Knockdown of astrocytic Grin2a aggravates β‐amyloid‐induced memory and cognitive deficits through regulating nerve growth factor

Synapse degeneration correlates strongly with cognitive impairments in Alzheimer''s disease (AD) patients. Soluble Amyloid‐beta (Aβ) oligomers are thought as the major trigger of synaptic malfunctions. Our earlier studies have demonstrated that Aβ oligomers interfere with synaptic function through N‐methyl‐D‐aspartate receptors (NMDARs). Our recent in vitro study found the neuroprotective role of astrocytic GluN2A in the promotion of synapse survival and identified nerve growth factor (NGF) derived from astrocytes, as a likely mediator of astrocytic GluN2A buffering against Aβ synaptotoxicity. Our present in vivo study focused on exploring the precise mechanism of astrocytic GluN2A influencing Aβ synaptotoxicity through regulating NGF. We generated an adeno‐associated virus (AAV) expressing an astrocytic promoter (GfaABC1D) shRNA targeted to Grin2a (the gene encoding GluN2A) to perform astrocyte‐specific Grin2a knockdown in the hippocampal dentate gyrus, after 3 weeks of virus vector expression, Aβ were bilaterally injected into the intracerebral ventricle. Our results showed that astrocyte‐specific knockdown of Grin2a and Aβ application both significantly impaired spatial memory and cognition, which associated with the reduced synaptic proteins PSD95, synaptophysin and compensatory increased NGF. The reduced astrocytic GluN2A can counteract Aβ‐induced compensatory protective increase of NGF through regulating pNF‐κB, Furin and VAMP3, which modulating the synthesis, mature and secretion of NGF respectively. Our present data reveal, for the first time, a novel mechanism of astrocytic GluN2A in exerting protective effects on synapses at the early stage of Aβ exposure, which may contribute to establish new targets for AD prevention and early therapy.     

Disassembly of Shank and Homer Synaptic Clusters Is Driven by Soluble β-Amyloid1-40 through Divergent NMDAR-Dependent Signalling Pathways

Disruption of the postsynaptic density (PSD), a network of scaffold proteins located in dendritic spines, is thought to be responsible for synaptic dysfunction and loss in early-stage Alzheimer''s disease (AD). Extending our previous demonstration that derangement of the PSD by soluble amyloid-β (Aβ) involves proteasomal degradation of PSD-95, a protein important for ionotropic glutamate receptor trafficking, we now show that Aβ also disrupts two other scaffold proteins, Homer1b and Shank1, that couple PSD-95 with ionotropic and metabotropic glutamate receptors. Treatment of fronto-cortical neurons with soluble Aβ results in rapid (within 1 h) and significant thinning of the PSD, decreased synaptic levels of Homer1b and Shank1, and reduced synaptic mGluR1 levels. We show that de novo protein synthesis is required for the declustering effects of Aβ on Homer1b (but not Shank1) and that, in contrast to PSD-95, Aβ-induced Homer1b and Shank1 cluster disassembly does not depend on proteasome activity. The regulation of Homer1b and Shank1 by Aβ diverges in two other respects: i) whereas the activity of both NMDAR and VDCC is required for Aβ-induced declustering of Homer1b, Aβ-induced declustering of Shank1 only requires NMDAR activity; and ii) whereas the effects of Aβ on Homer1b involve engagement of the PI-3K pathway and calcineurin phosphatase (PP2B) activity, those on Shank1 involve activation of the ERK pathway. In summary, soluble Aβ recruits discrete signalling pathways to rapidly reduce the synaptic localization of major components of the PSD and to regulate the availability of mGluR1 in the synapse.     

