Aβ oligomers inhibit synapse remodelling necessary for memory consolidation

Novel Anti-Neuroinflammatory Properties of a Thiosemicarbazone–Pyridylhydrazone Copper(II) Complex

Neuroinflammation has a major role in several brain disorders including Alzheimer’s disease (AD), yet at present there are no effective anti-neuroinflammatory therapeutics available. Copper(II) complexes of bis(thiosemicarbazones) (Cu^II(gtsm) and Cu^II(atsm)) have broad therapeutic actions in preclinical models of neurodegeneration, with Cu^II(atsm) demonstrating beneficial outcomes on neuroinflammatory markers in vitro and in vivo. These findings suggest that copper(II) complexes could be harnessed as a new approach to modulate immune function in neurodegenerative diseases. In this study, we examined the anti-neuroinflammatory action of several low-molecular-weight, charge-neutral and lipophilic copper(II) complexes. Our analysis revealed that one compound, a thiosemicarbazone–pyridylhydrazone copper(II) complex (CuL⁵), delivered copper into cells in vitro and increased the concentration of copper in the brain in vivo. In a primary murine microglia culture, CuL⁵ was shown to decrease secretion of pro-inflammatory cytokine macrophage chemoattractant protein 1 (MCP-1) and expression of tumor necrosis factor alpha (Tnf), increase expression of metallothionein (Mt1), and modulate expression of Alzheimer’s disease-associated risk genes, Trem2 and Cd33. CuL⁵ also improved the phagocytic function of microglia in vitro. In 5xFAD model AD mice, treatment with CuL⁵ led to an improved performance in a spatial working memory test, while, interestingly, increased accumulation of amyloid plaques in treated mice. These findings demonstrate that CuL⁵ can induce anti-neuroinflammatory effects in vitro and provide selective benefit in vivo. The outcomes provide further support for the development of copper-based compounds to modulate neuroinflammation in brain diseases.

AT2R's (Angiotensin II Type 2 Receptor's) Role in Cognitive and Cerebrovascular Deficits in a Mouse Model of Alzheimer Disease

Antihypertensive medications targeting the renin-angiotensin system have lowered the incidence and progression of Alzheimer disease. Understanding how these medications function could lead to novel therapeutic strategies. AT4Rs (angiotensin IV receptors) have been associated with angiotensin receptor blockers' cognitive, cerebrovascular, and neuroinflammatory rescue in Alzheimer disease models. Yet, whether AT4Rs act alone or with AT2Rs remains unknown. Here, we investigated whether AT2Rs contribute to losartan's benefits and whether chronic AT2R activation could mimic angiotensin receptor blocker benefits in transgenic mice overexpressing familial Alzheimer disease mutations of the human APP (amyloid precursor protein). Losartan-treated mice (10 mg/kg per day, drinking water, 7 months) received intracerebroventricular (1 month) administration of vehicle or AT2R antagonist PD123319 (1.6 nmol/day). PD123319 countered losartan's benefits on spatial learning and memory, neurovascular coupling, and hampered those on oxidative stress and nitric oxide bioavailability. PD123319 did not oppose losartan's benefits on short-term memory and vasodilatory function and had no benefit on neuroinflammation or Aβ (amyloid β) pathology. Mice receiving either vehicle or selective AT2R agonist compound 21 (intracerebroventricular: 1 nmol/day, 1 month or drinking water: 10 mg/kg per day, 7 months), showed no improvement in memory, vasodilatory function, or nitric oxide bioavailability. Compound 21 treatment normalized neurovascular coupling, reduced astrogliosis independent of persisting microgliosis, and exacerbated oxidative stress in APP mice. Compound 21 reduced dense core Aβ plaques, but not diffuse plaques or Aβ species. Our findings suggest that targeting AT2Rs is not an ideal strategy for restoring Aβ-related cognitive and cerebrovascular deficits.

Nanomechanical insights: Amyloid beta oligomer-induced senescent brain endothelial cells

Senescent cells accumulate in various peripheral tissues during aging and have been shown to exacerbate age-related inflammatory responses. We recently showed that exposure to neurotoxic amyloid β (Aβ1-42) oligomers can readily induce a senescence phenotype in human brain microvascular endothelial cells (HBMECs). In the present work, we used atomic force microscopy (AFM) to further characterize the morphological properties such as cell membrane roughness and cell height and nanomechanical properties such as Young's modulus of the membrane (membrane stiffness) and adhesion resulting from the interaction between AFM tip and cell membrane in Aβ1-42 oligomer-induced senescent human brain microvascular endothelial cells. Morphological imaging studies showed a flatter and spread-out nucleus in the senescent HBMECs, both characteristic features of a senescent phenotype. Furthermore, the mean cell body roughness and mean cell height were lower in senescent HBMECs compared to untreated normal HBMECs. We also observed increased stiffness and alterations in the adhesion properties in Aβ1-42 oligomer-induced senescent endothelial cells compared to the untreated normal HBMECs suggesting dynamic reorganization of cell membrane. We then show that vascular endothelial growth factor receptor 1 (VEGFR-1) knockdown or overexpression of Rho GTPase Rac 1 in the endothelial cells inhibited senescence and reversed these nanomechanical alterations, confirming a direct role of these pathways in the senescent brain endothelial cells. These results illustrate that nanoindentation and topographic analysis of live senescent brain endothelial cells can provide insights into cerebrovascular dysfunction in neurodegenerative diseases such as Alzheimer's disease.

NMR-based site-resolved profiling of β-amyloid misfolding reveals structural transitions from pathologically relevant spherical oligomer to fibril

Increasing evidence highlights the central role of neurotoxic oligomers of the 42-residue-long β-amyloid (Aβ42) in Alzheimer's disease (AD). However, very limited information is available on the structural transition from oligomer to fibril, particularly for pathologically relevant amyloids. To the best of our knowledge, we present here the first site-specific structural characterization of Aβ42 misfolding, from toxic oligomeric assembly yielding a similar conformation to an AD-associated Aβ42 oligomer, into a fibril. Transmission EM (TEM) analysis revealed that a spherical amyloid assembly (SPA) of Aβ42 with a 15.6 ± 2.1-nm diameter forms in a ∼30-μm Aβ42 solution after a ∼10-h incubation at 4 °C, followed by a slow conversion into fibril at ∼180 h. Immunological analysis suggested that the SPA has a surface structure similar to that of amylospheroid (ASPD), a patient-derived toxic Aβ oligomer, which had a diameter of 10-15 nm in negative-stain TEM. Solid-state NMR analyses indicated that the SPA structure involves a β-loop-β motif, which significantly differed from the triple-β motif observed for the Aβ42 fibril. The comparison of the ¹³C chemical shifts of SPA with those of the fibril prepared in the above conditions and interstrand distance measurements suggested a large conformational change involving rearrangements of intermolecular β-sheet into in-register parallel β-sheet during the misfolding. A comparison of the SPA and ASPD ¹³C chemical shifts indicated that SPA is structurally similar to the ASPD relevant to AD. These observations provide insights into the architecture and key structural transitions of amyloid oligomers relevant for AD pathology.

