Deleterious effects of amyloid beta oligomers acting as an extracellular scaffold for mGluR5

Neurotoxicity and memory deficits induced by soluble low-molecular-weight amyloid-β1-42 oligomers are revealed in vivo by using a novel animal model

Neuronal and synaptic degeneration are the best pathological correlates for memory decline in Alzheimer's disease (AD). Although the accumulation of soluble low-molecular-weight amyloid-β (Aβ) oligomers has been suggested to trigger neurodegeneration in AD, animal models overexpressing or infused with Aβ lack neuronal loss at the onset of memory deficits. Using a novel in vivo approach, we found that repeated hippocampal injections of small soluble Aβ(1-42) oligomers in awake, freely moving mice were able to induce marked neuronal loss, tau hyperphosphorylation, and deficits in hippocampus-dependent memory. The neurotoxicity of small Aβ(1-42) species was observed in vivo as well as in vitro in association with increased caspase-3 activity and reduced levels of the NMDA receptor subunit NR2B. We found that the sequestering agent transthyretin is able to bind the toxic Aβ(1-42) species and attenuated the loss of neurons and memory deficits. Our novel mouse model provides evidence that small, soluble Aβ(1-42) oligomers are able to induce extensive neuronal loss in vivo and initiate a cascade of events that mimic the key neuropathological hallmarks of AD.

Postsynaptic dysfunction is associated with spatial and object recognition memory loss in a natural model of Alzheimer's disease

Alzheimer's disease (AD) is an age-related neurodegenerative disorder associated with progressive memory loss, severe dementia, and hallmark neuropathological markers, such as deposition of amyloid-β (Aβ) peptides in senile plaques and accumulation of hyperphosphorylated tau proteins in neurofibrillary tangles. Recent evidence obtained from transgenic mouse models suggests that soluble, nonfibrillar Aβ oligomers may induce synaptic failure early in AD. Despite their undoubted value, these transgenic models rely on genetic manipulations that represent the inherited and familial, but not the most abundant, sporadic form of AD. A nontransgenic animal model that still develops hallmarks of AD would be an important step toward understanding how sporadic AD is initiated. Here we show that starting between 12 and 36 mo of age, the rodent Octodon degus naturally develops neuropathological signs of AD, such as accumulation of Aβ oligomers and phosphorylated tau proteins. Moreover, age-related changes in Aβ oligomers and tau phosphorylation levels are correlated with decreases in spatial and object recognition memory, postsynaptic function, and synaptic plasticity. These findings validate O. degus as a suitable natural model for studying how sporadic AD may be initiated.

Alzheimer amyloid-β oligomer bound to postsynaptic prion protein activates Fyn to impair neurons

Amyloid-beta (Aβ) oligomers are thought to trigger Alzheimer’s disease (AD) pathophysiology. Cellular Prion Protein (PrP^C) selectively binds oligomeric Aβ and can mediate AD-related phenotypes. Here, we examined the specificity, distribution and signaling from Aβ/PrP complexes, seeking to explain how they might alter the function of NMDA receptors in neurons. PrP^C is enriched in post-synaptic densities, and Aβ/PrP^C interaction leads to Fyn kinase activation. Soluble Aβ assemblies derived from human AD brain interact with PrP^C to activate Fyn. Aβ engagement of PrP^C/Fyn signaling yields phosphorylation of the NR2B subunit of NMDA-receptors, which is coupled to an initial increase and then loss of surface NMDA-receptors. Aβ-induced LDH release and dendritic spine loss require both PrP^C and Fyn, and human familial AD transgene-induced convulsive seizures do not occur in mice lacking PrP^C. These results delineate an Aβ oligomer signal transduction pathway requiring PrP^C and Fyn to alter synaptic function with relevance to 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.

Amyloid and Alzheimer's disease: inside and out

Alzheimer's disease (AD) is poised to become the most serious healthcare issue of our generation. The leading theory of AD pathophysiology is the Amyloid Cascade Hypothesis, and clinical trials are now proceeding based on this hypothesis. Here, we review the original evidence for the Amyloid Hypothesis, which was originally focused on the extracellular deposition of beta amyloid peptides (Aβ) in large fibrillar aggregates, as well as how this theory has been extended in recent years to focus on highly toxic small soluble amyloid oligomers. We will also examine emerging evidence that Aβ may actually begin to accumulate intracellularly in lysosomes, and the role for intracellular Aβ and lysosomal dysfunction may play in AD pathophysiology. Finally, we will review the clinical implications of these findings.

TRAIL death receptors DR4 and DR5 mediate cerebral microvascular endothelial cell apoptosis induced by oligomeric Alzheimer's Aβ

Vascular deposition of amyloid-β (Aβ) in sporadic and familial Alzheimer's disease, through poorly understood molecular mechanisms, leads to focal ischemia, alterations in cerebral blood flow, and cerebral micro-/macro-hemorrhages, significantly contributing to cognitive impairment. Here, we show that tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) death receptors DR4 and DR5 specifically mediate oligomeric Aβ induction of extrinsic apoptotic pathways in human microvascular cerebral endothelial cells with activation of both caspase-8 and caspase-9. The caspase-8 inhibitor cellular FLICE-like inhibitory protein (cFLIP) is downregulated, and mitochondrial paths are engaged through BH3-interacting domain death agonist (Bid) cleavage. Upregulation of DR4 and DR5 and colocalization with Aβ at the cell membrane suggests their involvement as initiators of the apoptotic machinery. Direct binding assays using receptor chimeras confirm the specific interaction of oligomeric Aβ with DR4 and DR5 whereas apoptosis protection achieved through RNA silencing of both receptors highlights their active role in downstream apoptotic pathways unveiling new targets for therapeutic intervention.

