loss in PDD and DLB is less severe than in Alzheimer's disease. Previously, we found that familial Parkinson's disease-linked human mutant A53T αS causes aberrant localization of the microtubule-associated protein tau to postsynaptic spines in neurons, leading to postsynaptic deficits. Thus, we directly tested if the synaptic and memory deficits in a mouse model of α-synucleinopathy (TgA53T) are mediated by tau. TgA53T mice exhibit progressive memory deficits associated with postsynaptic deficits in the absence of obvious neuropathological and neurodegenerative changes in the hippocampus. Significantly, removal of endogenous mouse tau expression in TgA53T mice (TgA53T/mTau-/-), achieved by mating TgA53T mice to mouse tau-knockout mice, completely ameliorates cognitive dysfunction and concurrent synaptic deficits without affecting αS expression or accumulation of selected toxic αS oligomers. Among the known tau-dependent effects, memory deficits in TgA53T mice were associated with hippocampal circuit remodeling linked to chronic network hyperexcitability. This remodeling was absent in TgA53T/mTau-/- mice, indicating that postsynaptic deficits, aberrant network hyperactivity, and memory deficits are mechanistically linked. Our results directly implicate tau as a mediator of specific human mutant A53T αS-mediated abnormalities related to deficits in hippocampal neurotransmission and suggest a mechanism for memory impairment that occurs as a consequence of synaptic dysfunction rather than synaptic or neuronal loss. We hypothesize that these initial synaptic deficits contribute to network hyperexcitability which, in turn, exacerbate cognitive dysfunction. Our results indicate that these synaptic changes present potential therapeutic targets for amelioration of memory deficits in α-synucleinopathies.

Axonal transport defects and neurodegeneration: Molecular mechanisms and therapeutic implications

Because of the extremely polarized morphology, the proper functioning of neurons largely relies on the efficient cargo transport along the axon. Axonal transport defects have been reported in multiple neurodegenerative diseases as an early pathological feature. The discovery of mutations in human genes involved in the transport machinery provide a direct causative relationship between axonal transport defects and neurodegeneration. Here, we summarize the current genetic findings related to axonal transport in neurodegenerative diseases, and we discuss the relationship between axonal transport defects and other pathological changes observed in neurodegeneration. In addition, we summarize the therapeutic approaches targeting the axonal transport machinery in studies of neurodegenerative diseases. Finally, we review the technical advances in tracking axonal transport both in vivo and in vitro.

Production of seedable Amyloid-β peptides in model of prion diseases upon PrPSc-induced PDK1 overactivation

The presence of amyloid beta (Aβ) plaques in the brain of some individuals with Creutzfeldt-Jakob or Gertsmann-Straussler-Scheinker diseases suggests that pathogenic prions (PrP^Sc) would have stimulated the production and deposition of Aβ peptides. We here show in prion-infected neurons and mice that deregulation of the PDK1-TACE α-secretase pathway reduces the Amyloid Precursor Protein (APP) α-cleavage in favor of APP β-processing, leading to Aβ40/42 accumulation. Aβ predominates as monomers, but is also found as trimers and tetramers. Prion-induced Aβ peptides do not affect prion replication and infectivity, but display seedable properties as they can deposit in the mouse brain only when seeds of Aβ trimers are co-transmitted with PrP^Sc. Importantly, brain Aβ deposition accelerates death of prion-infected mice. Our data stress that PrP^Sc, through deregulation of the PDK1-TACE-APP pathway, provokes the accumulation of Aβ, a prerequisite for the onset of an Aβ seeds-induced Aβ pathology within a prion-infectious context.

Aβ plaques have been detected in brains of patients with prion diseases. Here, using mice, the authors show that prion infection enhances Aβ production via a PDK1-TACE mechanism and that brain deposition of Aβ induced by Aβ seeds co-transmitted with PrP^Sc contributes to mortality in prion disease.