Intraventricular murine Aβ infusion elicits hippocampal inflammation and disrupts the consolidation, but not retrieval, of conditioned fear in C57BL6/J mice

Although one of the defining characteristics of Alzheimer's disease is the presence of amyloid-beta (Aβ) plaques, the early accumulation of soluble Aβ oligomers (AβOs) may disrupt synaptic function and trigger cognitive impairments long before the appearance of plaques. Furthermore, murine models aimed at understanding how AβOs alter formation and retrieval of associative memories are conducted using human Aβ species, which are more neurotoxic in the mouse brain than the native murine species. Unfortunately, there is currently a lack of attention in the literature as to what the murine version of the peptide (mAβ) does to synaptic function and how it impacts the consolidation and retrieval of associative memories. In the current study, adult mice were infused with mAβ 0, 2, 6, or 46 h after contextual-fear conditioning, and were tested 2-48 h later. Interestingly, only mAβ infusions within 2 h of training reduced freezing behavior at test, indicating that mAβ disrupted the consolidation, but not retrieval of fear memory. This consolidation deficit coincided with increased IL-1β and reduced synaptophysin mRNA levels, without disrupting other synaptic signaling-related genes here examined. Despite differences between murine and human Aβ, the deleterious functional outcomes of early-stage synaptic oligomer presence are similar. Thus, models utilizing or inducing the production of mAβ in non-transgenic animals are useful in exploring the role of dysregulated synaptic plasticity and resultant learning deficits induced by Aβ oligomers.

Doxycycline for Alzheimer's Disease: Fighting β-Amyloid Oligomers and Neuroinflammation

Alzheimer's disease (AD) is the most widespread form of dementia, affecting about 45 million people worldwide. Although the β-amyloid peptide (Aβ) remains the most acknowledged culprit of AD, the multiple failures of Aβ-centric therapies call for alternative therapeutic approaches. Conceivably, the complexity of the AD neuropathological scenario cannot be solved with single-target therapies, so multiple-target approaches are needed. Core targets of AD to date are soluble oligomeric Aβ species and neuroinflammation, in an intimate detrimental dialogue. Aβ oligomers, the most neurotoxic species, appear to induce synaptic and cognitive dysfunction through the activation of glial cells. Anti-inflammatory drugs can prevent the action of Aβ oligomers. Neuroinflammation is a chronic event whose perpetuation leads to the continuous release of pro-inflammatory cytokines, promoting neuronal cell death and gross brain atrophy. Among the possible multi-target therapeutic alternatives, this review highlights the antibiotic tetracyclines, which besides antimicrobial activity also have pleiotropic action against amyloidosis, neuroinflammation, and oxidative stress. A particular focus will be on doxycycline (Doxy), a second-generation tetracycline that crosses the blood-brain barrier more easily and has a safer clinical profile. Doxy emerged as a promising preventive strategy in prion diseases and gave compelling pre-clinical results in mouse models of AD against Aβ oligomers and neuroinflammation. This strongly supports its therapeutic potential and calls for deciphering its exact mechanisms of action so as to maximize its effects in the clinic.

VEGF receptor-1 modulates amyloid β 1-42 oligomer-induced senescence in brain endothelial cells

Aggregated amyloid β (Aβ) peptides in the Alzheimer's disease (AD) brain are hypothesized to trigger several downstream pathologies, including cerebrovascular dysfunction. Previous studies have shown that Aβ peptides can have antiangiogenic properties, which may contribute to vascular dysfunction in the early stages of the disease process. We have generated data showing that brain endothelial cells (ECs) exposed to toxic Aβ1-42 oligomers can readily enter a senescence phenotype. To determine the effect of Aβ oligomers on brain ECs, we treated early passaged human brain microvascular ECs and HUVECs with high MW Aβ1-42 oligomers (5 µM, for 72 h). For controls, we used no peptide treatment, 5 µM Aβ1-42 monomers, and 5 µM Aβ1-42 fibrils, respectively. Brain ECs treated with Aβ1-42 oligomers showed increased senescence-associated β-galactosidase staining and increased senescence-associated p21/p53 expression. Treatment with either Aβ1-42 monomer or Aβ1-42 fibrils did not induce senescence in this assay. We then measured vascular endothelial growth factor receptor (VEGFR) expression in the Aβ1-42 oligomer-treated ECs, and these cells showed significantly increased VEGFR-1 expression and decreased VEGFR-2 levels. Overexpression of VEGFR-1 in brain ECs readily induced senescence, suggesting a direct role of VEGFR-1 signaling events in this paradigm. More importantly, small interfering RNA-mediated knockdown of VEGFR-1 expression in brain ECs was able to prevent up-regulation of p21 protein expression and significantly reduced induction of senescence following Aβ1-42 oligomer treatment. Our studies show that exposure to Aβ1-42 oligomers may impair vascular functions by altering VEGFR-1 expression and causing ECs to enter a senescent phenotype. Altered VEGFR expression has been documented in brains of AD patients and suggests that this pathway may play a role in AD disease pathogenesis. These studies suggest that modulating VEGFR-1 expression and signaling events could potentially prevent senescence and rejuvenate EC functions, and provides us with a novel target to pursue for prevention and treatment of cerebrovascular dysfunction in AD.-Angom, R. S., Wang, Y., Wang, E., Pal, K., Bhattacharya, S., Watzlawik, J. O., Rosenberry, T. L., Das, P., Mukhopadhyay, D. VEGF receptor-1 modulates amyloid β 1-42 oligomer-induced senescence in brain endothelial cells.

The Role of Glial Cells and Synapse Loss in Mouse Models of Alzheimer's Disease

Synapse loss has detrimental effects on cellular communication, leading to network disruptions within the central nervous system (CNS) such as in Alzheimer’s disease (AD). AD is characterized by a progressive decline of memory function, cognition, neuronal and synapse loss. The two main neuropathological hallmarks are amyloid-β (Aβ) plaques and neurofibrillary tangles. In the brain of AD patients and in mouse models of AD several morphological and functional changes, such as microgliosis and astrogliosis around Aβ plaques, as well as dendritic and synaptic alterations, are associated with these lesions. In this review article, we will summarize the current literature on synapse loss in mouse models of AD and discuss current and prospective treatments for AD.

Reduced retromer function results in the accumulation of amyloid-beta oligomers

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by a progressive loss of multiple cognitive functions. Accumulation of amyloid beta oligomers (oAβ) play a major role in the neurotoxicity associated with the disease process. One of the early affected brain regions is the hippocampus, wherein a reduction of the vacuolar protein sorting-associated protein 35 (VPS35), the core protein comprising the retromer complex involved in cellular cargo sorting, has been identified. To investigate the role of the retromer function on the accumulation and clearance of oAβ, we reduced retromer function by selectively inhibiting VPS35 gene expression using siRNA in differentiated neuronal SH-SY5Y cells. As cell-to-cell transfer of oAβ to new brain regions is believed to be important for disease progression we investigated the effect of VPS35 reduction both in cells with direct uptake of oAβ and in cells receiving oAβ from donor cells. We demonstrate that reduced retromer function increases oAβ accumulation in both cell systems, both the number of cells containing intracellular oAβ and the amount within them. This effect was shown at different time points and regardless if the oAβ originated from the extracellular milieu or via a direct neuronal cell-to-cell transfer. Interestingly, not only did reduced VPS35 cause oAβ accumulation, but oAβ treatment alone also lead to a reduction of VPS35 protein content. The accumulated oAβ seems to co-localize with VPS35 and early endosome markers. Together, these findings provide evidence that reduced retromer function decreases the ability for neurons to transport and clear neurotoxic oAβ received through different routes resulting in the accumulation of oAβ. Thus, enhancing retromer function may be a potential therapeutic strategy to slow down the pathophysiology associated with the progression of AD.

