Phosphorylated amyloid-beta: the toxic intermediate in alzheimer's disease neurodegeneration

Effect of astaxanthin in type-2 diabetes -induced APPxhQC transgenic and NTG mice

OBJECTIVES

Aggregation and misfolding of amyloid beta (Aβ) and tau proteins, suggested to arise from post-translational modification processes, are thought to be the main cause of Alzheimer's disease (AD). Additionally, a plethora of evidence exists that links metabolic dysfunctions such as obesity, type 2 diabetes (T2D), and dyslipidemia to the pathogenesis of AD. We thus investigated the combinatory effect of T2D and human glutaminyl cyclase activity (pyroglutamylation), on the pathology of AD and whether astaxanthin (ASX) treatment ameliorates accompanying pathophysiological manifestations.

METHODS

Male transgenic AD mice, APPxhQC, expressing human APP751 with the Swedish and the London mutation and human glutaminyl cyclase (hQC) enzyme and their non-transgenic (NTG) littermates were used. Both APPxhQC and NTG mice were allocated to 3 groups, control, T2D-control, and T2D-ASX. Mice were fed control or high fat diet ± ASX for 13 weeks starting at an age of 11-12 months. High fat diet fed mice were further treated with streptozocin for T2D induction. Effects of genotype, T2D induction, and ASX treatment were evaluated by analysing glycemic readouts, lipid concentration, Aβ deposition, hippocampus-dependent cognitive function and nutrient sensing using immunosorbent assay, ELISA-based assays, western blotting, immunofluorescence staining, and behavioral testing via Morris water maze (MWM), respectively.

RESULTS

APPxhQC mice presented a higher glucose sensitivity compared to NTG mice. T2D-induced brain dysfunction was more severe in NTG compared to the APPxhQC mice. T2D induction impaired memory functions while increasing hepatic LC3B, ABCA1, and p65 levels in NTG mice. T2D induction resulted in a progressive shift of Aβ from the soluble to insoluble form in APPxhQC mice. ASX treatment reversed T2D-induced memory dysfunction in NTG mice and in parallel increased hepatic pAKT while decreasing p65 and increasing cerebral p-S6rp and p65 levels. ASX treatment reduced soluble Aβ38 and Aβ40 and insoluble Aβ40 levels in T2D-induced APPxhQC mice.

CONCLUSIONS

We demonstrate that T2D induction in APPxhQC mice poses additional risk for AD pathology as seen by increased Aβ deposition. Although ASX treatment reduced Aβ expression in T2D-induced APPxhQC mice and rescued T2D-induced memory impairment in NTG mice, ASX treatment alone may not be effective in cases of T2D comorbidity and AD.

When Good Kinases Go Rogue: GSK3, p38 MAPK and CDKs as Therapeutic Targets for Alzheimer's and Huntington's Disease

Alzheimer's disease (AD) is a mostly sporadic brain disorder characterized by cognitive decline resulting from selective neurodegeneration in the hippocampus and cerebral cortex whereas Huntington's disease (HD) is a monogenic inherited disorder characterized by motor abnormalities and psychiatric disturbances resulting from selective neurodegeneration in the striatum. Although there have been numerous clinical trials for these diseases, they have been unsuccessful. Research conducted over the past three decades by a large number of laboratories has demonstrated that abnormal actions of common kinases play a key role in the pathogenesis of both AD and HD as well as several other neurodegenerative diseases. Prominent among these kinases are glycogen synthase kinase (GSK3), p38 mitogen-activated protein kinase (MAPK) and some of the cyclin-dependent kinases (CDKs). After a brief summary of the molecular and cell biology of AD and HD this review covers what is known about the role of these three groups of kinases in the brain and in the pathogenesis of the two neurodegenerative disorders. The potential of targeting GSK3, p38 MAPK and CDKS as effective therapeutics is also discussed as is a brief discussion on the utilization of recently developed drugs that simultaneously target two or all three of these groups of kinases. Multi-kinase inhibitors either by themselves or in combination with strategies currently being used such as immunotherapy or secretase inhibitors for AD and knockdown for HD could represent a more effective therapeutic approach for these fatal neurodegenerative diseases.