Phosphorylation in two discrete tau domains regulates a stepwise process leading to postsynaptic dysfunction

KEY POINTS

Tau mislocalization to dendritic spines and associated postsynaptic deficits are mediated through different and non-overlapping phosphorylation sites. Tau mislocalization to dendritic spines depends upon the phosphorylation of either Ser396 or Ser404 in the C-terminus. Postsynaptic dysfunction instead depends upon the phosphorylation of at least one of five residues in the proline-rich region of tau. The blockade of both glycogen synthetase kinase 3β and cyclin-dependent kinase 5 is required to prevent P301L-induced tau mislocalization to dendritic spines, supporting redundant pathways that control tau mislocalization to spines.

ABSTRACT

Tau protein consists of an N-terminal projection domain, a microtubule-binding domain and a C-terminal domain. In neurodegenerative diseases, including Alzheimer's disease and frontotemporal dementia, the hyperphosphorylation of tau changes its shape, binding partners and resulting function. An early consequence of tau phosphorylation by proline-directed kinases is postsynaptic dysfunction associated with the mislocalization of tau to dendritic spines. The specific phosphorylation sites leading to these abnormalities have not been elucidated. Here, using imaging and electrophysiological techniques to study cultured rat hippocampal neurons, we show that postsynaptic dysfunction results from a sequential process involving differential phosphorylation in the N-terminal and C-terminal domains. First, tau mislocalizes to dendritic spines, in a manner that depends upon the phosphorylation of either Ser396 or Ser404 in the C-terminal domain. The blockade of both glycogen synthetase kinase 3β and cyclin-dependent kinase 5 prevents tau mislocalization to dendritic spines. Second, a reduction of functional AMPA receptors depends upon the phosphorylation of at least one of five residues (Ser202, Thr205, Thr212, Thr217 and Thr231) in the proline-rich region of the N-terminal domain. This is the first report of differential phosphorylation in distinct tau domains governing separate, but linked, steps leading to synaptic dysfunction.

Aβ(M1-40) and Wild-Type Aβ40 Self-Assemble into Oligomers with Distinct Quaternary Structures

Amyloid-β oligomers (AβOs) self-assemble into polymorphic species with diverse biological activities that are implicated causally to Alzheimer’s disease (AD). Synaptotoxicity of AβO species is dependent on their quaternary structure, however, low-abundance and environmental sensitivity of AβOs in vivo have impeded a thorough assessment of structure–function relationships. We developed a simple biochemical assay to quantify the relative abundance and morphology of cross-linked AβOs. We compared oligomers derived from synthetic Aβ40 (wild-type (WT) Aβ40) and a recombinant source, called Aβ(M1–40). Both peptides assemble into oligomers with common sizes and morphology, however, the predominant quaternary structures of Aβ(M1–40) oligomeric states were more diverse in terms of dispersity and morphology. We identified self-assembly conditions that stabilize high-molecular weight oligomers of Aβ(M1–40) with apparent molecular weights greater than 36 kDa. Given that mixtures of AβOs derived from both peptides have been shown to be potent neurotoxins that disrupt long-term potentiation, we anticipate that the diverse quaternary structures reported for Aβ(M1–40) oligomers using the assays reported here will facilitate research efforts aimed at isolating and identifying common toxic species that contribute to synaptic dysfunction.

Past to Future: What Animal Models Have Taught Us About Alzheimer's Disease

Alzheimer's disease (AD) impairs memory and causes significant cognitive deficits. The disease course is prolonged, with a poor prognosis, and thus exacts an enormous economic and social burden. Over the past two decades, genetically engineered mouse models have proven indispensable for understanding AD pathogenesis, as well as for discovering new therapeutic targets. Here we highlight significant studies from our laboratory that have helped advance the AD field by elucidating key pathogenic processes operative in AD and exploring a variety of aspects of the disease which may yield novel therapeutic strategies for combatting this burdensome disease.