Death-associated protein kinase 1 mediates Aβ42 aggregation-induced neuronal apoptosis and tau dysregulation in Alzheimer's disease

The aggregation of amyloid-β (Aβ) peptides into oligomers and fibrils is a key pathological feature of Alzheimer''s disease (AD). An increasing amount of evidence suggests that oligomeric Aβ might be the major culprit responsible for various neuropathological changes in AD. Death-associated protein kinase 1 (DAPK1) is abnormally elevated in brains of AD patients and plays an important role in modulating tau homeostasis by regulating prolyl isomerase Pin1 phosphorylation. However, it remains elusive whether and how Aβ species influence the function of DAPK1, and whether this may further affect the function and phosphorylation of tau in neurons. Herein, we demonstrated that Aβ aggregates (both oligomers and fibrils) prepared from synthetic Aβ42 peptides were able to upregulate DAPK1 protein levels and thereby its function through heat shock protein 90 (HSP90)-mediated protein stabilization. DAPK1 activation not only caused neuronal apoptosis, but also phosphorylated Pin1 at the Ser71 residue, leading to tau accumulation and phosphorylation at multiple AD-related sites in primary neurons. Both DAPK1 knockout (KO) and the application of a specific DAPK1 inhibitor could effectively protect primary neurons against Aβ aggregate-induced cell death and tau dysregulation, corroborating the critical role of DAPK1 in mediating Aβ aggregation-induced neuronal damage. Our study suggests a mechanistic link between Aβ oligomerization and tau hyperphosphorylation mediated by DAPK1, and supports the role of DAPK1 as a promising target for early intervention in AD.     

A Lifespan Observation of a Novel Mouse Model: In Vivo Evidence Supports Aβ Oligomer Hypothesis

Transgenic mouse models are powerful tools in exploring the mechanisms of AD. Most current transgenic models of AD mimic the memory impairment and the main pathologic features, among which the formation of beta-amyloid (Aβ) plaques is considered a dominant pathologic event. Recently, Aβ oligomers have been identified as more neurotoxic than Aβ plaques. However, no ideal transgenic mouse model directly support Aβ oligomers as a neurotoxic species due to the puzzling effects of amyloid plaques in the more widely-used models. Here, we constructed a single-mutant transgenic (Tg) model harboring the PS1_V97L mutation and used Non-Tg littermates as a control group. Employing the Morris water maze, electrophysiology, immunohistochemistry, biochemistry, and electron microscopy, we investigated behavioral changes and pathology progression in our single-mutant transgenic model. We discovered the pathological alteration of intraneuronal accumulation of Aβ oligomers without Aβ plaques in the PS1_V97L-Tg mouse model, which might be the result of PS1 gene mutation. Following Aβ oligomers, we detected synaptic alteration, tau hyperphosphorylation and glial activation. This model supports an initial role for Aβ oligomers in the onset of AD and suggests that Aβ plaques may not be the only prerequisite. This model provides a useful tool for studying the role of Aβ oligomers in AD pathogenesis.     

Advances on the Understanding of the Origins of Synaptic Pathology in AD

Although Alzheimer’s disease (AD) was first discovered a century ago, we are still facing a lack of definitive diagnosis during the patient’s lifetime and are unable to prescribe a curative treatment. However, the past 10 years have seen a “revamping” of the main hypothesis about AD pathogenesis and the hope to foresee possible treatment. AD is no longer considered an irreversible disease. A major refinement of the classic β-amyloid cascade describing amyloid fibrils as neurotoxins has been made to integrate the key scientific evidences demonstrating that the first pathological event occurring in AD early stages affects synaptic function and maintenance. A concept fully compatible with synapse loss being the best pathological correlate of AD rather than other described neuropathological hallmarks (amyloid plaques, neurofibrillary tangles or neuronal death). The notion that synaptic alterations might be reverted, thus offering a potential curability, was confirmed by immunotherapy experiments targeting β-amyloid protein in transgenic AD mice in which cognitive functions were improved despite no reduction in the amyloid plaques burden. The updated amyloid cascade now integrates the synapse failure triggered by soluble Aβ-oligomers. Still no consensus has been reached on the most toxic Aβ conformations, neither on their site of production nor on their extra- versus intra-cellular actions. Evidence shows that soluble Aβ oligomers or ADDLs bind selectively to neurons at their synaptic loci, and trigger major changes in synapse composition and morphology, which ultimately leads to dendritic spine loss. However, the exact mechanism is not yet fully understood but is suspected to involve some membrane receptor(s).     