Novel Phosphorylation-State Specific Antibodies Reveal Differential Deposition of Ser26 Phosphorylated Aβ Species in a Mouse Model of Alzheimer's Disease

Aggregation and deposition of amyloid-β (Aβ) peptides in extracellular plaques and in the cerebral vasculature are prominent neuropathological features of Alzheimer's disease (AD) and closely associated with the pathogenesis of AD. Amyloid plaques in the brains of most AD patients and transgenic mouse models exhibit heterogeneity in the composition of Aβ deposits, due to the occurrence of elongated, truncated, and post-translationally modified Aβ peptides. Importantly, changes in the deposition of these different Aβ variants are associated with the clinical disease progression and considered to mark sequential phases of plaque and cerebral amyloid angiopathy (CAA) maturation at distinct stages of AD. We recently showed that Aβ phosphorylated at serine residue 26 (pSer26Aβ) has peculiar characteristics in aggregation, deposition, and neurotoxicity. In the current study, we developed and thoroughly validated novel monoclonal and polyclonal antibodies that recognize Aβ depending on the phosphorylation-state of Ser26. Our results demonstrate that selected phosphorylation state-specific antibodies were able to recognize Ser26 phosphorylated and non-phosphorylated Aβ with high specificity in enzyme-linked immunosorbent assay (ELISA) and Western Blotting (WB) assays. Furthermore, immunofluorescence analyses with these antibodies demonstrated the occurrence of pSer26Aβ in transgenic mouse brains that show differential deposition as compared to non-phosphorylated Aβ (npAβ) or other modified Aβ species. Notably, pSer26Aβ species were faintly detected in extracellular Aβ plaques but most prominently found intraneuronally and in cerebral blood vessels. In conclusion, we developed new antibodies to specifically differentiate Aβ peptides depending on the phosphorylation state of Ser26, which are applicable in ELISA, WB, and immunofluorescence staining of mouse brain tissues. These site- and phosphorylation state-specific Aβ antibodies represent novel tools to examine phosphorylated Aβ species to further understand and dissect the complexity in the age-related and spatio-temporal deposition of different Aβ variants in transgenic mouse models and human AD brains.

Phosphorylation Promotes Aβ25-35 Peptide Aggregation within the DMPC Bilayer

The consequences of phosphorylation of the Aβ25-35 peptide at the position Ser26 on its aggregation have not been examined. To investigate them, we performed all-atom replica exchange simulations probing the binding of phosphorylated Aβ25-35 (pAβ25-35) peptides to the dimyristoyl phosphatidylcholine (DMPC) bilayer and their subsequent aggregation. As a control, we used our previous study of unmodified peptides. We found that phosphorylation moderately reduces the helical propensity in pAβ25-35 and its binding affinity to the DMPC bilayer. Phosphorylation preserves the bimodal binding observed for unmodified Aβ25-35, which features a preferred inserted state and a less probable surface bound state. Phosphorylation also retains the inserted dimer with a head-to-tail helical aggregation interface as the most thermodynamically stable state. Importantly, this post-translation modification strengthens interpeptide interactions by adding a new aggregation "hot spot" created by cross-bridging phosphorylated Ser26 with water, cationic ions, or Lys28. The cross-bridging constitutes the molecular mechanism behind most structural phosphorylation effects. In addition, phosphorylation eliminates pAβ25-35 monomers and diversifies the pool of aggregated species. As a result, it changes the binding and aggregation mechanism by multiplying pathways leading to stable inserted dimers. These findings offer a plausible molecular rationale for experimental observations, including enhanced production of low molecular weight oligomers and cytotoxicity of phosphorylated Aβ peptides.