Alzheimer culprits: cellular crossroads and interplay

Alzheimer's disease (AD) is the primary cause of dementia in the elderly and one of the major health problems worldwide. Since its first description by Alois Alzheimer in 1907, noticeable but insufficient scientific comprehension of this complex pathology has been achieved. All the research that has been pursued takes origin from the identification of the pathological hallmarks in the forms of amyloid-β (Aβ) deposits (plaques), and aggregated hyperphosphorylated tau protein filaments (named neurofibrillary tangles). Since this discovery, many hypotheses have been proposed to explain the origin of the pathology. The "amyloid cascade hypothesis" is the most accredited theory. The mechanism suggested to be one of the initial causes of AD is an imbalance between the production and the clearance of Aβ peptides. Therefore, Amyloid Precursor Protein (APP) synthesis, trafficking and metabolism producing either the toxic Aβ peptide via the amyloidogenic pathway or the sAPPα fragment via the non amyloidogenic pathway have become appealing subjects of study. Being able to reduce the formation of the toxic Aβ peptides is obviously an immediate approach in the trial to prevent AD. The following review summarizes the most relevant discoveries in the field of the last decades.

Synapses and Alzheimer's disease

Alzheimer's disease (AD) is a major cause of dementia in the elderly. Pathologically, AD is characterized by the accumulation of insoluble aggregates of Aβ-peptides that are proteolytic cleavage products of the amyloid-β precursor protein ("plaques") and by insoluble filaments composed of hyperphosphorylated tau protein ("tangles"). Familial forms of AD often display increased production of Aβ peptides and/or altered activity of presenilins, the catalytic subunits of γ-secretase that produce Aβ peptides. Although the pathogenesis of AD remains unclear, recent studies have highlighted two major themes that are likely important. First, oligomeric Aβ species have strong detrimental effects on synapse function and structure, particularly on the postsynaptic side. Second, decreased presenilin function impairs synaptic transmission and promotes neurodegeneration. The mechanisms underlying these processes are beginning to be elucidated, and, although their relevance to AD remains debated, understanding these processes will likely allow new therapeutic avenues to AD.

Gephyrin-independent GABA(A)R mobility and clustering during plasticity

The activity-dependent modulation of GABA-A receptor (GABA(A)R) clustering at synapses controls inhibitory synaptic transmission. Several lines of evidence suggest that gephyrin, an inhibitory synaptic scaffold protein, is a critical factor in the regulation of GABA(A)R clustering during inhibitory synaptic plasticity induced by neuronal excitation. In this study, we tested this hypothesis by studying relative gephyrin dynamics and GABA(A)R declustering during excitatory activity. Surprisingly, we found that gephyrin dispersal is not essential for GABA(A)R declustering during excitatory activity. In cultured hippocampal neurons, quantitative immunocytochemistry showed that the dispersal of synaptic GABA(A)Rs accompanied with neuronal excitation evoked by 4-aminopyridine (4AP) or N-methyl-D-aspartic acid (NMDA) precedes that of gephyrin. Single-particle tracking of quantum dot labeled-GABA(A)Rs revealed that excitation-induced enhancement of GABA(A)R lateral mobility also occurred before the shrinkage of gephyrin clusters. Physical inhibition of GABA(A)R lateral diffusion on the cell surface and inhibition of a Ca(2+) dependent phosphatase, calcineurin, completely eliminated the 4AP-induced decrease in gephyrin cluster size, but not the NMDA-induced decrease in cluster size, suggesting the existence of two different mechanisms of gephyrin declustering during activity-dependent plasticity, a GABA(A)R-dependent regulatory mechanism and a GABA(A)R-independent one. Our results also indicate that GABA(A)R mobility and clustering after sustained excitatory activity is independent of gephyrin.

Inhibition of choline acetyltransferase as a mechanism for cholinergic dysfunction induced by amyloid-β peptide oligomers

Dysregulated cholinergic signaling is an early hallmark of Alzheimer disease (AD), usually ascribed to degeneration of cholinergic neurons induced by the amyloid-β peptide (Aβ). It is now generally accepted that neuronal dysfunction and memory deficits in the early stages of AD are caused by the neuronal impact of soluble Aβ oligomers (AβOs). AβOs build up in AD brain and specifically attach to excitatory synapses, leading to synapse dysfunction. Here, we have investigated the possibility that AβOs could impact cholinergic signaling. The activity of choline acetyltransferase (ChAT, the enzyme that carries out ACh production) was inhibited by ~50% in cultured cholinergic neurons exposed to low nanomolar concentrations of AβOs. 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) reduction, lactate dehydrogenase release, and [(3)H]choline uptake assays showed no evidence of neuronal damage or loss of viability that could account for reduced ChAT activity under these conditions. Glutamate receptor antagonists fully blocked ChAT inhibition and oxidative stress induced by AβOs. Antioxidant polyunsaturated fatty acids had similar effects, indicating that oxidative damage may be involved in ChAT inhibition. Treatment with insulin, previously shown to down-regulate neuronal AβO binding sites, fully prevented AβO-induced inhibition of ChAT. Interestingly, we found that AβOs selectively bind to ~50% of cultured cholinergic neurons, suggesting that ChAT is fully inhibited in AβO-targeted neurons. Reduction in ChAT activity instigated by AβOs may thus be a relevant event in early stage AD pathology, preceding the loss of cholinergic neurons commonly observed in AD brains.