Gain of PITRM1 peptidase in cortical neurons affords protection of mitochondrial and synaptic function in an advanced age mouse model of Alzheimer’s disease

Mitochondrial dysfunction is one of the early pathological features of Alzheimer''s disease (AD). Accumulation of cerebral and mitochondrial Aβ links to mitochondrial and synaptic toxicity. We have previously demonstrated the mechanism by which presequence peptidase (PITRM1)‐mediated clearance of mitochondrial Aβ contributes to mitochondrial and cerebral amyloid pathology and mitochondrial and synaptic stress in adult transgenic AD mice overexpressing Aβ up to 12 months old. Here, we investigate the effect of PITRM1 in an advanced age AD mouse model (up to 19–24 months) to address the fundamental unexplored question of whether restoration/gain of PITRM1 function protects against mitochondrial and synaptic dysfunction associated with Aβ accumulation and whether this protection is maintained even at later ages featuring profound amyloid pathology and synaptic failure. Using newly developed aged PITRM1/Aβ‐producing AD mice, we first uncovered reduction in PITRM1 expression in AD‐affected cortex of AD mice at 19–24 months of age. Increasing neuronal PITRM1 activity/expression re‐established mitochondrial respiration, suppressed reactive oxygen species, improved synaptic function, and reduced loss of synapses even at advanced ages (up to 19–24 months). Notably, loss of PITRM1 proteolytic activity resulted in Aβ accumulation and failure to rescue mitochondrial and synaptic function, suggesting that PITRM1 activity is required for the degradation and clearance of mitochondrial Aβ and Aβ deposition. These data indicate that augmenting PITRM1 function results in persistent life‐long protection against Aβ toxicity in an AD mouse model. Therefore, augmenting PITRM1 function may enhance Aβ clearance in mitochondria, thereby maintaining mitochondrial integrity and ultimately slowing the progression of AD.     

CaMKIIα-driven,phosphatase-checked postsynaptic plasticity via phase separation

Ca²⁺/calmodulin-dependent kinase IIα (CaMKIIα) is essential for synaptic plasticity and learning by decoding synaptic Ca²⁺ oscillations. Despite decades of extensive research, new mechanisms underlying CaMKIIα’s function in synapses are still being discovered. Here, we discover that Shank3 is a specific binding partner for autoinhibited CaMKIIα. We demonstrate that Shank3 and GluN2B, via combined actions of Ca²⁺ and phosphatases, reciprocally bind to CaMKIIα. Under basal condition, CaMKIIα is recruited to the Shank3 subcompartment of postsynaptic density (PSD) via phase separation. Rise of Ca²⁺ concentration induces GluN2B-mediated recruitment of active CaMKIIα and formation of the CaMKIIα/GluN2B/PSD-95 condensates, which are autonomously dispersed upon Ca²⁺ removal. Protein phosphatases control the Ca²⁺-dependent shuttling of CaMKIIα between the two PSD subcompartments and PSD condensate formation. Activation of CaMKIIα further enlarges the PSD assembly and induces structural LTP. Thus, Ca²⁺-induced and phosphatase-checked shuttling of CaMKIIα between distinct PSD nano-domains can regulate phase separation-mediated PSD assembly and synaptic plasticity.Subject terms: Cell biology, Molecular biology     

A Highlights from MBoC Selection: Dendritic and axonal mechanisms of Ca2+ elevation impair BDNF transport in Aβ oligomer–treated hippocampal neurons

Disruption of fast axonal transport (FAT) and intracellular Ca²⁺ dysregulation are early pathological events in Alzheimer''s disease (AD). Amyloid-β oligomers (AβOs), a causative agent of AD, impair transport of BDNF independent of tau by nonexcitotoxic activation of calcineurin (CaN). Ca²⁺-dependent mechanisms that regulate the onset, severity, and spatiotemporal progression of BDNF transport defects from dendritic and axonal AβO binding sites are unknown. Here we show that BDNF transport defects in dendrites and axons are induced simultaneously but exhibit different rates of decline. The spatiotemporal progression of FAT impairment correlates with Ca²⁺ elevation and CaN activation first in dendrites and subsequently in axons. Although many axonal pathologies have been described in AD, studies have primarily focused only on the dendritic effects of AβOs despite compelling reports of presynaptic AβOs in AD models and patients. Indeed, we observe that dendritic CaN activation converges on Ca²⁺ influx through axonal voltage-gated Ca²⁺ channels to impair FAT. Finally, FAT defects are prevented by dantrolene, a clinical compound that reduces Ca²⁺ release from the ER. This work establishes a novel role for Ca²⁺ dysregulation in BDNF transport disruption and tau-independent Aβ toxicity in early AD.     