Immunotherapy to improve cognition and reduce pathological species in an Alzheimer's disease mouse model

Background

Alzheimer’s disease (AD) is characterized by physiologically endogenous proteins amyloid beta (Aβ) and tau undergoing a conformational change and accumulating as soluble oligomers and insoluble aggregates. Tau and Aβ soluble oligomers, which contain extensive β-sheet secondary structure, are thought to be the most toxic forms. The objective of this study was to determine the ability of TWF9, an anti-β-sheet conformation antibody (aβComAb), to selectively recognize pathological Aβ and phosphorylated tau in AD human tissue compared with cognitively normal age-matched controls and to improve the performance of old 3xTg-AD mice with advanced pathology in behavioral testing after acute treatment with TWF9.

Methods

In this study, we used immunohistochemistry, immunoprecipitation, and enzyme-linked immunosorbent assay (ELISA) to characterize TWF9 specificity. We further assessed cognitive performance in old (18–22 months) 3xTg-AD mice using both a Barnes maze and novel object recognition after intraperitoneal administration of TWF9 (4 mg/kg) biweekly for 2 weeks before the start of behavioral testing. Injections continued for the duration of the behavioral testing, which lasted 2 weeks.

Results

Histological analysis of TWF9 in formalin-fixed paraffin-embedded human control and AD (ABC score: A3B3C3) brain tissue revealed preferential cytoplasmic immunoreactivity in neurons in the AD tissue compared with controls (p < 0.05). Furthermore, ELISA using oligomeric and monomeric Aβ showed a preferential affinity for oligomeric Aβ. Immunoprecipitation studies showed that TWF9 extracted both phosphorylated tau (p < 0.01) and Aβ (p < 0.01) from fresh frozen brain tissues. Results show that treated old 3xTg-AD mice have an enhanced novel object recognition memory (p < 0.01) and Barnes maze performance (p = 0.05) compared with control animals. Overall plaque burden, neurofibrillary tangles, microgliosis, and astrocytosis remained unchanged. Soluble phosphorylated tau was significantly reduced in TWF9-treated mice (p < 0.05), and there was a trend for a reduction in soluble Aβ levels in the brain homogenates of female 3xTg-AD mice (p = 0.06).

Conclusions

This study shows that acute treatment with an aβComAb can effectively improve performance in behavioral testing without reduction of amyloid plaque burden, and that peripherally administered IgG can affect levels of pathological species in the brain.

Review: Neuropathology and behavioural features of transgenic murine models of Alzheimer's disease

Our understanding of the underlying biology of Alzheimer's disease (AD) has been steadily progressing; however, this is yet to translate into a successful treatment in humans. The use of transgenic mouse models has helped to develop our understanding of AD, not only in terms of disease pathology, but also with the associated cognitive impairments typical of AD. Plaques and neurofibrillary tangles are often among the last pathological changes in AD mouse models, after neuronal loss and gliosis. There is a general consensus that successful treatments need to be applied before the onset of these pathologies and associated cognitive symptoms. This review discusses the different types of AD mouse models in terms of the temporal progression of the disease, how well they replicate the pathological changes seen in human AD and their cognitive defects. We provide a critical assessment of the behavioural tests used with AD mice to assess cognitive changes and decline, and discuss how successfully they correlate with cognitive impairments in humans with AD. This information is an important tool for AD researchers when deciding on appropriate mouse models, and when selecting measures to assess behavioural and cognitive change.

Peripheral Interventions Enhancing Brain Glutamate Homeostasis Relieve Amyloid β- and TNFα- Mediated Synaptic Plasticity Disruption in the Rat Hippocampus

Dysregulation of glutamate homeostasis in the interstitial fluid of the brain is strongly implicated in causing synaptic dysfunction in many neurological and psychiatric illnesses. In the case of Alzheimer's disease (AD), amyloid β (Aβ)-mediated disruption of synaptic plasticity and memory can be alleviated by interventions that directly remove glutamate or block certain glutamate receptors. An alternative strategy is to facilitate the removal of excess glutamate from the nervous system by activating peripheral glutamate clearance systems. One such blood-based system, glutamate oxaloacetate transaminase (GOT), is activated by oxaloacetate, which acts as a co-substrate. We report here that synthetic and AD brain-derived Aβ-mediated inhibition of synaptic long-term potentiation in the hippocampus is alleviated by oxaloacetate. Moreover the effect of oxaloacetate was GOT-dependent. The disruptive effects of a general inhibitor of excitatory amino acid transport or TNFα, a pro-inflammatory mediator of Aβ action, were also reversed by oxaloacetate. Furthermore, another intervention that increases peripheral glutamate clearance, peritoneal dialysis, mimicked the beneficial effect of oxaloacetate. These findings lend support to the promotion of the peripheral clearance of glutamate as a means to alleviate synaptic dysfunction that is caused by impaired glutamate homeostasis in the brain.

Structural and kinetic basis for the selectivity of aducanumab for aggregated forms of amyloid-β

Aducanumab, a human-derived antibody targeting amyloid-β (Aβ), is in Phase 3 clinical trials for the treatment of Alzheimer’s disease. Biochemical and structural analyses show that aducanumab binds a linear epitope formed by amino acids 3–7 of the Aβ peptide. Aducanumab discriminates between monomers and oligomeric or fibrillar aggregates based on weak monovalent affinity, fast binding kinetics and strong avidity for epitope-rich aggregates. Direct comparative studies with analogs of gantenerumab, bapineuzumab and solanezumab demonstrate clear differentiation in the binding properties of these antibodies. The crystal structure of the Fab fragment of aducanumab bound to its epitope peptide reveals that aducanumab binds to the N terminus of Aβ in an extended conformation, distinct from those seen in structures with other antibodies that target this immunodominant epitope. Aducanumab recognizes a compact epitope that sits in a shallow pocket on the antibody surface. In silico analyses suggest that aducanumab interacts weakly with the Aβ monomer and may accommodate a variety of peptide conformations, further supporting its selectivity for Aβ aggregates. Our studies provide a structural rationale for the low affinity of aducanumab for non-pathogenic monomers and its greater selectivity for aggregated forms than is seen for other Aβ-targeting antibodies.

Antibody Engineering for Optimized Immunotherapy in Alzheimer's Disease

There are nearly 50 million people with Alzheimer's disease (AD) worldwide and currently no disease modifying treatment is available. AD is characterized by deposits of Amyloid-β (Aβ), neurofibrillary tangles, and neuroinflammation, and several drug discovery programmes studies have focussed on Aβ as therapeutic target. Active immunization and passive immunization against Aβ leads to the clearance of deposits in humans and transgenic mice expressing human Aβ but have failed to improve memory loss. This review will discuss the possible explanations for the lack of efficacy of Aβ immunotherapy, including the role of a pro-inflammatory response and subsequent vascular side effects, the binding site of therapeutic antibodies and the timing of the treatment. We further discuss how antibodies can be engineered for improved efficacy.

