Always Around, Never the Same: Pathways of Amyloid Beta Induced Neurodegeneration Throughout the Pathogenic Cascade of Alzheimers Disease

Mapping pathogenic processes contributing to neurodegeneration in Drosophila models of Alzheimer's disease

Alzheimer's disease (AD) is the most common form of dementia, affecting millions of people and currently lacking available disease‐modifying treatments. Appropriate disease models are necessary to investigate disease mechanisms and potential treatments. Drosophila melanogaster models of AD include the Aβ fly model and the AβPP‐BACE1 fly model. In the Aβ fly model, the Aβ peptide is fused to a secretion sequence and directly overexpressed. In the AβPP‐BACE1 model, human AβPP and human BACE1 are expressed in the fly, resulting in in vivo production of Aβ peptides and other AβPP cleavage products. Although these two models have been used for almost two decades, the underlying mechanisms resulting in neurodegeneration are not yet clearly understood. In this study, we have characterized toxic mechanisms in these two AD fly models. We detected neuronal cell death and increased protein carbonylation (indicative of oxidative stress) in both AD fly models. In the Aβ fly model, this correlates with high Aβ_1–42 levels and down‐regulation of the levels of mRNA encoding lysosomal‐associated membrane protein 1, lamp1 (a lysosomal marker), while in the AβPP‐BACE1 fly model, neuronal cell death correlates with low Aβ_1–42 levels, up‐regulation of lamp1 mRNA levels and increased levels of C‐terminal fragments. In addition, a significant amount of AβPP/Aβ antibody (4G8)‐positive species, located close to the endosomal marker rab5, was detected in the AβPP‐BACE1 model. Taken together, this study highlights the similarities and differences in the toxic mechanisms which result in neuronal death in two different AD fly models. Such information is important to consider when utilizing these models to study AD pathogenesis or screening for potential treatments.

Physiological Aβ Concentrations Produce a More Biomimetic Representation of the Alzheimer's Disease Phenotype in iPSC Derived Human Neurons

Alzheimer's disease (AD) is characterized by slow, progressive neurodegeneration leading to severe neurological impairment, but current drug development efforts are limited by the lack of robust, human-based disease models. Amyloid-β (Aβ) is known to play an integral role in AD progression as it has been shown to interfere with neurological function. However, studies into AD pathology commonly apply Aβ to neurons for short durations at nonphysiological concentrations to induce an exaggerated dysfunctional phenotype. Such methods are unlikely to elucidate early stage disease dysfunction, when treatment is still possible, since damage to neurons by these high concentrations is extensive. In this study, we investigated chronic, pathologically relevant Aβ oligomer concentrations to induce an electrophysiological phenotype that is more representative of early AD progression compared to an acute high-dose application in human cortical neurons. The high, acute oligomer dose resulted in severe neuronal toxicity as well as upregulation of tau and phosphorylated tau. Chronic, low-dose treatment produced significant functional impairment without increased cell death or accumulation of tau protein. This in vitro phenotype more closely mirrors the status of early stage neural decline in AD pathology and could provide a valuable tool to further understanding of early stage AD pathophysiology and for screening potential therapeutic compounds.

Stress-Induced Synaptic Dysfunction and Neurotransmitter Release in Alzheimer's Disease: Can Neurotransmitters and Neuromodulators be Potential Therapeutic Targets?