The Amyloid-β Oligomer Hypothesis: Beginning of the Third Decade

The amyloid-β oligomer (AβO) hypothesis was introduced in 1998. It proposed that the brain damage leading to Alzheimer’s disease (AD) was instigated by soluble, ligand-like AβOs. This hypothesis was based on the discovery that fibril-free synthetic preparations of AβOs were potent CNS neurotoxins that rapidly inhibited long-term potentiation and, with time, caused selective nerve cell death (Lambert et al., 1998). The mechanism was attributed to disrupted signaling involving the tyrosine-protein kinase Fyn, mediated by an unknown toxin receptor. Over 4,000 articles concerning AβOs have been published since then, including more than 400 reviews. AβOs have been shown to accumulate in an AD-dependent manner in human and animal model brain tissue and, experimentally, to impair learning and memory and instigate major facets of AD neuropathology, including tau pathology, synapse deterioration and loss, inflammation, and oxidative damage. As reviewed by Hayden and Teplow in 2013, the AβO hypothesis “has all but supplanted the amyloid cascade.” Despite the emerging understanding of the role played by AβOs in AD pathogenesis, AβOs have not yet received the clinical attention given to amyloid plaques, which have been at the core of major attempts at therapeutics and diagnostics but are no longer regarded as the most pathogenic form of Aβ. However, if the momentum of AβO research continues, particularly efforts to elucidate key aspects of structure, a clear path to a successful disease modifying therapy can be envisioned. Ensuring that lessons learned from recent, late-stage clinical failures are applied appropriately throughout therapeutic development will further enable the likelihood of a successful therapy in the near-term.

Amyloid beta-mediated KIF5A deficiency disrupts anterograde axonal mitochondrial movement

Mitochondria are crucial organelles for neurophysiology and brain mitochondrial defects constitute a characteristic of Alzheimer's disease (AD). Impaired axonal mitochondrial traffic, especially the anterograde axonal mitochondrial transport is a pronouncing mitochondrial defect that underlies synaptic failure in AD-related conditions. However, the detailed molecular mechanisms of such axonal mitochondrial abnormality have not been fully understood. KIF5A is a key isoform of kinesin-1, which is a key molecular machinery in facilitating anterograde axonal mitochondrial transport. In this study, we have determined a downregulation of KIF5A in postmortem AD temporal lobes. Further experiments on amyloid beta (Aβ)-treated primary neuron culture and 5 × FAD mice suggest a close association of Aβ toxicity and KIF5A loss. Downregulation of KIF5A mimics Aβ-induced axonal mitochondrial transport deficits, indicating a potential role of KIF5A deficiency in AD-relevant axonal mitochondrial traffic abnormalities. Importantly, the restoration of KIF5A corrects Aβ-induced impairments in axonal mitochondrial transport, especially the anterograde traffic, with little or no impact on retrograde axonal mitochondrial motility. Our findings suggest a novel KIF5A-associated mechanism conferring Aβ toxicity to axonal mitochondrial deficits. Furthermore, the results implicate a potential therapeutic avenue by protecting KIF5A function for the treatment of AD.

Can magnesium reduce central neurodegeneration in Alzheimer's disease? Basic evidences and research needs

Magnesium (Mg) is a crucial divalent cation with more than 300 cellular functions. This ion shows therapeutic properties in several neurological diseases. Although there are numerous basic evidences showing that Mg can inhibit pathological processes involved in neuroglial degeneration, this low-cost option is not well-considered in clinical research and practice for now. Nevertheless, none of the expensive drugs currently recommended by the classic guidelines (in addition to physiological rehabilitation) had shown exceptional effectiveness. Herein, focusing on Alzheimer's disease (AD), we analyze the therapeutic pathways that support the use of Mg for neurogenesis and neuroprotection. According to experimental findings reviewed, Mg shows interesting abilities to facilitate toxin clearance, reduce neuroinflammation, inhibit the pathologic processing of amyloid protein precursor (APP) as well as the abnormal tau protein phosphorylation, and to reverse the deregulation of N-methyl-D-aspartate receptors. Currently, some crucial details of the mechanisms involved in these proved effects remain elusive and clinical background is poor. Therefore, further studies are required to enable a better overview on pharmacodynamic targets of Mg and thus, to find optimal pharmacologic strategies for clinical use of this ion.