Neurogenesis and morphological-neural alterations closely related to amyloid β-peptide (25-35)-induced memory impairment in male rats

Memory impairment by the Amyloid-β 25-35 (Aβ_25-35) peptide in animal models has provided an understanding of the causes behind the similar deterioration that occurs in Alzheimer's disease. However, it is uncertain if a decrease of dendritic spines and neurogenesis conduces to cognitive impairment by an impairment in the retrieval of stored memory. The aim of this study was to evaluate the consequences of impairment on spatial memory caused by the administration of the Aβ_25-35 peptide in the hippocampus, which is associated whit morphological changes and neurogenesis in the dentate gyrus (DG). The vehicle or Aβ_25-35 peptide (0.1μg/μL) were bilaterally administered in the CA1 subfield of the rat hippocampus. The animals were tested for spatial learning and memory in the Morris Water Maze. In the day's 11, 18 and 32 after administration of the Aβ_25-35 peptide were examined the morphological changes in the DG using a Golgi-Cox stain. In the day 32, the neurogenesis was evaluated by the immunoreactivity to 5-bromo-2'-deoxyuridine (BrdU; 100mg/kg, i.p.) that corresponding to cellular proliferation post damage, the neuronal specific nuclear protein (NeuN) and doublecortin (DCX). This study found a memory retrieval impairment occurring at day 17, a cognitive deficit which had increased significantly at day 31 after the administration of Aβ_25-35 peptide. These results are related to morphological changes in the granular cells of the DG, such as a shorter dendritic length and a decrease in the number of dendritic spines. In neurogenesis, the total number of cells positive to BrdU, NeuN and DCX in the hippocampal granule cell layer was found to have declined in animals treated with Aβ_25-35. The results suggest that the Aβ_25-35 peptide impairs memory retrieval by decreasing the number of dendritic spines and altering neurogenesis in the DG.

Immunization Against Specific Fragments of Neurotrophin p75 Receptor Protects Forebrain Cholinergic Neurons in the Olfactory Bulbectomized Mice

Alzheimer’s disease (AD) is characterized by progressive cognitive impairment associated with marked cholinergic neuron loss and amyloid-β (Aβ) peptide accumulation in the brain. The cytotoxicity in AD is mediated, at least in part, by Aβ binding with the extracellular domain of the p75 neurotrophin receptor (p75NTR), localized predominantly in the membranes of acetylcholine-producing neurons in the basal forebrain. Hypothesizing that an open unstructured loop of p75NTR might be the effective site for Aβ binding, we have immunized both olfactory bulbectomized (OBX) and sham-operated (SO) mice (n = 82 and 49, respectively) with synthetic peptides, structurally similar to different parts of the loops, aiming to block them by specific antibodies. OBX-mice have been shown in previous studies, and confirmed in the present one, to be characterized by typical behavioral, morphological, and biochemical AD hallmarks, including cholinergic deficits in forebrain neurons. Immunization of OBX- or SO-mice with KLH conjugated fragments of p75NTR induced high titers of specific serum antibodies for each of nine chosen fragments. However, maximal protective effects on spatial memory, evaluated in a Morris water maze, and on activity of choline acetyltransferase in forebrain neurons, detected by immunoreactivity to specific antibodies, were revealed only for peptides with amino acid residue sequences of 155–164 and 167–176. We conclude that the approach based on immunological blockade of specific p75NTR sites, linked with the cytotoxicity, is a useful and effective tool for study of AD-associated mechanisms and for development of highly selective therapy of cholinergic malfunctioning in AD patients.

Alzheimer's disease as oligomeropathy

Alzheimer's disease (AD) is the most common age-related neurodegenerative disorder and is characterized by pathological aggregates of amyloid β-protein (Aβ) and tau protein. On the basis of genetic evidence, biochemical data, and animal models, Aβ has been suggested to be responsible for the pathogenesis of AD (the amyloid hypothesis). Aβ molecules tend to aggregate to form oligomers, protofibrils, and mature fibrils. Although mature fibrils in the final stage have been thought to be the cause of AD pathogenesis, recent studies using synthetic Aβ peptides, a cell culture model, Aβ precursor protein transgenic mice models, and human samples, such as cerebrospinal fluids and postmortem brains of AD patients, suggest that pre-fibrillar forms (oligomers of Aβ) are more deleterious than are extracellular fibril forms. Based on this recent evidence showing that oligomers have a central role in the pathogenesis of AD, the term "oligomeropathy" could be used to define AD and other protein-misfolding diseases. In this review, I discuss recent developments in the "oligomer hypothesis" including our research findings regarding the pathogenesis of AD.

Angiotensin IV Receptors Mediate the Cognitive and Cerebrovascular Benefits of Losartan in a Mouse Model of Alzheimer's Disease

The use of angiotensin receptor blockers (ARBs) correlates with reduced onset and progression of Alzheimer's disease (AD). The mechanism depicting how ARBs such as losartan restore cerebrovascular and cognitive deficits in AD is unknown. Here, we propose a mechanism underlying losartan's benefits by selectively blocking the effects of angiotensin IV (AngIV) at its receptor (AT4R) with divalinal in mice overexpressing the AD-related Swedish and Indiana mutations of the human amyloid precursor protein (APP mice) and WT mice. Young (3-month-old) mice were treated with losartan (∼10 mg/kg/d, 4 months), followed by intracerebroventricular administration of vehicle or divalinal in the final month of treatment. Spatial learning and memory were assessed using Morris water mazes at 3 and 4 months of losartan treatment. Cerebrovascular reactivity and whisker-evoked neurovascular coupling responses were measured at end point (∼7 months of age), together with biomarkers related to neuronal and vascular oxidative stress (superoxide dismutase-2), neuroinflammation (astroglial and microglial activation), neurogenesis (BrdU-labeled newborn cells), and amyloidosis [soluble amyloid-β (Aβ) species and Aβ plaque load]. Divalinal countered losartan's capacity to rescue spatial learning and memory and blocked losartan's benefits on dilatory function and baseline nitric oxide bioavailability. Divalinal reverted losartan's anti-inflammatory effects, but failed to modify losartan-mediated reductions in oxidative stress. Neither losartan nor divalinal affected arterial blood pressure or significantly altered the amyloid pathology in APP mice. Our findings identify activation of the AngIV/AT4R cascade as the underlying mechanism in losartan's benefits and a target that could restore Aβ-related cognitive and cerebrovascular deficits in AD.SIGNIFICANCE STATEMENT Antihypertensive medications that target the renin angiotensin system, such as angiotensin receptor blockers (ARBs), have been associated with lower incidence and progression of Alzheimer's disease (AD) in cohort studies. However, the manner by which ARBs mediate their beneficial effects is unknown. Here, the angiotensin IV receptor (AT4R) was identified as mediating the cognitive and cerebrovascular rescue of losartan, a commonly prescribed ARB, in a mouse model of AD. The AT4R was further implicated in mediating anti-inflammatory benefits. AT4R-mediated effects were independent from changes in blood pressure, amyloidosis, and oxidative stress. Overall, our results implicate the angiotensin IV/AT4R cascade as a promising candidate for AD intervention.

Structure-dependent effects of amyloid-β on long-term memory in Lymnaea stagnalis

Amyloid‐β (Aβ) peptides are implicated in the causation of memory loss, neuronal impairment, and neurodegeneration in Alzheimer's disease. Our recent work revealed that Aβ 1–42 and Aβ 25–35 inhibit long‐term memory (LTM) recall in Lymnaea stagnalis (pond snail) in the absence of cell death. Here, we report the characterization of the active species prepared under different conditions, describe which Aβ species is present in brain tissue during the behavioral recall time point and relate the sequence and structure of the oligomeric species to the resulting neuronal properties and effect on LTM. Our results suggest that oligomers are the key toxic Aβ1–42 structures, which likely affect LTM through synaptic plasticity pathways, and that Aβ 1–42 and Aβ 25–35 cannot be used as interchangeable peptides.