The communication between neurons at synaptic junctions is an intriguing process that monitors the transmission of various electro-chemical signals in the central nervous system. Albeit any aberration in the mechanisms associated with transmission of these signals leads to loss of synaptic contacts in both the neocortex and hippocampus thereby causing insidious cognitive decline and memory dysfunction. Compelling evidence suggests that soluble amyloid-β (Aβ) and hyperphosphorylated tau serve as toxins in the dysfunction of synaptic plasticity and aberrant neurotransmitter (NT) release at synapses consequently causing a cognitive decline in Alzheimer's disease (AD). Further, an imbalance between excitatory and inhibitory neurotransmission systems induced by impaired redox signaling and altered mitochondrial integrity is also amenable for such abnormalities. Defective NT release at the synaptic junction causes several detrimental effects associated with altered activity of synaptic proteins, transcription factors, Ca2+ homeostasis, and other molecules critical for neuronal plasticity. These detrimental effects further disrupt the normal homeostasis of neuronal cells and thereby causing synaptic loss. Moreover, the precise mechanistic role played by impaired NTs and neuromodulators (NMs) and altered redox signaling in synaptic dysfunction remains mysterious, and their possible interlink still needs to be investigated. Therefore, this review elucidates the intricate role played by both defective NTs/NMs and altered redox signaling in synaptopathy. Further, the involvement of numerous pharmacological approaches to compensate neurotransmission imbalance has also been discussed, which may be considered as a potential therapeutic approach in synaptopathy associated with AD.

Cholesterol overload induces apoptosis in SH-SY5Y human neuroblastoma cells through the up regulation of flotillin-2 in the lipid raft and the activation of BDNF/Trkb signaling

Epidemiological investigations have shown that Alzheimer's disease (AD) is one of the most common neurodegenerative diseases. It has been indicated that the cholesterol concentration in the brain of AD patients is higher than that in normal people. In this study, we investigated the effects of cholesterol concentrations, 0, as the control, 3.125, 12.5, and 25μM, on cholesterol metabolism, neuron survival, AD-related protein expressions, and cell morphology and apoptosis using SH-SY5Y human neuroblastoma cells. We observed that expressions of cholesterol hydroxylase (Cyp46), flotillin-2 (a marker of lipid raft content), and truncated tyrosine kinase B (TrkBtc) increased, while expressions of brain-derived neurotrophic factor (BDNF) and full-length TrkB (TrkBfl) decreased as the concentration of cholesterol loading increased. Down-regulation of the PI3K-Akt-glycogen synthase kinase (GSK)-3β cascade and cell apoptosis were also observed at higher concentrations of cholesterol, along with elevated levels of β-amyloid (Aβ), β-secretase (BACE), and reactive oxygen species (ROS). In conclusion, we found that cholesterol overload in neuronal cells imbalanced the cholesterol homeostasis and increased the protein expressions causing cell apoptosis, which illustrates the neurodegenerative pathology of abnormally elevated cholesterol concentrations found in AD patients.

Alzheimer's disease vaccine development: A new strategy focusing on immune modulation

Despite significant advances in the development of Alzheimer's disease (AD) vaccines effective in animal models, these prototypes have been clinically unsuccessful; apparently the result of using immunogens modified to prevent inflammation. Hence, a new paradigm is needed that uses entire AD-associated immunogens, a notion supported by recent successful passive immunotherapy results, with adjuvants that induce Th2-only while inhibiting without abrogating Th1 immunity. Here, we discuss the obstacles to AD vaccine development and Th2-adjuvants that by acting on dendritic and T cells, would elicit regardless of the antigen a safe and effective antibody response, while preventing damaging neuroinflammation and ameliorating immunosenescence.

Attenuation of the early events of α-synuclein aggregation: a fluorescence correlation spectroscopy and laser scanning microscopy study in the presence of surface-coated Fe3O4 nanoparticles

The aggregation of α-synuclein (A-syn) has been implicated in the pathogenesis of Parkinson's disease (PD). Although the early events of aggregation and not the matured amyloid fibrils are believed to be responsible for the toxicity, it has been difficult to probe the formation of early oligomers experimentally. We studied the effect of Fe3O4 nanoparticle (NP) in the early stage of aggregation of A-syn using fluorescence correlation spectroscopy (FCS) and laser scanning microscopy. The binding between the monomeric protein and NPs was also studied using FCS at single-molecule resolution. Our data showed that the addition of bare Fe3O4 NPs accelerated the rate of early aggregation, and it did not bind the monomeric A-syn. In contrast, L-lysine (Lys)-coated Fe3O4 NPs showed strong binding with the monomeric A-syn, inhibiting the early events of aggregation. Lys-coated Fe3O4 NPs showed significantly less cell toxicity compared with bare Fe3O4 NPs and can be explored as a possible strategy to develop therapeutic application against PD. To the best of our knowledge, this report is the first example of using a small molecule to attenuate the early (and arguably the most relevant in terms of PD pathogenesis) events of A-syn aggregation.