Receptor-selective diffusion barrier enhances sensitivity of astrocytic processes to metabotropic glutamate receptor stimulation

Metabotropic glutamate receptor (mGluR)-dependent calcium ion (Ca²+) signaling in astrocytic processes regulates synaptic transmission and local blood flow essential for brain function. However, because of difficulties in imaging astrocytic processes, the subcellular spatial organization of mGluR-dependent Ca²+ signaling is not well characterized and its regulatory mechanism remains unclear. Using genetically encoded Ca²+ indicators, we showed that despite global stimulation by an mGluR agonist, astrocyte processes intrinsically exhibited a marked enrichment of Ca²+ responses. Immunocytochemistry indicated that these polarized Ca²+ responses could be attributed to increased density of surface mGluR5 on processes relative to the soma. Single-particle tracking of surface mGluR5 dynamics revealed a membrane barrier that blocked the movement of mGluR5 between the processes and the soma. Overexpression of mGluR or expression of its carboxyl terminus enabled diffusion of mGluR5 between the soma and the processes, disrupting the polarization of mGluR5 and of mGluR-dependent Ca²+ signaling. Together, our results demonstrate an mGluR5-selective diffusion barrier between processes and soma that compartmentalized mGluR Ca²+ signaling in astrocytes and may allow control of synaptic and vascular activity in specific subcellular domains.

Alzheimer mechanisms and therapeutic strategies

There are still no effective treatments to prevent, halt, or reverse Alzheimer's disease, but research advances over the past three decades could change this gloomy picture. Genetic studies demonstrate that the disease has multiple causes. Interdisciplinary approaches combining biochemistry, molecular and cell biology, and transgenic modeling have revealed some of its molecular mechanisms. Progress in chemistry, radiology, and systems biology is beginning to provide useful biomarkers, and the emergence of personalized medicine is poised to transform pharmaceutical development and clinical trials. However, investigative and drug development efforts should be diversified to fully address the multifactoriality of the disease.

The levels of water-soluble and triton-soluble Aβ are increased in Alzheimer's disease brain

Although plaques composed of the amyloid β-protein (Aβ) are considered a defining feature of Alzheimer's disease (AD), they are also found in cognitively normal individuals and extensive evidence suggests that non-plaque, water-soluble forms of Aβ may play a role in AD pathogenesis. However, the relationship between the levels of water-soluble Aβ and the clinical severity of disease has never been investigated. Here, we present results of a pilot study designed to examine the levels of water-soluble forms of Aβ in brains of individuals who died at clinically distinct stages of AD. Using a serial extraction method, we also investigated the levels of triton-soluble and formic acid-soluble Aβ. We found that water-soluble and detergent-soluble Aβ monomer and SDS-stable dimer were elevated in AD and that the levels of water soluble Aβ did not increase with plaque pathology. These results support the notion that both water- and detergent-soluble Aβ are important in AD and are not simply released from plaques by mechanical disruption. Moreover, the fact that the levels of water- and triton-soluble Aβ were similar in very mild/mild AD and moderate/severe AD suggests that once a certain level of these species is attained, further accumulation is not necessary for the disease to progress. Consequently, therapeutic targeting of water-soluble Aβ should best benefit individuals in earliest phases of the disease process.

Intraneuronally injected amyloid β inhibits long-term potentiation in rat hippocampal slices

Extracellular accumulation of amyloid beta (Aβ) is a hallmark of Alzheimer's disease (AD). It has been reported that extracellular perfusion of Aβ inhibits long-term potentiation (LTP), which is strongly related to memory in animal models. However, it has recently been proposed that intracellular Aβ may be the first pathological change to occur in AD. Here, we have investigated the effect on LTP of intracellular injection of Aβ (Aβ(1-40), Aβ(1-42)) into hippocampal pyramidal cells using patch-clamp technique. We found that injection of 1 nM Aβ(1-42) completely blocked LTP, and extracellular perfusion of a p38 MAPK inhibitor or a metabotropic glutamate receptor blocker reversed these blocking effects on LTP. Furthermore, we have examined the effects of different concentrations of Aβ(1-40) and Aβ(1-42) on LTP and showed that Aβ(1-40) required a 1,000-fold higher concentration to attenuate LTP than 1 nM Aβ(1-42). These results indicate that LTP is impaired by Aβ injected into genetically wild-type neurons in the sliced hippocampus, suggesting an acute action of intracellular Aβ on the intracellular LTP-inducing machinery.

Structures of Aβ17-42 trimers in isolation and with five small-molecule drugs using a hierarchical computational procedure

The amyloid-β protein (Aβ) oligomers are believed to be the main culprits in the cytoxicity of Alzheimer's disease (AD) and p3 peptides (Aβ17-42 fragments) are present in AD amyloid plaques. Many small-molecule or peptide-based inhibitors are known to slow down Aβ aggregation and reduce the toxicity in vitro, but their exact modes of action remain to be determined since there has been no atomic level of Aβ(p3)-drug oligomers. In this study, we have determined the structure of Aβ17-42 trimers both in aqueous solution and in the presence of five small-molecule inhibitors using a multiscale computational study. These inhibitors include 2002-H20, curcumin, EGCG, Nqtrp, and resveratrol. First, we used replica exchange molecular dynamics simulations coupled to the coarse-grained (CG) OPEP force field. These CG simulations reveal that the conformational ensemble of Aβ17-42 trimer can be described by 14 clusters with each peptide essentially adopting turn/random coil configurations, although the most populated cluster is characterized by one peptide with a β-hairpin at Phe19-Leu31. Second, these 14 dominant clusters and the less-frequent fibril-like state with parallel register of the peptides were subjected to atomistic Autodock simulations. Our analysis reveals that the drugs have multiple binding modes with different binding affinities for trimeric Aβ17-42 although they interact preferentially with the CHC region (residues 17-21). The compounds 2002-H20 and Nqtrp are found to be the worst and best binders, respectively, suggesting that the drugs may interfere at different stages of Aβ oligomerization. Finally, explicit solvent molecular dynamics of two predicted Nqtrp-Aβ17-42 conformations describe at atomic level some possible modes of action for Nqtrp.