Alzheimer's Toxic Amyloid Beta Oligomers: Unwelcome Visitors to the Na/K ATPase alpha3 Docking Station

Toxic amyloid beta oligomers (AβOs) are known to accumulate in Alzheimer's disease (AD) and in animal models of AD. Their structure is heterogeneous, and they are found in both intracellular and extracellular milieu. When given to CNS cultures or injected ICV into non-human primates and other non-transgenic animals, AβOs have been found to cause impaired synaptic plasticity, loss of memory function, tau hyperphosphorylation and tangle formation, synapse elimination, oxidative and ER stress, inflammatory microglial activation, and selective nerve cell death. Memory loss and pathology in transgenic models are prevented by AβO antibodies, while Aducanumab, an antibody that targets AβOs as well as fibrillar Aβ, has provided cognitive benefit to humans in early clinical trials. AβOs have now been investigated in more than 3000 studies and are widely thought to be the major toxic form of Aβ. Although much has been learned about the downstream mechanisms of AβO action, a major gap concerns the earliest steps: How do AβOs initially interact with surface membranes to generate neuron-damaging transmembrane events? Findings from Ohnishi et al (PNAS 2005) combined with new results presented here are consistent with the hypothesis that AβOs act as neurotoxins because they attach to particular membrane protein docks containing Na/K ATPase-α3, where they inhibit ATPase activity and pathologically restructure dock composition and topology in a manner leading to excessive Ca++ build-up. Better understanding of the mechanism that makes attachment of AβOs to vulnerable neurons a neurotoxic phenomenon should open the door to therapeutics and diagnostics targeting the first step of a complex pathway that leads to neural damage and dementia.

Complement and microglia mediate early synapse loss in Alzheimer mouse models

Synapse loss in Alzheimer's disease (AD) correlates with cognitive decline. Involvement of microglia and complement in AD has been attributed to neuroinflammation, prominent late in disease. Here we show in mouse models that complement and microglia mediate synaptic loss early in AD. C1q, the initiating protein of the classical complement cascade, is increased and associated with synapses before overt plaque deposition. Inhibition of C1q, C3, or the microglial complement receptor CR3 reduces the number of phagocytic microglia, as well as the extent of early synapse loss. C1q is necessary for the toxic effects of soluble β-amyloid (Aβ) oligomers on synapses and hippocampal long-term potentiation. Finally, microglia in adult brains engulf synaptic material in a CR3-dependent process when exposed to soluble Aβ oligomers. Together, these findings suggest that the complement-dependent pathway and microglia that prune excess synapses in development are inappropriately activated and mediate synapse loss in AD.

Intrahippocampal administration of a domain antibody that binds aggregated amyloid-β reverses cognitive deficits produced by diet-induced obesity

BACKGROUND

The prevalence of high fat diets (HFD), diet-induced obesity (DIO) and Type 2 diabetes continues to increase, associated with cognitive impairment in both humans and rodent models. Mechanisms transducing these impairments remain largely unknown: one possibility is that a common mechanism may be involved in the cognitive impairment seen in obese and/or diabetic states and in dementia, specifically Alzheimer's disease (AD). DIO is well established as a risk factor for development of AD. Oligomeric amyloid-β (Aβ) is neurotoxic, and we showed that intrahippocampal oligomeric Aβ produces cognitive and metabolic dysfunction similar to that seen in DIO or diabetes. Moreover, animal models of DIO show elevated brain Aβ, a hallmark of AD, suggesting that this may be one source of cognitive impairment in both conditions.

METHODS

Intrahippocampal administration of a novel anti-Aβ domain antibody for aggregated Aβ, or a control domain antibody, to control or HFD-induced DIO rats. Spatial learning measured in a conditioned contextual fear (CCF) task after domain antibody treatment; postmortem, hippocampal NMDAR and AMPAR were measured.

RESULTS

DIO caused impairment in CCF, and this impairment was eliminated by intrahippocampal administration of the active domain antibody. Measurement of hippocampal proteins suggests that DIO causes dysregulation of hippocampal AMPA receptors, which is also reversed by acute domain antibody administration.

CONCLUSIONS

Our findings support the concept that oligomeric Aβ within the hippocampus of DIO animals may not only be a risk factor for development of AD but may also cause cognitive impairment before the development of dementia.

GENERAL SIGNIFICANCE AND INTEREST

Our work integrates the engineering of domain antibodies with conformational- and sequence-specificity for oligomeric amyloid beta with a clinically relevant model of diet-induced obesity in order to demonstrate not only the pervasive effects of obesity on several aspects of brain biochemistry and behavior, but also the bioengineering of a successful treatment against the long-term detrimental effects of a pre-diabetic state on the brain. We show for the first time that cognitive impairment linked to obesity and/or insulin resistance may be due to early accumulation of oligomeric beta-amyloid in the brain, and hence may represent a pre-Alzheimer's state.

Actin dynamics and cofilin-actin rods in alzheimer disease

Cytoskeletal abnormalities and synaptic loss, typical of both familial and sporadic Alzheimer disease (AD), are induced by diverse stresses such as neuroinflammation, oxidative stress, and energetic stress, each of which may be initiated or enhanced by proinflammatory cytokines or amyloid-β (Aβ) peptides. Extracellular Aβ-containing plaques and intracellular phospho-tau-containing neurofibrillary tangles are postmortem pathologies required to confirm AD and have been the focus of most studies. However, AD brain, but not normal brain, also have increased levels of cytoplasmic rod-shaped bundles of filaments composed of ADF/cofilin-actin in a 1:1 complex (rods). Cofilin, the major ADF/cofilin isoform in mammalian neurons, severs actin filaments at low cofilin/actin ratios and stabilizes filaments at high cofilin/actin ratios. It binds cooperatively to ADP-actin subunits in F-actin. Cofilin is activated by dephosphorylation and may be oxidized in stressed neurons to form disulfide-linked dimers, required for bundling cofilin-actin filaments into stable rods. Rods form within neurites causing synaptic dysfunction by sequestering cofilin, disrupting normal actin dynamics, blocking transport, and exacerbating mitochondrial membrane potential loss. Aβ and proinflammatory cytokines induce rods through a cellular prion protein-dependent activation of NADPH oxidase and production of reactive oxygen species. Here we review recent advances in our understanding of cofilin biochemistry, rod formation, and the development of cognitive deficits. We will then discuss rod formation as a molecular pathway for synapse loss that may be common between all three prominent current AD hypotheses, thus making rods an attractive therapeutic target. © 2016 Wiley Periodicals, Inc.

Curcumin Ameliorates Memory Decline via Inhibiting BACE1 Expression and β-Amyloid Pathology in 5×FAD Transgenic Mice

Alzheimer's disease (AD) is the most common dementia and the trigger of its pathological cascade is widely believed to be the overproduction and accumulation of β-amyloid protein (Aβ) in the affected brain. However, effective AD remedies are still anxiously awaited. Recent evidence suggests that curcumin may be a potential agent for AD treatment. In this study, we used 5×FAD transgenic mice as an AD model to investigate the effects of curcumin on AD. Our results showed that curcumin administration (150 or 300 mg/kg/day, intragastrically, for 60 days) dramatically reduced Aβ production by downregulating BACE1 expression, preventing synaptic degradation, and improving spatial learning and memory impairment of 5×FAD mice. These findings suggest that curcumin is a potential candidate for AD treatment.