Regulation of a TrkB Alternative Transcript by microRNAs

Background/Aims

Tropomyosin-related kinase B receptor (TrkB)-mediated signaling is vital for neuronal differentiation, survival, plasticity, and cognition. In this study, the focus was placed on TrkB-Shc, a neuron-specific transcript, to determine if microRNAs (miRNAs) play a role in TrkB-Shc regulation.

Methods

A combination of bioinformatics and molecular gene expression analysis techniques was used to assess the effect of miR-409-3p and miR-216b on TrkB-Shc expression.

Results

miR-409-3p and miR-216b were found to regulate the TrkB-Shc 3′UTR through the identified putative binding sites. When the effect of the miRNAs on TrkB was assessed using SHSY5Y neuronal cells, differential effects were observed between mRNA and protein expression.

Conclusion

This study highlights the importance of miRNA-mediated regulation in TrkB signaling.

Trafficking regulation of proteins in Alzheimer's disease

The β-amyloid (Aβ) peptide has been postulated to be a key determinant in the pathogenesis of Alzheimer’s disease (AD). Aβ is produced through sequential cleavage of the β-amyloid precursor protein (APP) by β- and γ-secretases. APP and relevant secretases are transmembrane proteins and traffic through the secretory pathway in a highly regulated fashion. Perturbation of their intracellular trafficking may affect dynamic interactions among these proteins, thus altering Aβ generation and accelerating disease pathogenesis. Herein, we review recent progress elucidating the regulation of intracellular trafficking of these essential protein components in AD.

Disruption of neurogenesis by hypothalamic inflammation in obesity or aging

Adult neural stem cells contribute to neurogenesis and plasticity of the brain which is essential for central regulation of systemic homeostasis. Damage to these homeostatic components, depending on locations in the brain, poses threat to impaired neurogenesis, neurodegeneration, cognitive loss and energy imbalance. Recent research has identified brain metabolic inflammation via proinflammatory IκB kinase-β (IKKβ) and its downstream nuclear transcription factor NF-κB pathway as a non-classical linker of metabolic and neurodegenerative disorders. Chronic activation of the pathway results in impairment of energy balance and nutrient metabolism, impediment of neurogenesis, neural stem cell proliferation and differentiation, collectively converging on metabolic and cognitive decline. Hypothalamic IKKβ/NF-κB via inflammatory crosstalk between microglia and neurons has been discovered to direct systemic aging by inhibiting the production of gonadotropin-releasing hormone (GnRH) and inhibition of inflammation or GnRH therapy could revert aging related degenerative symptoms at least in part. This article reviews the crucial role of hypothalamic inflammation in affecting neural stem cells which mediates the neurodegenerative mechanisms of causing metabolic derangements as well as aging-associated disorders or diseases.

Cytosolic phospholipase A2α upregulation mediates apoptotic neuronal death induced by aggregated amyloid-β peptide1-42