APP/Aβ structural diversity and Alzheimer's disease pathogenesis

The amyloid cascade hypothesis of Alzheimer's disease (AD) proposes amyloid- β (Aβ) is a chief pathological element of dementia. AD therapies have targeted monomeric and oligomeric Aβ 1-40 and 1-42 peptides. However, alternative APP proteolytic processing produces a complex roster of Aβ species. In addition, Aβ peptides are subject to extensive posttranslational modification (PTM). We propose that amplified production of some APP/Aβ species, perhaps exacerbated by differential gene expression and reduced peptide degradation, creates a diverse spectrum of modified species which disrupt brain homeostasis and accelerate AD neurodegeneration. We surveyed the literature to catalog Aβ PTM including species with isoAsp at positions 7 and 23 which may phenocopy the Tottori and Iowa Aβ mutations that result in early onset AD. We speculate that accumulation of these alterations induce changes in secondary and tertiary structure of Aβ that favor increased toxicity, and seeding and propagation in sporadic AD. Additionally, amyloid-β peptides with a pyroglutamate modification at position 3 and oxidation of Met35 make up a substantial portion of sporadic AD amyloid deposits. The intrinsic physical properties of these species, including resistance to degradation, an enhanced aggregation rate, increased neurotoxicity, and association with behavioral deficits, suggest their emergence is linked to dementia. The generation of specific 3D-molecular conformations of Aβ impart unique biophysical properties and a capacity to seed the prion-like global transmission of amyloid through the brain. The accumulation of rogue Aβ ultimately contributes to the destruction of vascular walls, neurons and glial cells culminating in dementia. A systematic examination of Aβ PTM and the analysis of the toxicity that they induced may help create essential biomarkers to more precisely stage AD pathology, design countermeasures and gauge the impacts of interventions.

Phosphorylation at Ser8 as an Intrinsic Regulatory Switch to Regulate the Morphologies and Structures of Alzheimer's 40-residue β-Amyloid (Aβ40) Fibrils

Polymorphism of amyloid-β (Aβ) fibrils, implying different fibril structures, may play important pathological roles in Alzheimer's disease (AD). Morphologies of Aβ fibrils were found to be sensitive to fibrillation conditions. Herein, the Ser⁸-phosphorylated Aβ (pAβ), which is assumed to specially associate with symptomatic AD, is reported to modify the morphology, biophysical properties, cellular toxicity, and structures of Aβ fibrils. Under the same fibrillation conditions, pAβ favors the formation of fibrils (F_pβ), which are different from the wild-type Aβ fibrils (F_β). Both F_β and F_pβ fibrils show single predominant morphologies. Compared with F_β, F_pβ exhibits higher propagation efficiency and higher neuronal cell toxicity. The residue-specific structural differences between the F_β- and F_pβ-seeded Aβ fibrils were identified using magic angle spin NMR. Our results suggest a potential regulatory mechanism of phosphorylation on Aβ fibril formation in AD and imply that the post-translationally modified Aβ, especially the phosphorylated Aβ, may be an important target for the diagnosis or treatment of AD at specific stages.

Association of status redox with demographic, clinical and imaging parameters in patients with Huntington's disease

Huntington's disease (HD) is an autosomal dominant, progressive neurodegenerative disorder, caused by an expanded trinucleotide CAG sequence of the huntingtin (Htt) gene, which encodes a stretch of glutamines in the Htt protein. The mechanisms of neurodegeneration associated with the accumulation of Htt aggregates still remains unclear.

OBJECTIVES

To determine oxidative stress biomarkers in HD patients and their relationship with clinical, demographic and neuroimaging parameters.

DESIGN AND METHODS

Fourteen patients and 39 controls paired by age and sex participated in this study. Oxidative damage was assayed in blood by measuring malondialdehyde (MDA) and advanced oxidative protein products (AOPPs). Antioxidant status was determined by activities of superoxide dismutase (SOD), catalase (CAT), glutathione peroxidase (GPx), glutathione reductase (GR), reduced glutathione (GSH), protein thiols and total antioxidant capacity (FRAP). The Unified Huntington Disease Rating Scale (UHDRS) and neuroimaging studies were also employed.

RESULTS

MDA, AOPP and GPx were significantly increased in HD patients with respect to the control group, while GR activity was decreased. FRAP correlated with age of disease onset, AOPP with motor severity (UHDRS score), age of patients and age of disease onset. Caudate atrophy was associated with lower plasma concentrations of GSH.