In vitro validation of effects of BDNF-expressing mesenchymal stem cells on neurodegeneration in primary cultured neurons of APP/PS1 mice

Alzheimer's disease (AD), the most common type of dementia, is characterized by the presence of senile plaques, neurofibrillary tangles, and neuronal loss in defined regions of the brain including the hippocampus and cortex. Transplantation of bone marrow-derived mesenchymal stem cells (BM-MSCs) offers a safe and potentially effective tool for treating neurodegenerative disorders. However, the therapeutic effects of BM-MSCs on AD pathology remain unclear and their mechanisms at cellular and molecular levels still need to be addressed. In this study, we developed a unique neuronal culture made from 5xFAD mouse, an APP/PS1 transgenic mouse model (FAD neurons) to investigate progressive neurodegeneration associated with AD pathology and efficacy of brain-derived neurotrophic factor expressing-MSCs (BDNF-MSCs). Analyses of the expression of brain-derived neurotrophic factor (BDNF), synaptic markers and survival/apoptotic signals indicate that pathological features of cultured neurons made from these mice accurately mimic AD pathology, suggesting that our protocol provided a valid in vitro model of AD. We also demonstrated amelioration of AD pathology by MSCs in vitro when these FAD neurons were co-cultured with MSCs, a paradigm that mimics the in vivo environment of post-transplantation of MSCs into damaged regions of brains. To overcome failed delivery of BDNF to the brain and to enhance MSCs releasing BDNF effect, we created BDNF-MSCs and found that MSCs protection was enhanced by BDNF-MSCs. This protection was abolished by BDNF-blocking peptides, suggesting that BDNF supply from BDNF-MSCs was enough to prevent AD pathology.

Neuropsychological and neuroimaging characteristics of amnestic mild cognitive impairment subtypes: a selective overview

Alzheimer's disease (AD) is a progressive age-related neurodegenerative disease. Amnestic mild cognitive impairment (aMCI) is considered to represent early AD. Various aMCI clinical subtypes have been identified as either single domain (SD) or multidomain (MD). The various subtypes represent heterogeneous syndrome, indicating the different probability of progression to AD. Understanding the heterogeneous concept of aMCI can help to construct potential biomarkers to monitor the progression of aMCI to AD. This review provides an overview of various neuroimaging measures for subtypes of aMCI. Focusing on neuropsychological, structural, and functional neuroimaging findings, we found that aMCI showed differences in clinical progression and the abnormalities in MD-aMCI were distributed across temporal, frontal, and parietal cortices, which is similar to AD. This is also compatible with the notion that MD-aMCI is a transition stage between SD-aMCI and AD. Our review provided a framework for the diagnosis of clinical subtypes of aMCI and early detection and intervention of the progression from aMCI to AD.

A γ-secretase inhibitor, but not a γ-secretase modulator, induced defects in BDNF axonal trafficking and signaling: evidence for a role for APP

Clues to Alzheimer disease (AD) pathogenesis come from a variety of different sources including studies of clinical and neuropathological features, biomarkers, genomics and animal and cellular models. An important role for amyloid precursor protein (APP) and its processing has emerged and considerable interest has been directed at the hypothesis that Aβ peptides induce changes central to pathogenesis. Accordingly, molecules that reduce the levels of Aβ peptides have been discovered such as γ-secretase inhibitors (GSIs) and modulators (GSMs). GSIs and GSMs reduce Aβ levels through very different mechanisms. However, GSIs, but not GSMs, markedly increase the levels of APP CTFs that are increasingly viewed as disrupting neuronal function. Here, we evaluated the effects of GSIs and GSMs on a number of neuronal phenotypes possibly relevant to their use in treatment of AD. We report that GSI disrupted retrograde axonal trafficking of brain-derived neurotrophic factor (BDNF), suppressed BDNF-induced downstream signaling pathways and induced changes in the distribution within neuronal processes of mitochondria and synaptic vesicles. In contrast, treatment with a novel class of GSMs had no significant effect on these measures. Since knockdown of APP by specific siRNA prevented GSI-induced changes in BDNF axonal trafficking and signaling, we concluded that GSI effects on APP processing were responsible, at least in part, for BDNF trafficking and signaling deficits. Our findings argue that with respect to anti-amyloid treatments, even an APP-specific GSI may have deleterious effects and GSMs may serve as a better alternative.

Tailoring the antibody response to aggregated Aß using novel Alzheimer-vaccines

Recent evidence suggests Alzheimer-Disease (AD) to be driven by aggregated Aß. Capitalizing on the mechanism of molecular mimicry and applying several selection layers, we screened peptide libraries for moieties inducing antibodies selectively reacting with Aß-aggregates. The technology identified a pool of peptide candidates; two, AFFITOPES AD01 and AD02, were assessed as vaccination antigens and compared to Aβ1-6, the targeted epitope. When conjugated to Keyhole Limpet Hemocyanin (KLH) and adjuvanted with aluminum, all three peptides induced Aß-targeting antibodies (Abs). In contrast to Aß1-6, AD01- or AD02-induced Abs were characterized by selectivity for aggregated forms of Aß and absence of reactivity with related molecules such as Amyloid Precursor Protein (APP)/ secreted APP-alpha (sAPPa). Administration of AFFITOPE-vaccines to APP-transgenic mice was found to reduce their cerebral amyloid burden, the associated neuropathological alterations and to improve their cognitive functions. Thus, the AFFITOME-technology delivers vaccines capable of inducing a distinct Ab response. Their features may be beneficial to AD-patients, a hypothesis currently tested within a phase-II-study.

Sleep, cognition, and normal aging: integrating a half century of multidisciplinary research

Sleep is implicated in cognitive functioning in young adults. With increasing age, there are substantial changes to sleep quantity and quality, including changes to slow-wave sleep, spindle density, and sleep continuity/fragmentation. A provocative question for the field of cognitive aging is whether such changes in sleep physiology affect cognition (e.g., memory consolidation). We review nearly a half century of research across seven diverse correlational and experimental domains that historically have had little crosstalk. Broadly speaking, sleep and cognitive functions are often related in advancing age, though the prevalence of null effects in healthy older adults (including correlations in the unexpected, negative direction) indicates that age may be an effect modifier of these associations. We interpret the literature as suggesting that maintaining good sleep quality, at least in young adulthood and middle age, promotes better cognitive functioning and serves to protect against age-related cognitive declines.

Synapto-depressive effects of amyloid beta require PICK1

Amyloid beta (Aβ), a key component in the pathophysiology of Alzheimer's disease, is thought to target excitatory synapses early in the disease. However, the mechanism by which Aβ weakens synapses is not well understood. Here we showed that the PDZ domain protein, protein interacting with C kinase 1 (PICK1), was required for Aβ to weaken synapses. In mice lacking PICK1, elevations of Aβ failed to depress synaptic transmission in cultured brain slices. In dissociated cultured neurons, Aβ failed to reduce surface α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor subunit 2, a subunit of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors that binds with PICK1 through a PDZ ligand-domain interaction. Lastly, a novel small molecule (BIO922) discovered through structure-based drug design that targets the specific interactions between GluA2 and PICK1 blocked the effects of Aβ on synapses and surface receptors. We concluded that GluA2-PICK1 interactions are a key component of the effects of Aβ on synapses.

Endoplasmic reticulum stress as a novel neuronal mediator in Alzheimer's disease

Alzheimer's disease (AD) is one of the most common types of progressive dementias. The typical neuropathological changes in AD include extracellular senile plaques, intracellular neurofibrillary tangles, and loss of neurons. The pathogenetic mechanism of this disease is not comprehensively understood yet. Recently, endoplasmic reticulum stress (ER stress) has been considered as a potential event involved in AD development. Some AD-related factors, such as misfolded protein and Ca(2+) depletion, could disrupt the homeostasis of ER lumen. In AD, the aggregated amyloid-beta peptide (Abeta) could induce ER stress in an assembly dependent way. The presenilin has been identified as a Ca(2+) channel. Mutations of presenilin could change the balance of Ca(2+) in ER lumen and thus disrupts the ER homeostasis. Furthermore, the ER stress could lead to cellular disorders like inflammation. Through activating the expression of inflammatory factors, ER stress triggers inflammatory response in AD pathology. Herein, we reviewed the recent progress of ER stress-induced unfolded protein response (UPR) and the roles of ER stress in AD pathological process.