Increased cytosolic phospholipase A2α (cPLA2α) immunoreactivity and transcript were observed in Alzheimer's disease (AD) brain associated with amyloid deposits. Thus, the present study examined whether cPLA2α upregulation participate in cortical neuron damage induced by aggregated Aβ1-42 and determined its role in the signaling events leading to damage, using an antisense technology. Exposure of primary cortical neurons to 1μM aggregated Aβ1-42 for 24h induced up-regulation and activation of cPLA2α and apoptotic cell death of about 30% as detected by: cell count, MTT reduction, caspases-3 and -8 activation, DAPI and TUNEL staining, that were prevented by inhibition of cPLA2α up-regulation and activity in the presence of antisense against cPLA2α (AS). cPLA2α was rapidly activated upon addition of aggregated Aβ1-42, as determined by its phosphorylated form on serine 505, and this activity was dependent on NADPH oxidase activity. NOX2- and NOX4-NADPH oxidase upregulation at 24h of aggregated Aβ1-42 exposure was not affected by the presence of AS, but superoxide production was reduced, probably due to NOX2 inhibition. cPLA2α upregulation led to activation of neutral sphingomyelinase (N-SMase) as its activity was inhibited in the presence of AS, and could be restored by addition of arachidonic acid. Addition of ceramide analog induced caspase-8 activation leading to caspase-3 activation and apoptotic neuronal death. In conclusion, our results suggest that cPLA2α activity plays a crucial role in the signaling cascade leading to apoptotic neuronal death by aggregated Aβ1-42 probably via activation of N-SMase, ceramide production and caspases-3 and -8.

Intracellular accumulation of aggregated pyroglutamate amyloid beta: convergence of aging and Aβ pathology at the lysosome

Deposition of aggregated amyloid beta (Aβ) is a major hallmark of Alzheimer's disease (AD)-a common age-related neurodegenerative disorder. Typically, Aβ is generated as a peptide of varying lengths. However, a major fraction of Aβ peptides in the brains of AD patients has undergone posttranslational modifications, which often radically change the properties of the peptides. Aβ3(pE)-42 is an N-truncated, pyroglutamate-modified variant that is abundantly present in AD brain and was suggested to play a role early in the pathogenesis. Here we show that intracellular accumulation of oligomeric aggregates of Aβ3(pE)-42 results in loss of lysosomal integrity. Using a novel antibody specific for aggregates of AβpE3, we show that in postmortem human brain tissue, aggregated AβpE3 is predominantly found in the lysosomes of both neurons and glial cells. Our data further demonstrate that AβpE3 is relatively resistant to lysosomal degradation, which may explain its accumulation in the lysosomes. The intracellular AβpE3 aggregates increase in an age-dependent manner. The results presented in this study support a model where Aβ pathology and aging converge, leading to accumulation of the degradation-resistant pE-modified Aβ in the lysosomes, lysosomal dysfunction, and neurodegeneration.

Resolution of oligomeric species during the aggregation of Aβ1-40 using (19)F NMR

In the commonly used nucleation-dependent model of protein aggregation, aggregation proceeds only after a lag phase in which the concentration of energetically unfavorable nuclei reaches a critical value. The formation of oligomeric species prior to aggregation can be difficult to detect by current spectroscopic techniques. By using real-time (19)F NMR along with other techniques, we are able to show that multiple oligomeric species can be detected during the lag phase of Aβ1-40 fiber formation, consistent with a complex mechanism of aggregation. At least six types of oligomers can be detected by (19)F NMR. These include the reversible formation of large β-sheet oligomer immediately after solubilization at high peptide concentration, a small oligomer that forms transiently during the early stages of the lag phase, and four spectroscopically distinct forms of oligomers with molecular weights between ∼30 and 100 kDa that appear during the later stages of aggregation. The ability to resolve individual oligomers and track their formation in real-time should prove fruitful in understanding the aggregation of amyloidogenic proteins and in isolating potentially toxic nonamyloid oligomers.

Time-Dependent Compensatory Responses to Chronic Neuroinflammation in Hippocampus and Brainstem: The Potential Role of Glutamate Neurotransmission