Neuronal receptors as targets for the action of amyloid-beta protein (Aβ) in the brain

Accumulation of neurotoxic soluble amyloid-beta protein (Aβ) oligomers in the brains of patients with Alzheimer disease (AD) and their role in AD pathogenesis have emerged as topics of considerable interest in recent years. Soluble Aβ oligomers impair synaptic and neuronal function, leading to neurodegeneration that is clinically manifested by memory and cognitive dysfunction. The precise mechanisms whereby Aβ oligomers cause neurotoxicity remain unknown. Emerging insights into the mechanistic link between neuronal receptors and soluble Aβ oligomers highlight the potential role of these receptors in Aβ-mediated neurotoxicity in AD. The current review focuses on studies describing interactions between soluble Aβ oligomers and neuronal receptors, and their role in AD pathogenesis. Furthermore, these studies provide insight into potential therapies for AD using compounds directed at putative target receptors for the action of Aβ in the central nervous system. We focus on interactions of Aβ with subtypes of acetylcholine and glutamatergic receptors. Additionally, neuronal receptors such as insulin, amylin and receptor for advanced glycation end products could be potential targets for soluble Aβ-oligomer-mediated neurotoxicity. Aβ interactions with other receptors such as the p75 neurotrophin receptors, which are highly expressed on cholinergic basal forebrain neurons lost in AD, are also highlighted.

NMDA receptors and BAX are essential for Aβ impairment of LTP

Accumulation of amyloid-β (Aβ) is a hallmark of Alzheimer’s disease, a neurodegenerative disorder in which synapse loss and dysfunction are early features. Acute exposure of hippocampal slices to Aβ leads to changes in synaptic plasticity, specifically reduced long-term potentiation (LTP) and enhanced long-term depression (LTD), with no change in basal synaptic transmission. We also report here that D-AP5, a non-selective NMDA receptor antagonist, completely prevented Aβ-mediated inhibition of LTP in area CA1 of the hippocampus. Ro25-6981, an antagonist selective for GluN2B (NR2B) NMDA receptors, only partially prevented this Aβ action, suggesting that GluN2A and GluN2B receptors may both contribute to Aβ suppression of LTP. The effect of Aβ on LTP was also examined in hippocampal slices from BAX −/− mice and wild-type littermates. Aβ failed to block LTP in hippocampal slices from BAX −/− mice, indicating that BAX is essential for Aβ inhibition of LTP.

Successful cognitive aging in rats: a role for mGluR5 glutamate receptors, homer 1 proteins and downstream signaling pathways

Normal aging is associated with impairments in cognition, especially learning and memory. However, major individual differences are known to exist. Using the classical Morris Water Maze (MWM) task, we discriminated a population of 24-months old Long Evans aged rats in two groups--memory-impaired (AI) and memory-unimpaired (AU) in comparison with 6-months old adult animals. AI rats presented deficits in learning, reverse memory and retention. At the molecular level, an increase in metabotropic glutamate receptors 5 (mGluR5) was observed in post-synaptic densities (PSD) in the hippocampus of AU rats after training. Scaffolding Homer 1b/c proteins binding to group 1 mGluR facilitate coupling with its signaling effectors while Homer 1a reduces it. Both Homer 1a and 1b/c levels were up-regulated in the hippocampus PSD of AU animals following MWM task. Using immunohistochemistry we further demonstrated that mGluR5 as well as Homer 1b/c stainings were enhanced in the CA1 hippocampus sub-field of AU animals. In fact mGluR5 and Homer 1 isoforms were more abundant and co-localized in the hippocampal dendrites in AU rats. However, the ratio of Homer 1a/Homer 1b/c bound to mGluR5 in the PSD was four times lower for AU animals compared to AI rats. Consequently, AU animals presented higher PKCγ, ERK, p70S6K, mTOR and CREB activation. Finally the expression of immediate early gene Arc/Arg3.1 was shown to be higher in AU rats in accordance with its role in spatial memory consolidation. On the basis of these results, a model of successful cognitive aging with a critical role for mGluR5, Homer 1 proteins and downstream signalling pathways is proposed here.

Natural amyloid-β oligomers acutely impair the formation of a contextual fear memory in mice

Memory loss is one of the hallmark symptoms of Alzheimer's disease (AD). It has been proposed that soluble amyloid-beta (Abeta) oligomers acutely impair neuronal function and thereby memory. We here report that natural Abeta oligomers acutely impair contextual fear memory in mice. A natural Abeta oligomer solution containing Abeta monomers, dimers, trimers, and tetramers was derived from the conditioned medium of 7PA2 cells, a cell line that expresses human amyloid precursor protein containing the Val717Phe familial AD mutation. As a control we used 7PA2 conditioned medium from which Abeta oligomers were removed through immunodepletion. Separate groups of mice were injected with Abeta and control solutions through a cannula into the lateral brain ventricle, and subjected to fear conditioning using two tone-shock pairings. One day after fear conditioning, mice were tested for contextual fear memory and tone fear memory in separate retrieval trials. Three experiments were performed. For experiment 1, mice were injected three times: 1 hour before and 3 hours after fear conditioning, and 1 hour before context retrieval. For experiments 2 and 3, mice were injected a single time at 1 hour and 2 hours before fear conditioning respectively. In all three experiments there was no effect on tone fear memory. Injection of Abeta 1 hour before fear conditioning, but not 2 hours before fear conditioning, impaired the formation of a contextual fear memory. In future studies, the acute effect of natural Abeta oligomers on contextual fear memory can be used to identify potential mechanisms and treatments of AD associated memory loss.

Synaptic dysfunction in Alzheimer's disease

Generation of amyloid peptide (Aβ) is at the beginning of a cascade that leads to Alzheimer's disease (AD). Amyloid precursor protein (APP), as well as β- and γ-secretases, is the principal player involved in Aβ production, while α-secretase cleavage on APP prevents Aβ deposition. Recent studies suggested that soluble assembly states of Aβ peptides can cause cognitive problems by disrupting synaptic function in the absence of significant neurodegeneration. Therefore, current research investigates the relative importance of these various soluble Aβ assemblies in causing synaptic dysfunction and cognitive deficits. Several Aβ oligomers targets and cellular mechanisms responsible of Aβ-induced synaptic failure have been identified. The first and most important mechanism impugns a toxic gain of function for Aβ which results due to self-association and attainment of new structures capable of novel interactions that lead to impaired plasticity. Other scenarios predicate that Aβ has a normal physiological role. On the one hand, insufficient Aβ could lead to a loss of normal function, whereas excess Aβ may precipitate dysfunction. How this occurs and which the main target/s is/are for the synaptic action of Aβ remains to be fully understood and would certainly represent one of the main challenges to future AD research.