Structural diversity of Alzheimer's disease amyloid-β dimers and their role in oligomerization and fibril formation

Alzheimer's disease (AD) is associated with the formation of toxic amyloid-β (Aβ)42 oligomers, and recent evidence supports a role for Aβ dimers as building blocks for oligomers. Molecular dynamics simulation studies have identified clans for the dominant conformations of Aβ42 forming dimers; however, it is unclear if a larger spectrum of dimers is involved and which set(s) of dimers might evolve to oligomers verse fibrils. Therefore, for this study we generated multiple structural conformations of Aβ42, using explicit all-atom molecular dynamics, and then clustering the different structures based on key conformational similarities. Those matching a selection threshold were then used to model a process of oligomerization. Remarkably, we showed a greater diversity in Aβ dimers than previously described. Depending on the clan family, different types of Aβ dimers were obtained. While some had the tendency to evolve into oligomeric rings, others formed fibrils of diverse characteristics. Then we selected the dimers that would evolve to membranephilic annular oligomers. Nearly one third of the 28 evaluated annular oligomers had the dimer interfaces between the neighboring Aβ42 monomers with possible salt bridges between the residue K28 from one side and either residue E22 or D23 on the other. Based on these results, key amino acids were identified for point mutations that either enhanced or suppressed the formation and toxicity of oligomer rings. Our studies suggest a greater diversity of Aβ dimers. Understanding the structure of Aβ dimers might be important for the rationale design of small molecules that block formation of toxic oligomers.

Secreted amyloid β-proteins in a cell culture model include N-terminally extended peptides that impair synaptic plasticity

Evidence for a central role of amyloid β-protein (Aβ) in the genesis of Alzheimer’s disease (AD) has led to advanced human trials of Aβ-lowering agents. The “amyloid hypothesis” of AD postulates deleterious effects of small, soluble forms of Aβ on synaptic form and function. Because selectively targeting synaptotoxic forms of soluble Aβ could be therapeutically advantageous, it is important to understand the full range of soluble Aβ derivatives. We previously described a Chinese hamster ovary (CHO) cell line (7PA2 cells) that stably expresses mutant human amyloid precursor protein (APP). Here, we extend this work by purifying an sodium dodecyl sulfate (SDS)-stable, ∼8 kDa Aβ species from the 7PA2 medium. Mass spectrometry confirmed its identity as a noncovalently bonded Aβ40 homodimer that impaired hippocampal long-term potentiation (LTP) in vivo. We further report the detection of Aβ-containing fragments of APP in the 7PA2 medium that extend N-terminal from Asp1 of Aβ. These N-terminally extended Aβ-containing monomeric fragments are distinct from soluble Aβ oligomers formed from Aβ1-40/42 monomers and are bioactive synaptotoxins secreted by 7PA2 cells. Importantly, decreasing β-secretase processing of APP elevated these alternative synaptotoxic APP fragments. We conclude that certain synaptotoxic Aβ-containing species can arise from APP processing events N-terminal to the classical β-secretase cleavage site.

Targeting the proper amyloid-beta neuronal toxins: a path forward for Alzheimer's disease immunotherapeutics

Levels of amyloid-beta monomer and deposited amyloid-beta in the Alzheimer’s disease brain are orders of magnitude greater than soluble amyloid-beta oligomer levels. Monomeric amyloid-beta has no known direct toxicity. Insoluble fibrillar amyloid-beta has been proposed to be an in vivo mechanism for removal of soluble amyloid-beta and exhibits relatively low toxicity. In contrast, soluble amyloid-beta oligomers are widely reported to be the most toxic amyloid-beta form, both causing acute synaptotoxicity and inducing neurodegenerative processes. None of the amyloid-beta immunotherapies currently in clinical development selectively target soluble amyloid-beta oligomers, and their lack of efficacy is not unexpected considering their selectivity for monomeric or fibrillar amyloid-beta (or both) rather than soluble amyloid-beta oligomers. Because they exhibit acute, memory-compromising synaptic toxicity and induce chronic neurodegenerative toxicity and because they exist at very low in vivo levels in the Alzheimer’s disease brain, soluble amyloid-beta oligomers constitute an optimal immunotherapeutic target that should be pursued more aggressively.

Pathogenesis of synaptic degeneration in Alzheimer's disease and Lewy body disease

Considerable progress has been made in the past few years in the fight against Alzheimer's disease (AD) and Parkinson's disease (PD). Neuropathological studies in human brains and experimental in vivo and in vitro models support the notion that synapses are affected even at the earliest stages of the neurodegenerative process. The objective of this manuscript is to review some of the mechanisms of synaptic damage in AD and PD. Some lines of evidence support the notion that oligomeric neurotoxic species of amyloid β, α-synuclein, and Tau might contribute to the pathogenesis of synaptic failure at early stages of the diseases. The mechanisms leading to synaptic damage by oligomers might involve dysregulation of glutamate receptors and scaffold molecules that results in alterations in the axonal transport of synaptic vesicles and mitochondria that later on lead to dendritic and spine alterations, axonal dystrophy, and eventually neuronal loss. However, while some studies support a role of oligomers, there is an ongoing debate as to the exact nature of the toxic species. Given the efforts toward earlier clinical and preclinical diagnosis of these disorders, understanding the molecular and cellular mechanisms of synaptic degeneration is crucial toward developing specific biomarkers and new therapies targeting the synaptic apparatus of vulnerable neurons.

PDE9A inhibition rescues amyloid beta-induced deficits in synaptic plasticity and cognition

The cyclic nucleotide cGMP is an important intracellular messenger for synaptic plasticity and memory function in rodents. Therefore, inhibition of cGMP degrading phosphodiesterases, like PDE9A, has gained interest as potential target for treatment of cognition deficits in indications like Alzheimer's disease (AD). In fact, PDE9A inhibition results in increased hippocampal long-term potentiation and exhibits procognitive effects in rodents. To date, however, no evidence has been published linking PDE9A inhibition to the pathologic hallmarks of AD such as amyloid beta (Aβ) deposition. Therefore, we investigated the role of PDE9A inhibition in an AD relevant context by testing its effects on Aβ-related deficits in synaptic plasticity and cognition. The PDE9A inhibitor BAY 73-6691 was found to restore long-term potentiation impaired by Aβ42 oligomers. Furthermore, we demonstrated that BAY 73-6691 enhanced cGMP levels in the hippocampus of APP transgenic tg2576 mice and improved memory performance of these mice. Altogether, our results support the hypothesis that inhibition of PDE9A could be a beneficial approach for the treatment of memory impairment in AD patients.

Memory consolidation in aging and MCI after 1 week

OBJECTIVE

To assess consolidation in amnestic mild cognitive impairment (aMCI), controlling for differences in initial learning and using a protracted delay period for recall.

METHOD

15 individuals with aMCI were compared with 15 healthy older adult controls on a story learning task. Subjects were trained to criteria to equalize initial learning across subjects. Recall was tested at both the 30-min typically used delay and a 1-week delay used to target consolidation.

RESULTS

Using repeated measures ANOVAs adjusted for age, we found group × time point interactions across the entire task between the final trial and 30-min delay, and again between the 30-min and 1-week delay periods, with aMCI having greater declines in recall as compared with controls. Significant group main effects were also found, with aMCI recalling less than controls.