CONCLUSIONS

These findings point to a redox imbalance in HD patients. GR activity could be a potential biomarker for symptom onset in asymptomatic gene carriers, while plasmatic GSH could be useful in monitoring the progression of neurodegeneration - as an expression of caudate atrophy - during the course of the disease.

Phosphorylation as conformational switch from the native to amyloid state: Trp-cage as a protein aggregation model

The 20 residue long Trp-cage miniprotein is an excellent model for both computational and experimental studies of protein folding and stability. Recently, great attention emerged to study disease-related protein misfolding, aggregation, and amyloid formation, with the aim of revealing their structural and thermodynamic background. Trp-cage is sensitive to both environmental and structure-modifying effects. It aggregates with ease upon structure destabilization, and thus it is suitable for modeling aggregation and amyloid formation. Here, we characterize the amyloid formation of several sequence modified and side-chain phosphorylated Trp-cage variants. We applied NMR, circular dichroism (CD) and Fourier transform infrared (FTIR) spectroscopies, molecular dynamics (MD) simulations, and transmission electron microscopy (TEM) in conjunction with thioflavin-T (ThT) fluorescence measurements to reveal the structural consequences of side-chain phosphorylation. We demonstrate that the native fold is destabilized upon serine phosphorylation, and the resultant highly dynamic structures form amyloid-like ordered aggregates with high intermolecular β-structure content. The only exception is the D9S(P) variant, which follows an alternative aggregation process by forming thin fibrils, presenting a CD spectrum of PPII helix, and showing low ThT binding capability. We propose a complex aggregation model for these Trp-cage miniproteins. This model assumes an additional aggregated state, a collagen triple helical form that can precede amyloid formation. The phosphorylation of a single serine residue serves as a conformational switch, triggering aggregation, otherwise mediated by kinases in cell. We show that Trp-cage miniprotein is indeed a realistic model of larger globular systems of composite folding and aggregation landscapes and helps us to understand the fundamentals of deleterious protein aggregation and amyloid formation.

Immunolocalization of Kisspeptin Associated with Amyloid-β Deposits in the Pons of an Alzheimer's Disease Patient

The pons region of the Alzheimer's disease (AD) brain is one of the last to show amyloid-β (Aβ) deposits and has been suggested to contain neuroprotective compounds. Kisspeptin (KP) is a hormone that activates the hypothalamic-pituitary-gonadal axis and has been suggested to be neuroprotective against Aβ toxicity. The localization of KP, plus the established endogenous neuroprotective compounds corticotropin releasing hormone (CRH) and catalase, in tissue sections from the pons region of a male AD subject has been determined in relation to Aβ deposits. Results showed Aβ deposits also stained with KP, CRH, and catalase antibodies. At high magnification the staining of deposits was either KP or catalase positive, and there was only a limited area of the deposits with KP-catalase colocalization. The CRH does not bind Aβ, whilst both KP and catalase can bind Aβ, suggesting that colocalization in Aβ deposits is not restricted to compounds that directly bind Aβ. The neuroprotective actions of KP, CRH, and catalase were confirmed in vitro, and fibrillar Aβ preparations were shown to stimulate the release of KP in vitro. In conclusion, neuroprotective KP, CRH, and catalase all colocalize with Aβ plaque-like deposits in the pons region from a male AD subject.

Early intraneuronal accumulation and increased aggregation of phosphorylated Abeta in a mouse model of Alzheimer's disease

The progressive accumulation of extracellular amyloid plaques in the brain is a common hallmark of Alzheimer's disease (AD). We recently identified a novel species of Aβ phosphorylated at serine residue 8 with increased propensity to form toxic aggregates as compared to non-phosphorylated species. The age-dependent analysis of Aβ depositions using novel monoclonal phosphorylation-state specific antibodies revealed that phosphorylated Aβ variants accumulate first inside of neurons in a mouse model of AD already at 2 month of age. At higher ages, phosphorylated Aβ is also abundantly detected in extracellular plaques. Besides a large overlap in the spatiotemporal deposition of phosphorylated and non-phosphorylated Aβ species, fractionized extraction of Aβ from brains revealed an increased accumulation of phosphorylated Aβ in oligomeric assemblies as compared to non-phosphorylated Aβ in vivo. Thus, phosphorylated Aβ could represent an important species in the formation and stabilization of neurotoxic aggregates, and might be targeted for AD therapy and diagnosis.