Beyond amyloid: the future of therapeutics for Alzheimer's disease

Currently, the field is awaiting the results of several pivotal Phase III clinical Alzheimer's disease (AD) trials that target amyloid-β (Aβ). In light of the recent biomarker studies that indicate Aβ levels are at their most dynamic 5-10 years before the onset of clinical symptoms, it is becoming uncertain whether direct approaches to target Aβ will achieve desired clinical efficacy. AD is a complex neurodegenerative disease caused by dysregulation of numerous neurobiological networks and cellular functions, resulting in synaptic loss, neuronal loss, and ultimately impaired memory. While it is clear that Aβ plays a key role in the pathogenesis of AD, it may be a challenging and inefficient target for mid-to-late stage AD intervention. Throughout the course of AD, multiple pathways become perturbed, presenting a multitude of possible therapeutic avenues for design of AD intervention and prophylactic therapies. In this chapter, we sought to first provide an overview of Aβ-directed strategies that are currently in development, and the pivotal Aβ-targeted trials that are currently underway. Next, we delve into the biology and therapeutic designs associated with other key areas of research in the field including tau, protein trafficking and degradation pathways, ApoE, synaptic function, neurotrophic/neuroprotective strategies, and inflammation and energy utilization. For each area we have provided a comprehensive and balanced overview of the therapeutic strategies currently in preclinical and clinical development, which will shape the future therapeutic landscape of AD.

Surface traffic in synaptic membranes

The precision of signal transmission in chemical synapses is highly dependent on the structural alignment between pre- and postsynaptic components. The thermal agitation of transmembrane signaling molecules by surrounding lipid molecules and activity-driven changes in the local protein interaction affinities indicate a dynamic molecular traffic of molecules within synapses. The observation of local protein surface dynamics starts to be a useful tool to determine the contribution of intracellular and extracellular structures in organizing a plastic synapse. Local rearrangements by lateral diffusion in the synaptic and perisynaptic membrane induce fast density changes of signaling molecules and enable the synapse to change efficacy in short time scales. The degree of lateral mobility is restricted by many passive and active interactions inside and outside the membrane. AMPAR at the glutamatergic synapse are the best explored receptors in this respect and reviewed here as an example molecule. In addition, transsynaptic adhesion molecule complexes also appear highly dynamically in the synapse and do further support the importance of local surface traffic in subcellular compartments like synapses.

Diversity of metabotropic glutamate receptor-interacting proteins and pathophysiological functions

In the mammalian brain, the large majority of excitatory synapses express pre- and postsynaptic glutamate receptors. These are ion channels and G protein-coupled membrane proteins that are organized into functional signaling complexes. Here, we will review the nature and pathophysiological functions of the scaffolding proteins associated to these receptors, focusing on the G protein-coupled subtypes.

Soluble Aβ oligomer production and toxicity

For nearly 100 years following the first description of this neurological disorder by Dr Alois Alzheimer, amyloid plaques and neurofibrillary tangles have been hypothesized to cause neuronal loss. With evidence that the extent of insoluble, deposited amyloid poorly correlated with cognitive impairment, research efforts focused on soluble forms of Aβ, also referred as Aβ oligomers. Following a decade of studies, soluble oligomeric forms of Aβ are now believed to induce the deleterious cascade(s) involved in the pathophysiology of Alzheimer's disease. In this review, we will discuss our current understanding about endogenous oligomeric Aβ production, their relative toxicity in vivo and in vitro, and explore the potential future directions needed for the field.

Cellular prion protein is essential for oligomeric amyloid-β-induced neuronal cell death

In Alzheimer disease (AD), amyloid-β (Aβ) oligomer is suggested to play a critical role in imitating neurodegeneration, although its pathogenic mechanism remains to be determined. Recently, the cellular prion protein (PrP(C)) has been reported to be an essential co-factor in mediating the neurotoxic effect of Aβ oligomer. However, these previous studies focused on the synaptic plasticity in either the presence or the absence of PrP(C) and no study to date has reported whether PrP(C) is required for the neuronal cell death, the most critical element of neurodegeneration in AD. Here, we show that Prnp(-/-) mice are resistant to the neurotoxic effect of Aβ oligomer in vivo and in vitro. Furthermore, application of an anti-PrP(C) antibody or PrP(C) peptide prevents Aβ oligomer-induced neurotoxicity. These findings are the first to demonstrate that PrP(C) is required for Aβ oligomer-induced neuronal cell death, the pathology essential to cognitive loss.

The Aβ oligomer hypothesis for synapse failure and memory loss in Alzheimer's disease

Alzheimer's disease (AD) is the 3rd most costly disease and the leading cause of dementia. It can linger for many years, but ultimately is fatal, the 6th leading cause of death. Alzheimer's disease (AD) is fatal and affected individuals can sometimes linger many years. Current treatments are palliative and transient, not disease modifying. This article reviews progress in the search to identify the primary AD-causing toxins. We summarize the shift from an initial focus on amyloid plaques to the contemporary concept that AD memory failure is caused by small soluble oligomers of the Aβ peptide, toxins that target and disrupt particular synapses. Evidence is presented that links Aβ oligomers to pathogenesis in animal models and humans, with reference to seminal discoveries from cell biology and new ideas concerning pathogenic mechanisms, including relationships to diabetes and Fragile X. These findings have established the oligomer hypothesis as a new molecular basis for the cause, diagnosis, and treatment of AD.