CONCLUSION

Consolidation was impaired in aMCI as compared with controls. Our findings indicate that aMCI-related performance typically measured at 30 min underestimates aMCI-associated memory deficits. This is the first study to isolate consolidation by controlling for initial learning differences and using a protracted delay period to target consolidation in an aMCI sample.

Amyloid β-protein oligomers and Alzheimer's disease

The oligomer cascade hypothesis, which states that oligomers are the initiating pathologic agents in Alzheimer’s disease, has all but supplanted the amyloid cascade hypothesis, which suggested that fibers were the key etiologic agents in Alzheimer’s disease. We review here the results of in vivo, in vitro and in silico studies of amyloid β-protein oligomers, and discuss important caveats that should be considered in the evaluation of these results. This article is divided into four sections that mirror the main approaches used in the field to better understand oligomers: (1) attempts to locate and examine oligomers in vivo in situ; that is, without removing these species from their environment; (2) studies involving oligomers extracted from human or animal tissues and the subsequent characterization of their properties ex vivo; (3) studies of oligomers that have been produced synthetically and studied using a reductionist approach in relatively simple in vitro biophysical systems; and (4) computational studies of oligomers in silico. These multiple orthogonal approaches have revealed much about the molecular and cell biology of amyloid β-protein. However, as informative as these approaches have been, the amyloid β-protein oligomer system remains enigmatic.

Modifications of hippocampal circuits and early disruption of adult neurogenesis in the tg2576 mouse model of Alzheimer's disease

At advanced stages of Alzheimer's disease, cognitive dysfunction is accompanied by severe alterations of hippocampal circuits that may largely underlie memory impairments. However, it is likely that anatomical remodeling in the hippocampus may start long before any cognitive alteration is detected. Using the well-described Tg2576 mouse model of Alzheimer's disease that develops progressive age-dependent amyloidosis and cognitive deficits, we examined whether specific stages of the disease were associated with the expression of anatomical markers of hippocampal dysfunction. We found that these mice develop a complex pattern of changes in their dentate gyrus with aging. Those include aberrant expression of neuropeptide Y and reduced levels of calbindin, reflecting a profound remodeling of inhibitory and excitatory circuits in the dentate gyrus. Preceding these changes, we identified severe alterations of adult hippocampal neurogenesis in Tg2576 mice. We gathered converging data in Tg2576 mice at young age, indicating impaired maturation of new neurons that may compromise their functional integration into hippocampal circuits. Thus, disruption of adult hippocampal neurogenesis occurred before network remodeling in this mouse model and therefore may account as an early event in the etiology of Alzheimer's pathology. Ultimately, both events may constitute key components of hippocampal dysfunction and associated cognitive deficits occurring in Alzheimer's disease.

The missing link in the amyloid cascade of Alzheimer's disease - metal ions

Progressive deposition of amyloid beta (Aβ) peptides into amyloid plaques is the pathological hallmark of Alzheimer's disease (AD). The amyloid cascade hypothesis pins this deposition as the primary cause of the disease, but the mechanisms that causes this deposition remain elusive. An increasing amount of evidence shows that biometals Zn(II) and Cu(II) can interact with Aβ, thus influencing the fibrillization and toxicity. This review focuses on the role of Zn(II) and Cu(II) in AD, and revisits the amyloid cascade hypothesis demonstrating the possible roles of Zn(II) and Cu(II) in the disease pathogenesis.

Alzheimer brain-derived amyloid β-protein impairs synaptic remodeling and memory consolidation

Aggregation of the amyloid β-protein (Aβ) is believed to play a central role in initiating the molecular cascade that culminates in Alzheimer-type dementia (AD), a disease which in its early stage is characterized by synaptic loss and impairment of episodic memory. Here we show that intracerebroventricular injection of Aβ-containing water-soluble extracts of AD brain inhibits consolidation of the memory of avoidance learning in the rat and that this effect is highly dependent on the interval between learning and administration. When injected at 1 hour post training extracts from 2 different AD brains significantly impaired recall tested at 48 hours. Ultrastructural examination of hippocampi from animals perfused after 48 hours revealed that Aβ-mediated impairment of avoidance memory was associated with lower density of synapses and altered synaptic structure in the dentate gyrus and CA1 fields. These behavioral and ultrastructural data suggest that human brain-derived Aβ impairs formation of long-term memory by compromising the structural plasticity essential for consolidation and that Aβ targets processes initiated very early in the consolidation pathway.

Astrocyte senescence as a component of Alzheimer's disease

Aging is the main risk factor for Alzheimer’s disease (AD); however, the aspects of the aging process that predispose the brain to the development of AD are largely unknown. Astrocytes perform a myriad of functions in the central nervous system to maintain homeostasis and support neuronal function. In vitro, human astrocytes are highly sensitive to oxidative stress and trigger a senescence program when faced with multiple types of stress. In order to determine whether senescent astrocytes appear in vivo, brain tissue from aged individuals and patients with AD was examined for the presence of senescent astrocytes using p16^INK4a and matrix metalloproteinase-1 (MMP-1) expression as markers of senescence. Compared with fetal tissue samples (n = 4), a significant increase in p16^INK4a-positive astrocytes was observed in subjects aged 35 to 50 years (n = 6; P = 0.02) and 78 to 90 years (n = 11; P<10⁻⁶). In addition, the frontal cortex of AD patients (n = 15) harbored a significantly greater burden of p16^INK4a-positive astrocytes compared with non-AD adult control subjects of similar ages (n = 25; P = 0.02) and fetal controls (n = 4; P<10⁻⁷). Consistent with the senescent nature of the p16^INK4a-positive astrocytes, increased metalloproteinase MMP-1 correlated with p16^INK4a. In vitro, beta-amyloid 1–42 (Aβ_1–42) triggered senescence, driving the expression of p16^INK4a and senescence-associated beta-galactosidase. In addition, we found that senescent astrocytes produce a number of inflammatory cytokines including interleukin-6 (IL-6), which seems to be regulated by p38MAPK. We propose that an accumulation of p16^INK4a-positive senescent astrocytes may link increased age and increased risk for sporadic AD.

Alzheimer's disease Aβ assemblies mediating rapid disruption of synaptic plasticity and memory

Alzheimer’s disease (AD) is characterized by episodic memory impairment that often precedes clinical diagnosis by many years. Probing the mechanisms of such impairment may provide much needed means of diagnosis and therapeutic intervention at an early, pre-dementia, stage. Prior to the onset of significant neurodegeneration, the structural and functional integrity of synapses in mnemonic circuitry is severely compromised in the presence of amyloidosis. This review examines recent evidence evaluating the role of amyloid-ß protein (Aβ) in causing rapid disruption of synaptic plasticity and memory impairment. We evaluate the relative importance of different sizes and conformations of Aβ, including monomer, oligomer, protofibril and fibril. We pay particular attention to recent controversies over the relevance to the pathophysiology of AD of different water soluble Aβ aggregates and the importance of cellular prion protein in mediating their effects. Current data are consistent with the view that both low-n oligomers and larger soluble assemblies present in AD brain, some of them via a direct interaction with cellular prion protein, cause synaptic memory failure. At the two extremes of aggregation, monomers and fibrils appear to act in vivo both as sources and sinks of certain metastable conformations of soluble aggregates that powerfully disrupt synaptic plasticity. The same principle appears to apply to other synaptotoxic amyloidogenic proteins including tau, α-synuclein and prion protein.