Kisspeptin prevention of amyloid-β peptide neurotoxicity in vitro

Alzheimer's disease (AD) onset is associated with changes in hypothalamic-pituitary-gonadal (HPG) function. The 54 amino acid kisspeptin (KP) peptide regulates the HPG axis and alters antioxidant enzyme expression. The Alzheimer's amyloid-β (Aβ) is neurotoxic, and this action can be prevented by the antioxidant enzyme catalase. Here, we examined the effects of KP peptides on the neurotoxicity of Aβ, prion protein (PrP), and amylin (IAPP) peptides. The Aβ, PrP, and IAPP peptides stimulated the release of KP and KP 45-54. The KP peptides inhibited the neurotoxicity of Aβ, PrP, and IAPP peptides, via an action that could not be blocked by kisspeptin-receptor (GPR-54) or neuropeptide FF (NPFF) receptor antagonists. Knockdown of KiSS-1 gene, which encodes the KP peptides, in human neuronal SH-SY5Y cells with siRNA enhanced the toxicity of amyloid peptides, while KiSS-1 overexpression was neuroprotective. A comparison of the catalase and KP sequences identified a similarity between KP residues 42-51 and the region of catalase that binds Aβ. The KP peptides containing residues 45-50 bound Aβ, PrP, and IAPP, inhibited Congo red binding, and were neuroprotective. These results suggest that KP peptides are neuroprotective against Aβ, IAPP, and PrP peptides via a receptor independent action involving direct binding to the amyloid peptides.

Phosphorylation of amyloid-β peptide at serine 8 attenuates its clearance via insulin-degrading and angiotensin-converting enzymes

Accumulation of amyloid-β peptides (Aβ) in the brain is a common pathological feature of Alzheimer disease (AD). Aggregates of Aβ are neurotoxic and appear to be critically involved in the neurodegeneration during AD pathogenesis. Accumulation of Aβ could be caused by increased production, as indicated by several mutations in the amyloid precursor protein or the γ-secretase components presenilin-1 and presenilin-2 that cause familial early-onset AD. However, recent data also indicate a decreased clearance rate of Aβ in AD brains. We recently demonstrated that Aβ undergoes phosphorylation by extracellular or cell surface-localized protein kinase A, leading to increased aggregation. Here, we provide evidence that phosphorylation of monomeric Aβ at Ser-8 also decreases its clearance by microglial cells. By using mass spectrometry, we demonstrate that phosphorylation at Ser-8 inhibited the proteolytic degradation of monomeric Aβ by the insulin-degrading enzyme, a major Aβ-degrading enzyme released from microglial cells. Phosphorylation also decreased the degradation of Aβ by the angiotensin-converting enzyme. In contrast, Aβ degradation by plasmin was largely unaffected by phosphorylation. Thus, phosphorylation of Aβ could play a dual role in Aβ metabolism. It decreases its proteolytic clearance and also promotes its aggregation. The inhibition of extracellular Aβ phosphorylation, stimulation of protease expression and/or their proteolytic activity could be explored to promote Aβ degradation in AD therapy or prevention.

Phosphorylation of amyloid beta (Aβ) peptides - a trigger for formation of toxic aggregates in Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia and associated with the progressive accumulation of amyloid β-peptides (Aβ) in form of extracellular amyloid plaques in the human brain. A critical role of Aβ in the pathogenesis of AD is strongly supported by gene mutations that cause early-onset familial forms of the disease. Such mutations have been identified in the APP gene itself and in presenilin 1 and 2. Importantly, all the identified mutations commonly lead to early deposition of extracellular plaques likely by increasing the generation and/or aggregation of Aβ. However, such mutations are very rare and molecular mechanisms that might trigger aggregation and deposition of Aβ, in the most common late onset AD are largely unknown. We recently demonstrated that extracellular Aβ undergoes phosphorylation by a cell surface-localized or secreted form of protein kinase A. The phosphorylation of serine residue 8 promotes aggregation by stabilization of β-sheet conformation of Aβ and increased formation of oligomeric Aβ aggregates that represent nuclei for fibrillization. Phosphorylated Aβ was detected in the brains of transgenic mice and human AD brains and showed increased toxicity in Drosophila models as compared with non-phosphorylated Aβ. Together, these findings demonstrate a novel molecular mechanism that triggers aggregation and toxicity of Aβ. Thus, phosphorylation of Aβ could be relevant in the pathogenesis of late onset AD. The identification of extracellular protein kinase A should also stimulate pharmacological approaches to decrease Aβ phosphorylation in the therapy and/or prevention of AD.