Reversal of fragile X phenotypes by manipulation of AβPP/Aβ levels in Fmr1KO mice

Fragile X syndrome (FXS) is the most common form of inherited intellectual disability and the leading known genetic cause of autism. Fragile X mental retardation protein (FMRP), which is absent or expressed at substantially reduced levels in FXS, binds to and controls the postsynaptic translation of amyloid β-protein precursor (AβPP) mRNA. Cleavage of AβPP can produce β-amyloid (Aβ), a 39-43 amino acid peptide mis-expressed in Alzheimer's disease (AD) and Down syndrome (DS). Aβ is over-expressed in the brain of Fmr1(KO) mice, suggesting a pathogenic role in FXS. To determine if genetic reduction of AβPP/Aβ rescues characteristic FXS phenotypes, we assessed audiogenic seizures (AGS), anxiety, the ratio of mature versus immature dendritic spines and metabotropic glutamate receptor (mGluR)-mediated long-term depression (LTD) in Fmr1(KO) mice after removal of one App allele. All of these phenotypes were partially or completely reverted to normal. Plasma Aβ(1-42) was significantly reduced in full-mutation FXS males compared to age-matched controls while cortical and hippocampal levels were somewhat increased, suggesting that Aβ is sequestered in the brain. Evolving therapies directed at reducing Aβ in AD may be applicable to FXS and Aβ may serve as a plasma-based biomarker to facilitate disease diagnosis or assess therapeutic efficacy.

Ablation of cellular prion protein does not ameliorate abnormal neural network activity or cognitive dysfunction in the J20 line of human amyloid precursor protein transgenic mice

Previous studies suggested that the cellular prion protein (PrP(c)) plays a critical role in the pathogenesis of Alzheimer's disease (AD). Specifically, amyloid-β (Aβ) oligomers were proposed to cause synaptic and cognitive dysfunction by binding to PrP(c). To test this hypothesis, we crossed human amyloid precursor protein (hAPP) transgenic mice from line J20 onto a PrP(c)-deficient background. Ablation of PrP(c) did not prevent the premature mortality and abnormal neural network activity typically seen in hAPPJ20 mice. Furthermore, hAPPJ20 mice with or without PrP(c) expression showed comparably robust abnormalities in learning and memory and in other behavioral domains at 6-8 months of age. Notably, these abnormalities are not refractory to therapeutic manipulations in general: they can be effectively prevented by interventions that prevent Aβ-dependent neuronal dysfunction also in other lines of hAPP transgenic mice. Thus, at least in this model, PrP(c) is not an important mediator of Aβ-induced neurological impairments.

Amyloid-β-induced synapse damage is mediated via cross-linkage of cellular prion proteins

The cellular prion protein (PrP(C)), which is highly expressed at synapses, was identified as a receptor for the amyloid-β (Aβ) oligomers that are associated with dementia in Alzheimer disease. Here, we report that Aβ oligomers secreted by 7PA2 cells caused synapse damage in cultured neurons via a PrP(C)-dependent process. Exogenous PrP(C) added to Prnp knock-out((0/0)) neurons was targeted to synapses and significantly increased Aβ-induced synapse damage. In contrast, the synapse damage induced by a phospholipase A(2)-activating peptide was independent of PrP(C). In Prnp wild-type((+/+)) neurons Aβ oligomers activated synaptic cytoplasmic phospholipase A(2) (cPLA(2)). In these cells, the addition of Aβ oligomers triggered the translocation of cPLA(2) in synapses to cholesterol dense membranes (lipid rafts) where it formed a complex also containing Aβ and PrP(C). In contrast, the addition of Aβ to Prnp((0/0)) neurons did not activate synaptic cPLA(2), which remained in the cytoplasm and was not associated with Aβ. Filtration assays and non-denaturing gels demonstrated that Aβ oligomers cross-link PrP(C). We propose that it is the cross-linkage of PrP(C) by Aβ oligomers that triggers abnormal activation of cPLA(2) and synapse damage. This hypothesis was supported by our observation that monoclonal antibody mediated cross-linkage of PrP(C) also activated synaptic cPLA(2) and caused synapse damage.

[Alzheimer's disease, amyloid peptide and synaptic dysfunction]

Alzheimer's disease (AD) is the first cause of dementia that leads to insidious and progressive loss of memory and cognitive functions. In the early stages of AD, there is a strong correlation between memory impairment and cortical levels of soluble amyloid-β peptide oligomers (Aβ). It has become clear that Aβ disrupt glutamatergic synaptic function, which in turn may lead to the characteristic cognitive deficits. Conversely, experiments in rodents have conforted the notion that Aβo impairs synaptic transmission and plasticity, and that mouse models with increased production of these oligomers display cognitive impairment. Many studies have attempted to determine the mechanisms by which Aβo disrupt synaptic plasticity and mediate their detrimental effect, but the actual pathways are still poorly understood. Here we review this thriving area of research which aims at understanding the mechanisms of synaptic dysfunction in the early phase of AD, and its consequences on the activity of neural circuits.

Alzheimer's disease: synapses gone cold

Alzheimer's disease (AD) is a progressive neurodegenerative disease characterized by insidious cognitive decline and memory dysfunction. Synapse loss is the best pathological correlate of cognitive decline in AD and mounting evidence suggests that AD is primarily a disease of synaptic dysfunction. Soluble oligomeric forms of amyloid beta (Aβ), the peptide that aggregates to form senile plaques in the brain of AD patients, have been shown to be toxic to neuronal synapses both in vitro and in vivo. Aβ oligomers inhibit long-term potentiation (LTP) and facilitate long-term depression (LTD), electrophysiological correlates of memory formation. Furthermore, oligomeric Aβ has also been shown to induce synapse loss and cognitive impairment in animals. The molecular underpinnings of these observations are now being elucidated, and may provide clear therapeutic targets for effectively treating the disease. Here, we review recent findings concerning AD pathogenesis with a particular focus on how Aβ impacts synapses.