Bidirectional modulation of Alzheimer phenotype by alpha-synuclein in mice and primary neurons

α-Synuclein (αSyn) histopathology defines several neurodegenerative disorders, including Parkinson's disease, Lewy body dementia, and Alzheimer's disease (AD). However, the functional link between soluble αSyn and disease etiology remains elusive, especially in AD. We, therefore, genetically targeted αSyn in APP transgenic mice modeling AD and mouse primary neurons. Our results demonstrate bidirectional modulation of behavioral deficits and pathophysiology by αSyn. Overexpression of human wild-type αSyn in APP animals markedly reduced amyloid deposition but, counter-intuitively, exacerbated deficits in spatial memory. It also increased extracellular amyloid-β oligomers (AβOs), αSyn oligomers, exacerbated tau conformational and phosphorylation variants associated with AD, and enhanced neuronal cell cycle re-entry (CCR), a frequent prelude to neuron death in AD. Conversely, ablation of the SNCA gene encoding for αSyn in APP mice improved memory retention in spite of increased plaque burden. Reminiscent of the effect of MAPT ablation in APP mice, SNCA deletion prevented premature mortality. Moreover, the absence of αSyn decreased extracellular AβOs, ameliorated CCR, and rescued postsynaptic marker deficits. In summary, this complementary, bidirectional genetic approach implicates αSyn as an essential mediator of key phenotypes in AD and offers new functional insight into αSyn pathophysiology.

Tau-mediated synaptic and neuronal dysfunction in neurodegenerative disease

The accumulation of pathological tau in the brain is associated with neuronal deterioration and cognitive impairments in tauopathies including Alzheimer's disease. Tau, while primarily localized in the axons of healthy neurons, accumulates in the soma and dendrites of neurons under pathogenic conditions. Tau is found in both presynaptic and postsynaptic compartments of neurons in Alzheimer's disease. New research supports that soluble forms of tau trigger pathophysiology in the brain by altering properties of synaptic and neuronal function at the early stages of disease progression, before neurons die. Here we review the current understanding of how tau-mediated synaptic and neuronal dysfunction contributes to cognitive decline. Delineating the mechanisms by which pathogenic tau alters synapses, dendrites and axons will help lay the foundation for new strategies that can restore neuronal function in tauopathy.

Impact of amyloid-beta changes on cognitive outcomes in Alzheimer's disease: analysis of clinical trials using a quantitative systems pharmacology model

BACKGROUND

Despite a tremendous amount of information on the role of amyloid in Alzheimer's disease (AD), almost all clinical trials testing this hypothesis have failed to generate clinically relevant cognitive effects.

METHODS

We present an advanced mechanism-based and biophysically realistic quantitative systems pharmacology computer model of an Alzheimer-type neuronal cortical network that has been calibrated with Alzheimer Disease Assessment Scale, cognitive subscale (ADAS-Cog) readouts from historical clinical trials and simulated the differential impact of amyloid-beta (Aβ40 and Aβ42) oligomers on glutamate and nicotinic neurotransmission.

RESULTS

Preclinical data suggest a beneficial effect of shorter Aβ forms within a limited dose range. Such a beneficial effect of Aβ40 on glutamate neurotransmission in human patients is absolutely necessary to reproduce clinical data on the ADAS-Cog in minimal cognitive impairment (MCI) patients with and without amyloid load, the effect of APOE genotype effect on the slope of the cognitive trajectory over time in placebo AD patients and higher sensitivity to cholinergic manipulation with scopolamine associated with higher Aβ in MCI subjects. We further derive a relationship between units of Aβ load in our model and the standard uptake value ratio from amyloid imaging. When introducing the documented clinical pharmacodynamic effects on Aβ levels for various amyloid-related clinical interventions in patients with low Aβ baseline, the platform predicts an overall significant worsening for passive vaccination with solanezumab, beta-secretase inhibitor verubecestat and gamma-secretase inhibitor semagacestat. In contrast, all three interventions improved cognition in subjects with moderate to high baseline Aβ levels, with verubecestat anticipated to have the greatest effect (around ADAS-Cog value 1.5 points), solanezumab the lowest (0.8 ADAS-Cog value points) and semagacestat in between. This could explain the success of many amyloid interventions in transgene animals with an artificial high level of Aβ, but not in AD patients with a large variability of amyloid loads.

CONCLUSIONS

If these predictions are confirmed in post-hoc analyses of failed clinical amyloid-modulating trials, one should question the rationale behind testing these interventions in early and prodromal subjects with low or zero amyloid load.