The Pathogenic Aβ43 Is Enriched in Familial and Sporadic Alzheimer Disease

The amyloid-cascade hypothesis posits that the role of amyloid β-peptide (Aβ) in Alzheimer disease (AD) involves polymerization into structures that eventually are deposited as amyloid plaques. During this process, neurotoxic oligomers are formed that induce synaptic loss and neuronal death. Several different isoforms of Aβ are produced, of which the 40 and 42 residue variants (Aβ40 and Aβ42) are the most common. Aβ42 has a strong tendency to form neurotoxic aggregates and is involved in AD pathogenesis. Longer Aβ isoforms, like the less studied Aβ43, are gaining attention for their higher propensity to aggregate into neurotoxic oligomers. To further investigate Aβ43 in AD, we conducted a quantitative study on Aβ43 levels in human brain. We homogenized human brain tissue and prepared fractions of various solubility; tris buffered saline (TBS), sodium dodecyl sulfate (SDS) and formic acid (FA). Levels of Aβ43, as well as Aβ40 and Aβ42, were quantified using ELISA. We compared quantitative data showing Aβ levels in occipital and frontal cortex from sporadic (SAD) and familial (FAD) AD cases, as well as non-demented (ND) controls. Results showed Aβ43 present in each fraction from the SAD and FAD cases, while its level was lower than the detection limit in the majority of the ND-cases. Aβ42 and Aβ43 were enriched in the less soluble fractions (SDS and FA) of SAD and FAD cases in both occipital and frontal cortex. Thus, although the total levels of Aβ43 in human brain are low compared to Aβ40 and Aβ42, we suggest that Aβ43 could initiate the formation of oligomers and amyloid plaques and thereby be crucial to AD pathogenesis.     

Molecular insights into the primary nucleation of polymorphic amyloid β dimers in DOPC lipid bilayer membrane

Alzheimer''s disease (AD) pathology is characterized by loss of memory cognitive and behavioral deterioration. One of the hallmarks of AD is amyloid β (Aβ) plaques in the brain that consists of Aβ oligomers and fibrils. It is accepted that oligomers, particularly dimers, are toxic species that are produced extracellularly and intracellularly in membranes. It is believed that the disruption of membranes by polymorphic Aβ oligomers is the key for the pathology of AD. This is a first study that investigate the effect of polymorphic “α‐helix/random coil” and “fibril‐like” Aβ dimers on 1,2‐dioleoyl‐sn‐glycero‐3‐phosphocholine (DOPC) membrane. It has been found that the DOPC membrane promotes Aβ_1–42 “fibril‐like” dimers and impedes Aβ_1–42 “α‐helix/random coil” dimers. The N‐termini domains within Aβ_1–42 dimers play a role in Aβ aggregation in membrane milieus. In addition, the aromatic π–π interactions (involving residues F19 and F20 in Aβ_1–42) are the driving forces for the hydrophobic interactions that initiate the primary nucleation of polymorphic Aβ_1–42 dimers within DOPC membrane. Finally, the DOPC bilayer membrane thickness is locally decreased, and it is disrupted by an embedded distinct Aβ_1–42 dimer, due to relatively large contacts between Aβ_1–42 monomers and the DOPC membrane. This study reveals insights into the molecular mechanisms by which polymorphic early‐stage Aβ_1–42 dimers have distinct impacts on DOPC membrane.     