A glycine zipper motif mediates the formation of toxic β-amyloid oligomers in vitro and in vivo

Background

The β-amyloid peptide (Aβ) contains a Gly-XXX-Gly-XXX-Gly motif in its C-terminal region that has been proposed to form a "glycine zipper" that drives the formation of toxic Aβ oligomers. We have tested this hypothesis by examining the toxicity of Aβ variants containing substitutions in this motif using a neuronal cell line, primary neurons, and a transgenic C. elegans model.

Results

We found that a Gly37Leu substitution dramatically reduced Aβ toxicity in all models tested, as measured by cell dysfunction, cell death, synaptic alteration, or tau phosphorylation. We also demonstrated in multiple models that Aβ Gly37Leu is actually anti-toxic, thereby supporting the hypothesis that interference with glycine zipper formation blocks assembly of toxic Aβ oligomers. To test this model rigorously, we engineered second site substitutions in Aβ predicted by the glycine zipper model to compensate for the Gly37Leu substitution and expressed these in C. elegans. We show that these second site substitutions restore in vivo Aβtoxicity, further supporting the glycine zipper model.

Conclusions

Our structure/function studies support the view that the glycine zipper motif present in the C-terminal portion of Aβ plays an important role in the formation of toxic Aβ oligomers. Compounds designed to interfere specifically with formation of the glycine zipper could have therapeutic potential.

Alzheimer's disease brain-derived amyloid-β-mediated inhibition of LTP in vivo is prevented by immunotargeting cellular prion protein

Synthetic amyloid-β protein (Aβ) oligomers bind with high affinity to cellular prion protein (PrP(C)), but the role of this interaction in mediating the disruption of synaptic plasticity by such soluble Aβ in vitro is controversial. Here we report that intracerebroventricular injection of Aβ-containing aqueous extracts of Alzheimer's disease (AD) brain robustly inhibits long-term potentiation (LTP) without significantly affecting baseline excitatory synaptic transmission in the rat hippocampus in vivo. Moreover, the disruption of LTP was abrogated by immunodepletion of Aβ. Importantly, intracerebroventricular administration of antigen-binding antibody fragment D13, directed to a putative Aβ-binding site on PrP(C), prevented the inhibition of LTP by AD brain-derived Aβ. In contrast, R1, a Fab directed to the C terminus of PrP(C), a region not implicated in binding of Aβ, did not significantly affect the Aβ-mediated inhibition of LTP. These data support the pathophysiological significance of SDS-stable Aβ dimer and the role of PrP(C) in mediating synaptic plasticity disruption by soluble Aβ.

Endoplasmic reticulum Ca(2+) handling in excitable cells in health and disease

The endoplasmic reticulum (ER) is a morphologically and functionally diverse organelle capable of integrating multiple extracellular and internal signals and generating adaptive cellular responses. It plays fundamental roles in protein synthesis and folding and in cellular responses to metabolic and proteotoxic stress. In addition, the ER stores and releases Ca(2+) in sophisticated scenarios that regulate a range of processes in excitable cells throughout the body, including muscle contraction and relaxation, endocrine regulation of metabolism, learning and memory, and cell death. One or more Ca(2+) ATPases and two types of ER membrane Ca(2+) channels (inositol trisphosphate and ryanodine receptors) are the major proteins involved in ER Ca(2+) uptake and release, respectively. There are also direct and indirect interactions of ER Ca(2+) stores with plasma membrane and mitochondrial Ca(2+)-regulating systems. Pharmacological agents that selectively modify ER Ca(2+) release or uptake have enabled studies that revealed many different physiological roles for ER Ca(2+) signaling. Several inherited diseases are caused by mutations in ER Ca(2+)-regulating proteins, and perturbed ER Ca(2+) homeostasis is implicated in a range of acquired disorders. Preclinical investigations suggest a therapeutic potential for use of agents that target ER Ca(2+) handling systems of excitable cells in disorders ranging from cardiac arrhythmias and skeletal muscle myopathies to Alzheimer disease.

Structural and mechanistic commonalities of amyloid-β and the prion protein

Amyloid β (Aβ) is a major causative agent of Alzheime disease. This neurotoxic peptide is generated as a result of the cleavage of the Amyloid-Precursor-Protein (APP) by the action of beta secretase and gamma secretase. The neurotoxicity was previously thought to be the result of aggregation. However, recent studies suggest that the interaction of Aβ with numerous cell surface receptors such as N-methyl-D-aspartate (NMDA), receptor for advanced glycosylation end products (RAGE), P75 neurotrophin receptor (P75NTR) as well as cell surface proteins such as the cellular prion protein (PrP(c) ) and heparan sulfate proteoglycans (HSPG) strongly enhances Aβ induced apoptosis and thereby contributes to neurotoxicity. This review focuses on the molecular mechanism resulting in Aβ-shedding as well as Aβ-induced apoptotic processes, genetic risk factors for familial Alzheimer disease and interactions of Aβ with cell surface receptors and proteins, with particular emphasis on the cellular prion protein. Furthermore, comparisons are drawn between Alzheimer disease and prion disorders and the role of laminin, an extracellular matrix protein, glycosaminoglycans and the 37 kDa/67 kDa laminin receptor (LRP/LR) have been highlighted with regards to both neurodegenerative diseases.