Synaptic transmission block by presynaptic injection of oligomeric amyloid beta

Early Alzheimer's disease (AD) pathophysiology is characterized by synaptic changes induced by degradation products of amyloid precursor protein (APP). The exact mechanisms of such modulation are unknown. Here, we report that nanomolar concentrations of intraaxonal oligomeric (o)Abeta42, but not oAbeta40 or extracellular oAbeta42, acutely inhibited synaptic transmission at the squid giant synapse. Further characterization of this phenotype demonstrated that presynaptic calcium currents were unaffected. However, electron microscopy experiments revealed diminished docked synaptic vesicles in oAbeta42-microinjected terminals, without affecting clathrin-coated vesicles. The molecular events of this modulation involved casein kinase 2 and the synaptic vesicle rapid endocytosis pathway. These findings open the possibility of a new therapeutic target aimed at ameliorating synaptic dysfunction in AD.

The CDC2 I-G-T haplotype associated with the APOE ɛ4 allele increases the risk of sporadic Alzheimer's disease in Sicily

The cell division cycle 2 (CDC2) gene is a candidate susceptibility gene for Alzheimer's disease (AD). We investigated the CDC2 genotype, and allele and haplotype frequencies in AD patients and matched controls, distinguishing between apolipoprotein E (APOE) epsilon4 allele carriers and non-carriers. APOE epsilon4 is an established predictor of AD risk. APOE and CDC2 genotypes were examined in 109 sporadic AD patients and in 110 healthy age- and sex-matched controls from Sicily. The epsilon4 allele of APOE was predictive of AD risk in our study group (odds ratio: 5.37, 95% CI 2.77-10.41; P<0.0001). Genotype and allele frequencies of the three tested CDC2 polymorphisms (Ex6+7I/D, Ex7-15 G>A, Ex7-14 T>A) were not significantly different between AD patients and controls. However, a significant different distribution of a specific CDC2 haplotype (I-G-T) was found between AD patients and controls when analyzing APOE epsilon4-positive subjects (P=0.0288). Moreover, the combined presence of the I-G-T haplotype and the epsilon4 allele almost doubled the risk of AD (odds ratio: 10.09, 95% CI 3.88-26.25; P<0.0001) compared to carriers of epsilon4 alone. This study suggests that the I-G-T haplotype of the CDC2 gene increases the risk of AD in APOE epsilon4 carriers.

Cdh1-APC/C, cyclin B-Cdc2, and Alzheimer’s disease pathology

The anaphase-promoting complex/cyclosome (APC/C) is a key E3 ubiquitin ligase complex that functions in regulating cell cycle transitions in proliferating cells and has, as revealed recently, novel roles in postmitotic neurons. Regulated by its activator Cdh1 (or Hct1), whose level is high in postmitotic neurons, APC/C seems to have multiple functions at different cellular locations, modulating diverse processes such as synaptic development and axonal growth. These processes do not, however, appear to be directly connected to cell cycle regulation. It is now shown that Cdh1-APC/C activity may also have a basic role in suppressing cyclin B levels, thus preventing terminally differentiated neurons from aberrantly re-entering the cell cycle. The result of an aberrant cyclin B-induced S-phase entry, at least for some of these neurons, would be death via apoptosis. Cdh1 thus play an active role in maintaining the terminally differentiated, non-cycling state of postmitotic neurons--a function that could become impaired in Alzheimer's and other neurodegenerative diseases.