Chronic neuroinflammation is characteristic of neurodegenerative diseases and is present during very early stages, yet significant pathology and behavioral deficits do not manifest until advanced age. We investigated the consequences of experimentally-induced chronic neuroinflammation within the hippocampus and brainstem of young (4 mo) F-344 rats. Lipopolysaccharide (LPS) was infused continuously into the IV^th ventricle for 2, 4 or 8 weeks. The number of MHC II immunoreactive microglia in the brain continued to increase throughout the infusion period. In contrast, performance in the Morris water maze was impaired after 4 weeks but recovered by 8 weeks. Likewise, a transient loss of tyrosine hydroxylase immunoreactivity in the substantia nigra and locus coeruleus was observed after 2 weeks, but returned to control levels by 4 weeks of continuous LPS infusion. These data suggest that direct activation of microglia is sufficient to drive, but not sustain, spatial memory impairment and a decrease in tyrosine hydroxylase production in young rats. Our previous studies suggest that chronic neuroinflammation elevates extracellular glutamate and that this elevation underlies the spatial memory impairment. In the current study, increased levels of GLT1 and SNAP25 in the hippocampus corresponded with the resolution of performance deficit. Increased expression of SNAP25 is consistent with reduced glutamate release from axonal terminals while increased GLT1 is consistent with enhanced clearance of extracellular glutamate. These data demonstrate the capacity of the brain to compensate for the presence of chronic neuroinflammation, despite continued activation of microglia, through changes in the regulation of the glutamatergic system.

Role of prostaglandins in neuroinflammatory and neurodegenerative diseases

Increasing data demonstrates that inflammation participates in the pathophysiology of neurodegenerative diseases. Among the different inflammatory mediators involved, prostaglandins play an important role. The effects induced by prostaglandins might be mediated by activation of their known receptors or by nonclassical mechanisms. In the present paper, we discuss the evidences that link prostaglandins, as well as the enzymes that produce them, to some neurological diseases.

Inhibition of Alzheimer's amyloid toxicity with a tricyclic pyrone molecule in vitro and in vivo

Small beta-amyloid (Abeta) 1-42 aggregates are toxic to neurons and may be the primary toxic species in Alzheimer's disease (AD). Methods to reduce the level of Abeta, prevent Abeta aggregation, and eliminate existing Abeta aggregates have been proposed for treatment of AD. A tricyclic pyrone named CP2 is found to prevent cell death associated with Abeta oligomers. We studied the possible mechanisms of neuroprotection by CP2. Surface plasmon resonance spectroscopy shows a direct binding of CP2 with Abeta42 oligomer. Circular dichroism spectroscopy reveals monomeric Abeta42 peptide remains as a random coil/alpha-helix structure in the presence of CP2 over 48 h. Atomic force microscopy studies show CP2 exhibits similar ability to inhibit Abeta42 aggregation as that of Congo red and curcumin. Atomic force microscopy closed-fluid cell study demonstrates that CP2 disaggregates Abeta42 oligomers and protofibrils. CP2 also blocks Abeta fibrillations using a protein quantification method. Treatment of 5x familial Alzheimer's disease mice, a robust Abeta42-producing animal model of AD, with a 2-week course of CP2 resulted in 40% and 50% decreases in non-fibrillar and fibrillar Abeta species, respectively. Our results suggest that CP2 might be beneficial to AD patients by preventing Abeta aggregation and disaggregating existing Abeta oligomers and protofibrils.

Neuronal protein trafficking associated with Alzheimer disease: from APP and BACE1 to glutamate receptors

Aberrant and/or cumulative amyloid-beta (Abeta) production, resulting from proteolytic processing of the amyloid precursor protein (APP) by beta and gamma-secretases, have been postulated to be a main etiological basis of Alzheimer disease (AD). A number of proteins influence the subcellular trafficking itinerary of APP and the beta-site APP-cleaving enzyme (BACE1) between the cell surface, endosomes and the trans-Golgi network (TGN). Available evidence suggests that co-residence of APP and BACE1 in the endosomal compartments promotes amyloidogenesis. Retrograde transport of APP out of the endosome to the TGN reduces Abeta production, while APP routed to and kept at the cell surface enhances its non-amyloidogenic, alpha-secretase-mediated processing. Changes in post-Golgi membrane trafficking in aging neurons that may influence APP processing is particularly relevant to late-onset, idiopathic AD. Dystrophic axons are key features of AD pathology, and impaired axonal transport could play crucial roles in the pathogenesis of idiopathic AD. Recent evidence has also indicated that Abeta-induced synaptic defects and memory impairment could be explained by a loss of both AMPA and NMDA receptors through endocytosis. Detail understanding of factors that influence these neuronal trafficking processes will open up novel therapeutic avenues for preventing or delaying the onset of symptomatic AD.