The role of STEP in Alzheimer disease

Amyloid beta (Aβ), the putative causative agent in Alzheimer disease, is known to affect glutamate receptor trafficking. Previous studies have shown that Aβ downregulates the surface expression of N-methyl D-aspartate type glutamate receptors (NMDARs) by the activation of STriatal-Enriched protein tyrosine Phosphatase 61 (STEP₆₁). More recent findings confirm that STEP₆₁ plays an important role in Aβ-induced NMDAR endocytosis. STEP levels are elevated in human AD prefrontal cortex and in the cortex of several AD mouse models. The increase in STEP₆₁ levels and activity contribute to the removal of GluN1/GluN2B receptor complexes from the neuronal surface membranes. The elevation of STEP₆₁ is due to disruption in the normal degradation of STEP₆₁ by the ubiquitin proteasome system. Here, we briefly discuss additional studies in support of our hypothesis that STEP₆₁ contributes to aspects of the pathophysiology in Alzheimer''s disease. Exogenous application of Aβ-enriched conditioned medium (7PA2-CM) to wild-type cortical cultures results in a loss of GluN1/GluN2B subunits from neuronal membranes. Abeta-mediated NMDAR internalization does not occur in STEP knock-out cultures, but is rescued by the addition of active TAT-STEP to the cultures prior to Aβ treatment.Key words: Alzheimer disease, amyloid beta, NMDA receptor, protein tyrosine phosphatases, STEP, synaptic plasticityIn Alzheimer disease (AD), the abnormal accumulation of soluble Aβ peptides has a profound impact on cognitive function.¹ Aβ peptides disrupt synaptic plasticity, a molecular mechanism involved in learning and memory.^2,3 N-methyl D-aspartate type glutamate receptors (NMDAR) play an important role in the development of synaptic strengthening. Aβ downregulates the surface expression of NMDARs by activation of STriatal-Enriched protein tyrosine Phosphatase 61 (STEP₆₁).⁴ STEP₆₁ is a brain-specific phosphatase that opposes the development of synaptic strengthening.⁵ STEP₆₁ is present in postsynaptic terminals, immunoprecipitates with the NMDAR complex and decreases NMDA channel function.^6,7 The reduced channel function is mediated, at least in part, by an increased internalization of the NMDAR complex, as STEP dephosphorylates the GluN2B subunit at a regulatory tyrosine (tyr¹⁴⁷²) leading to NMDAR endocytosis. Knocking down STEP with interfering RNA increases NMDAR trafficking to synaptic membranes.^4,8 A previous study suggested that Aβ leads to the activation of STEP through a calcineurin-mediated pathway, which subsequently increased internalization of surface NMDAR.⁴ A recent study has demonstrated that STEP is also regulated by the ubiquitin proteasome system, and an Aβ-mediated disruption of the proteasome leads to increased STEP₆₁ levels in human Alzheimer''s disease (AD) brains and AD mouse models.⁹ Taken together, these studies suggest that an increase in the activity of STEP₆₁ contributes to the cognitive deficits in AD by increasing the internalization of NMDAR from synaptic membrane surfaces.     

Spreading of Alzheimer tau seeds is enhanced by aging and template matching with limited impact of amyloid-β

In Alzheimer''s disease (AD), deposition of pathological tau and amyloid-β (Aβ) drive synaptic loss and cognitive decline. The injection of misfolded tau aggregates extracted from human AD brains drives templated spreading of tau pathology within WT mouse brain. Here, we assessed the impact of Aβ copathology, of deleting loci known to modify AD risk (Ptk2b, Grn, and Tmem106b) and of pharmacological intervention with an Fyn kinase inhibitor on tau spreading after injection of AD tau extracts. The density and spreading of tau inclusions triggered by human tau seed were unaltered in the hippocampus and cortex of APPswe/PSEN1ΔE9 transgenic and App^NL-F/NL-F knock-in mice. In mice with human tau sequence replacing mouse tau, template matching enhanced neuritic tau burden. Human AD brain tau-enriched preparations contained aggregated Aβ, and the Aβ coinjection caused a redistribution of Aβ aggregates in mutant AD model mice. The injection-induced Aβ phenotype was spatially distinct from tau accumulation and could be ameliorated by depleting Aβ from tau extracts. These data suggest that Aβ and tau pathologies propagate by largely independent mechanisms after their initial formation. Altering the activity of the Fyn and Pyk2 (Ptk2b) kinases involved in Aβ-oligomer–induced signaling, or deleting expression of the progranulin and TMEM106B lysosomal proteins, did not alter the somatic tau inclusion burden or spreading. However, mouse aging had a prominent effect to increase the accumulation of neuritic tau after injection of human AD tau seeds into WT mice. These studies refine our knowledge of factors capable of modulating tau spreading.     