Religious Orders Study and Rush Memory and Aging Project

BACKGROUND

The Religious Orders Study and Rush Memory and Aging Project are both ongoing longitudinal clinical-pathologic cohort studies of aging and Alzheimer's disease (AD).

OBJECTIVES

To summarize progress over the past five years and its implications for understanding neurodegenerative diseases.

METHODS

Participants in both studies are older adults who enroll without dementia and agree to detailed longitudinal clinical evaluations and organ donation. The last review summarized findings through the end of 2011. Here we summarize progress and study findings over the past five years and discuss new directions for how these studies can inform on aging and AD in the future.

RESULTS

We summarize 1) findings on the relation of neurobiology to clinical AD; 2) neurobiologic pathways linking risk factors to clinical AD; 3) non-cognitive AD phenotypes including motor function and decision making; 4) the development of a novel drug discovery platform.

CONCLUSION

Complexity at multiple levels needs to be understood and overcome to develop effective treatments and preventions for cognitive decline and AD dementia.

The olfactory bulb as the entry site for prion-like propagation in neurodegenerative diseases

Olfactory deficits are present in numerous neurodegenerative disorders and are accompanied by pathology in related brain regions. In several of these disorders, olfactory disturbances appear early and are considered as prodromal symptoms of the disease. In addition, pathological protein aggregates affect olfactory regions prior to other regions, suggesting that the olfactory system might be particularly vulnerable to neurodegenerative diseases. Exposed to the external environment, the olfactory epithelium and olfactory bulb allow pathogen and toxin penetration into the brain, a process that has been proposed to play a role in neurodegenerative diseases. Determining whether the olfactory bulb could be a starting point of pathology and of pathology spread is crucial to understanding how neurodegenerative diseases evolve. We argue that pathological changes following environmental insults contribute to the initiation of protein aggregation in the olfactory bulb, which then triggers the spread of the pathology within the brain by a templating mechanism in a prion-like manner. We review the evidence for the early involvement of olfactory structures in neurodegenerative diseases and the relationship between neuropathology and olfactory function. We discuss the vulnerability and putative underlying mechanisms by which pathology could be initiated in the olfactory bulb, from the entry of pathogens (promoted by increased permeability of the olfactory epithelium with aging or inflammation) to the sensitivity of the olfactory system to oxidative stress and inflammation. Finally, we review changes in protein expression and neural excitability triggered by pathogenic proteins that can promote pathogenesis in the olfactory bulb and beyond.

Amyloid-beta-dependent phosphorylation of collapsin response mediator protein-2 dissociates kinesin in Alzheimer's disease

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by accumulation of amyloid plaques and neurofibrillary tangles. Prior to the development of these characteristic pathological hallmarks of AD, anterograde axonal transport is impaired. However, the key proteins that initiate these intracellular impairments remain elusive. The collapsin response mediator protein-2 (CRMP-2) plays an integral role in kinesin-1-dependent axonal transport and there is evidence that phosphorylation of CRMP-2 releases kinesin-1. Here, we tested the hypothesis that amyloid-beta (Aβ)-dependent phosphorylation of CRMP-2 disrupts its association with the kinesin-1 (an anterograde axonal motor transport protein) in AD. We found that brain sections and lysates from AD patients demonstrated elevated phosphorylation of CRMP-2 at the T555 site. Additionally, in the transgenic Tg2576 mouse model of familial AD (FAD) that exhibits Aβ accumulation in the brain with age, we found substantial co-localization of pT555CRMP-2 and dystrophic neurites. In SH-SY5Y differentiated neuronal cultures, Aβ-dependent phosphorylation of CRMP-2 at the T555 site was also elevated and this reduced the CRMP-2 association with kinesin-1. The overexpression of an unphosphorylatable form of CRMP-2 in neurons promoted the re-establishment of CRMP-2-kinesin association and axon elongation. These data suggest that Aβ-dependent phosphorylation of CRMP-2 at the T555 site may directly impair anterograde axonal transport protein function, leading to neuronal defects.