Abeta 1-42 induces mild endoplasmic reticulum stress in an aggregation state-dependent manner

Alzheimer's disease (AD) is characterized by the aggregation of misfolded proteins. Previously we reported activation of the unfolded protein response (UPR) in AD neurons. A potential source for UPR activation in AD neurons may be the increased levels of beta-amyloid (Abeta). In this study, we used preparations enriched in oligomeric or fibrillar Abeta (1-42) to investigate the role of the conformational state of Abeta in UPR activation in differentiated neuroblastoma cells. Both oligomeric and fibrillar Abeta (1-42) do not induce BiP expression to the extent that it can be detected in a pool of cells. However, using a fluorescent UPR reporter cell line that allows analysis of individual cells, we demonstrated mild activation of the UPR by oligomeric but not fibrillar Abeta (1-42). We showed that oligomeric Abeta (1-42) is significantly more toxic to cells primed for UPR than is fibrillar Abeta (1-42), indicating that activation of the UPR contributes to oligomer-specific Abeta (1-42) toxicity. Because UPR activation is observed in AD brain at a stage that precedes the massive fibrillar Abeta deposition and tangle formation, this may indicate a role for nonfibrillar Abeta in the induction of the UPR in AD neurons.

Branched KLVFF Tetramers Strongly Potentiate Inhibition of β-Amyloid Aggregation

The key pathogenic event in the onset of Alzheimer's disease (AD) is the aggregation of beta-amyloid (Abeta) peptides into toxic aggregates. Molecules that interfere with this process might act as therapeutic agents for the treatment of AD. The amino acid residues 16-20 (KLVFF) are known to be essential for the aggregation of Abeta. In this study, we have used a first-generation dendrimer as a scaffold for the multivalent display of the KLVFF peptide. The effect of four KLVFF peptides attached to the dendrimer (K(4)) on Abeta aggregation was compared to the effect of monomeric KLVFF (K(1)). Our data show that K(4) very effectively inhibits the aggregation of low-molecular-weight and protofibrillar Abeta(1-42) into fibrils, in a concentration-dependent manner, and much more potently than K(1). Moreover, we show that K(4) can lead to the disassembly of existing aggregates. Our data lead us to propose that conjugates that bear multiple copies of KLVFF might be useful as therapeutic agents for the treatment of Alzheimer's disease.

The significance of neuroinflammation in understanding Alzheimer's disease

The interest of scientists in the involvement of inflammation-related mechanisms in the pathogenesis of Alzheimer's disease (AD) goes back to the work of one of the pioneers of the study of this disease. About hundred years ago Oskar Fischer stated that the crucial step in the plaque formation is the extracellular deposition of a foreign substance that provokes an inflammatory reaction followed by a regenerative response of the surrounding nerve fibers. Eighty years later immunohistochemical studies revealed that amyloid plaques are indeed co-localized with a broad variety of inflammation-related proteins (complement factors, acute-phase proteins, pro-inflammatory cytokines) and clusters of activated microglia. These findings have led to the view that the amyloid plaque is the nidus of a non-immune mediated chronic inflammatory response locally induced by fibrillar A beta deposits. Recent neuropathological studies show a close relationship between fibrillar A beta deposits, inflammation and neuroregeneration in relatively early stages of AD pathology preceding late AD stages characterized by extensive tau-related neurofibrillary changes. In the present work we will review the role of inflammation in the early stage of AD pathology and particularly the role of inflammation in A beta metabolism and deposition. We also discuss the possibilities of inflammation-based therapeutic strategies in AD.