Cross-seeding of WT amyloid-β with Arctic but not Italian familial mutants accelerates fibril formation in Alzheimer's disease

Alzheimer’s disease (AD) involves the neurotoxic self-assembly of a 40 and 42 residue peptide, Amyloid-β (Aβ). Inherited early-onset AD can be caused by single point mutations within the Aβ sequence, including Arctic (E22G) and Italian (E22K) familial mutants. These mutations are heterozygous, resulting in an equal proportion of the WT and mutant Aβ isoform expression. It is therefore important to understand how these mixtures of Aβ isoforms interact with each other and influence the kinetics and morphology of their assembly into oligomers and fibrils. Using small amounts of nucleating fibril seeds, here, we systematically monitored the kinetics of fibril formation, comparing self-seeding with cross-seeding behavior of a range of isoform mixtures of Aβ42 and Aβ40. We confirm that Aβ40(WT) does not readily cross-seed Aβ42(WT) fibril formation. In contrast, fibril formation of Aβ40(Arctic) is hugely accelerated by Aβ42(WT) fibrils, causing an eight-fold reduction in the lag-time to fibrillization. We propose that cross-seeding between the more abundant Aβ40(Arctic) and Aβ42(WT) may be important for driving early-onset AD and will propagate fibril morphology as indicated by fibril twist periodicity. This kinetic behavior is not emulated by the Italian mutant, where minimal cross-seeding is observed. In addition, we studied the cross-seeding behavior of a C-terminal-amidated Aβ42 analog to probe the coulombic charge interplay between Glu22/Asp23/Lys28 and the C-terminal carboxylate. Overall, these studies highlight the role of cross-seeding between WT and mutant Aβ40/42 isoforms, which can impact the rate and structure of fibril assembly.     

The α5β1 Integrin Mediates Elimination of Amyloid-β Peptide and Protects Against Apoptosis

The amyloid-β peptide (Aβ) can mediate cell attachment by binding to β1 integrins through an arg-his-asp sequence. We show here that the α5β1 integrin, a fibronectin receptor, is an efficient binder of Aβ, and mediates cell attachment to nonfibrillar Aβ. Cells engineered to express α5β1 internalized and degraded more added Aβ1-40 than did α5β1-negative control cells. Deposition of an insoluble Aβ1-40 matrix around the α5β1-expressing cells was reduced, and the cells showed less apoptosis than the control cells. Thus, the α5β1 integrin may protect against Aβ deposition and toxicity, which is a course of Alzheimer's disease lesions.     

PSD95 Suppresses Dendritic Arbor Development in Mature Hippocampal Neurons by Occluding the Clustering of NR2B-NMDA Receptors