Practical considerations for choosing a mouse model of Alzheimer's disease

Alzheimer’s disease (AD) is behaviorally identified by progressive memory impairment and pathologically characterized by the triad of β-amyloid plaques, neurofibrillary tangles, and neurodegeneration. Genetic mutations and risk factors have been identified that are either causal or modify the disease progression. These genetic and pathological features serve as basis for the creation and validation of mouse models of AD. Efforts made in the past quarter-century have produced over 100 genetically engineered mouse lines that recapitulate some aspects of AD clinicopathology. These models have been valuable resources for understanding genetic interactions that contribute to disease and cellular reactions that are engaged in response. Here we focus on mouse models that have been widely used stalwarts of the field or that are recently developed bellwethers of the future. Rather than providing a summary of each model, we endeavor to compare and contrast the genetic approaches employed and to discuss their respective advantages and limitations. We offer a critical account of the variables which may contribute to inconsistent findings and the factors that should be considered when choosing a model and interpreting the results. We hope to present an insightful review of current AD mouse models and to provide a practical guide for selecting models best matched to the experimental question at hand.

Stabilization of dynamic microtubules by mDia1 drives Tau-dependent Aβ1-42 synaptotoxicity

Oligomeric Amyloid β_1-42 (Aβ) plays a crucial synaptotoxic role in Alzheimer's disease, and hyperphosphorylated tau facilitates Aβ toxicity. The link between Aβ and tau, however, remains controversial. In this study, we find that in hippocampal neurons, Aβ acutely induces tubulin posttranslational modifications (PTMs) and stabilizes dynamic microtubules (MTs) by reducing their catastrophe frequency. Silencing or acute inhibition of the formin mDia1 suppresses these activities and corrects the synaptotoxicity and deficits of axonal transport induced by Aβ. We explored the mechanism of rescue and found that stabilization of dynamic MTs promotes tau-dependent loss of dendritic spines and tau hyperphosphorylation. Collectively, these results uncover a novel role for mDia1 in Aβ-mediated synaptotoxicity and demonstrate that inhibition of MT dynamics and accumulation of PTMs are driving factors for the induction of tau-mediated neuronal damage.

The amyloid-β oligomer Aβ*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-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.

Synaptic Impairment in Alzheimer's Disease: A Dysregulated Symphony

Alzheimer's disease (AD) is characterized by memory loss, cognitive decline, and devastating neurodegeneration, not only as a result of the extracellular accumulation of beta-amyloid peptide (Aβ) and intracellular accumulation of tau, but also as a consequence of the dysfunction and loss of synapses. Although significant advances have been made in our understanding of the relationship of the pathological role of Aβ and tau in synapse dysfunction, several questions remain as to how Aβ and tau interdependently cause impairments in synaptic function in AD. Overall, more insight into these questions should enable researchers in this field to develop novel therapeutic targets to mitigate or delay the cognitive deficits associated with this devastating disease.

Stabilization, Assembly, and Toxicity of Trimers Derived from Aβ

Oligomers of the β-amyloid peptide Aβ have emerged as important contributors to neurodegeneration in Alzheimer’s disease. Mounting evidence suggests that Aβ trimers and higher-order oligomers derived from trimers have special significance in the early stages of Alzheimer’s disease. Elucidating the structures of these trimers and higher-order oligomers is paramount for understanding their role in neurodegeneration. This paper describes the design, synthesis, X-ray crystallographic structures, and biophysical and biological properties of two stabilized trimers derived from the central and C-terminal regions of Aβ. These triangular trimers are stabilized through three disulfide cross-links between the monomer subunits. The X-ray crystallographic structures reveal that the stabilized trimers assemble hierarchically to form hexamers, dodecamers, and annular porelike structures. Solution-phase biophysical studies reveal that the stabilized trimers assemble in solution to form oligomers that recapitulate some of the higher-order assemblies observed crystallographically. The stabilized trimers share many of the biological characteristics of oligomers of full-length Aβ, including toxicity toward a neuronally derived human cell line, activation of caspase-3 mediated apoptosis, and reactivity with the oligomer-specific antibody A11. These studies support the biological significance of the triangular trimer assembly of Aβ β-hairpins and may offer a deeper understanding of the molecular basis of Alzheimer’s disease.