Considerable evidence indicates that the NMDA receptor (NMDAR) subunits NR2A and NR2B are critical mediators of synaptic plasticity and dendritogenesis; however, how they differentially regulate these processes is unclear. Here we investigate the roles of the NR2A and NR2B subunits, and of their scaffolding proteins PSD-95 and SAP102, in remodeling the dendritic architecture of developing hippocampal neurons (2–25 DIV). Analysis of the dendritic architecture and the temporal and spatial expression patterns of the NMDARs and anchoring proteins in immature cultures revealed a strong positive correlation between synaptic expression of the NR2B subunit and dendritogenesis. With maturation, the pruning of dendritic branches was paralleled by a strong reduction in overall and synaptic expression of NR2B, and a significant elevation in synaptic expression of NR2A and PSD95. Using constructs that alter the synaptic composition, we found that either over-expression of NR2B or knock-down of PSD95 by shRNA-PSD95 augmented dendritogenesis in immature neurons. Reactivation of dendritogenesis could also be achieved in mature cultured neurons, but required both manipulations simultaneously, and was accompanied by increased dendritic clustering of NR2B. Our results indicate that the developmental increase in synaptic expression of PSD95 obstructs the synaptic clustering of NR2B-NMDARs, and thereby restricts reactivation of dendritic branching. Experiments with shRNA-PSD95 and chimeric NR2A/NR2B constructs further revealed that C-terminus of the NR2B subunit (tail) was sufficient to induce robust dendritic branching in mature hippocampal neurons, and suggest that the NR2B tail is important in recruiting calcium-dependent signaling proteins and scaffolding proteins necessary for dendritogenesis.     

Accumulation of Intraneuronal β-Amyloid 42 Peptides Is Associated with Early Changes in Microtubule-Associated Protein 2 in Neurites and Synapses

Pathologic aggregation of β-amyloid (Aβ) peptide and the axonal microtubule-associated protein tau protein are hallmarks of Alzheimer''s disease (AD). Evidence supports that Aβ peptide accumulation precedes microtubule-related pathology, although the link between Aβ and tau remains unclear. We previously provided evidence for early co-localization of Aβ42 peptides and hyperphosphorylated tau within postsynaptic terminals of CA1 dendrites in the hippocampus of AD transgenic mice. Here, we explore the relation between Aβ peptide accumulation and the dendritic, microtubule-associated protein 2 (MAP2) in the well-characterized amyloid precursor protein Swedish mutant transgenic mouse (Tg2576). We provide evidence that localized intraneuronal accumulation of Aβ42 peptides is spatially associated with reductions of MAP2 in dendrites and postsynaptic compartments of Tg2576 mice at early ages. Our data support that reduction in MAP2 begins at sites of Aβ42 monomer and low molecular weight oligomer (M/LMW) peptide accumulation. Cumulative evidence suggests that accumulation of M/LMW Aβ42 peptides occurs early, before high molecular weight oligomerization and plaque formation. Since synaptic alteration is the best pathologic correlate of cognitive dysfunction in AD, the spatial association of M/LMW Aβ peptide accumulation with pathology of MAP2 within neuronal processes and synaptic compartments early in the disease process reinforces the importance of intraneuronal Aβ accumulation in AD pathogenesis.     

Neurotransmitter receptor and time dependence of the synaptic plasticity disrupting actions of Alzheimer's disease Aβ in vivo

Many endogenous factors influence the time course and extent of the detrimental effects of amyloid β-protein (Aβ) on synaptic function. Here, we assessed the impact of varying endogenous glutamatergic and cholinergic transmission by pharmacological means on the disruption of plasticity at hippocampal CA3-to-CA1 synapses in the anaesthetized rat. NMDA receptors (NMDARs) are considered critical in mediating Aβ-induced inhibition of long-term potentiation (LTP). However, intracerebroventricular injection of Aβ_1–42 inhibited not only NMDAR-dependent LTP but also voltage-activated Ca²⁺-dependent LTP induced by strong conditioning stimulation during NMDAR blockade. On the other hand, another form of NMDAR-independent synaptic plasticity, endogenous acetylcholine-induced muscarinic receptor-dependent long-term enhancement, was not hindered by Aβ_1–42. Interestingly, augmenting endogenous acetylcholine activation of nicotinic receptors prior to the injection of Aβ_1–42 prevented the inhibition of NMDAR-dependent LTP, whereas the same intervention when introduced after the infusion of Aβ was ineffective. We also examined the duration of action of Aβ, including water soluble Aβ from Alzheimer''s disease (AD) brain. Remarkably, the inhibition of LTP induction caused by a single injection of sodium dodecyl sulfate-stable Aβ dimer-containing AD brain extract persisted for at least a week. These findings highlight the need to increase our understanding of non-NMDAR mechanisms and of developing novel means of overcoming, rather than just preventing, the deleterious synaptic actions of Aβ.