Addressing the complex etiology of Alzheimer’s disease: the role of p25/Cdk5

Alzheimer’s disease (AD) is an age-related neurodegenerative disorder characterized by the progressive loss of forebrain neurons and the deterioration of learning and memory. Therapies for AD have primarily focused upon either the inhibition of amyloid synthesis or its deposition in the brain, but clinical testing to date has not yet found an effective amelioration of cognitive symptoms. Synaptic loss closely correlates with the degree of dementia in AD patients. However, mouse AD models that target the amyloid-β pathway generally do not exhibit a profound loss of synapses, despite extensive synaptic dysfunction. The increased generation of p25, an activator of the cyclin-dependent kinase 5 (Cdk5) has been found in both human patients and mouse models of neurodegeneration. The current work reviews our knowledge, to date, on the role of p25/Cdk5 in Alzheimer’s disease, with a focus upon the interaction of amyloid-β and p25/Cdk5 in synaptic dysfunction and neuronal loss.

Impaired regulation of synaptic actin cytoskeleton in Alzheimer's disease

Representing the most common cause of dementia, Alzheimer's disease (AD) has dramatically impacted the neurological and economic health of our society. AD is a debilitating neurodegenerative disease that produces marked cognitive decline. Much evidence has accumulated over the past decade to suggest soluble oligomers of beta-amyloid (Aβ) have a critical role in mediating AD pathology early in the disease process by perturbing synaptic efficacy. Here we critically review recent research that implicates synapses as key sites of early pathogenesis in AD. Most excitatory synapses in the brain rely on dendritic spines as the sites for excitatory neurotransmission. The structure and function of dendritic spines are dynamically regulated by cellular pathways acting on the actin cytoskeleton. Numerous studies analyzing human postmortem tissue, animal models and cellular paradigms indicate that AD pathology has a deleterious effect on the pathways governing actin cytoskeleton stability. Based on the available evidence, we propose the idea that a contributing factor to synaptic pathology in early AD is an Aβ oligomer-initiated collapse of a "synaptic safety net" in spines, leading to dendritic spine degeneration and synaptic dysfunction. Spine stabilizing pathways may thus represent efficacious therapeutic targets for combating AD pathology.

Different β-amyloid oligomer assemblies in Alzheimer brains correlate with age of disease onset and impaired cholinergic activity

In this study, we examined the relationship between various β-amyloid (Aβ) oligomer assemblies in autopsy brain with the levels of fibrillar Aβ and cholinergic synaptic function. Brain tissues obtained from the frontal cortex of 14 Alzheimer's disease (AD) patients grouped into early-onset AD (EOAD) and late-onset AD (LOAD) and 12 age-matched control subjects were used to extract and quantify Aβ oligomers in soluble (TBS), detergent soluble (TBST), and insoluble (GuHCl) fractions. The predominant oligomeric Aβ assemblies detected were dodecamers, decamers, and pentamers, and different patterns of expression were observed between EOAD and LOAD patients. There was no association between any of the detected Aβ oligomer assemblies and fibrillar Aβ levels measured by N-methyl[(3)H] 2-(40-methylaminophenyl)-6-hydroxy-benzothiazole ([(3)H]PIB) binding. Levels of pentamers in the soluble fraction significantly correlated with a reduction in choline acetyltransferase activity in AD patients. The number of nicotinic acetylcholine receptors negatively correlated with the total amount of Aβ oligomers in the insoluble fraction in EOAD patients, and with decamers in the soluble fraction in LOAD patients. These novel findings suggest that distinct Aβ oligomers induce impairment of cholinergic neurotransmission in AD pathogenesis.

Interaction between prion protein and toxic amyloid β assemblies can be therapeutically targeted at multiple sites

A role for PrP in the toxic effect of oligomeric forms of Aβ, implicated in Alzheimer's disease (AD), has been suggested but remains controversial. Here we show that PrP is required for the plasticity-impairing effects of ex vivo material from human AD brain and that standardized Aβ-derived diffusible ligand (ADDL) preparations disrupt hippocampal synaptic plasticity in a PrP-dependent manner. We screened a panel of anti-PrP antibodies for their ability to disrupt the ADDL–PrP interaction. Antibodies directed to the principal PrP/Aβ-binding site and to PrP helix-1, were able to block Aβ binding to PrP suggesting that the toxic Aβ species are of relatively high molecular mass and/or may bind multiple PrP molecules. Two representative and extensively characterized monoclonal antibodies directed to these regions, ICSM-35 and ICSM-18, were shown to block the Aβ-mediated disruption of synaptic plasticity validating these antibodies as candidate therapeutics for AD either individually or in combination.

The ability of synthetic amyloid β-protein to bind to prion proteins and alter synaptic plasticity has been previously reported. Here the relevance of this binding is investigated in brains of Alzheimer's disease patients and the interaction is shown to be blocked by antibodies to two distinct regions of prion proteins.

Alzheimer's disease brain-derived amyloid-β-mediated inhibition of LTP in vivo is prevented by immunotargeting cellular prion protein

Synthetic amyloid-β protein (Aβ) oligomers bind with high affinity to cellular prion protein (PrP(C)), but the role of this interaction in mediating the disruption of synaptic plasticity by such soluble Aβ in vitro is controversial. Here we report that intracerebroventricular injection of Aβ-containing aqueous extracts of Alzheimer's disease (AD) brain robustly inhibits long-term potentiation (LTP) without significantly affecting baseline excitatory synaptic transmission in the rat hippocampus in vivo. Moreover, the disruption of LTP was abrogated by immunodepletion of Aβ. Importantly, intracerebroventricular administration of antigen-binding antibody fragment D13, directed to a putative Aβ-binding site on PrP(C), prevented the inhibition of LTP by AD brain-derived Aβ. In contrast, R1, a Fab directed to the C terminus of PrP(C), a region not implicated in binding of Aβ, did not significantly affect the Aβ-mediated inhibition of LTP. These data support the pathophysiological significance of SDS-stable Aβ dimer and the role of PrP(C) in mediating synaptic plasticity disruption by soluble Aβ.

Aβ oligomer-induced synapse degeneration in Alzheimer's disease

Aβ oligomers cause a collection of molecular events associated with memory loss in Alzheimer's disease, centering on disrupting the maintenance of synapse structure and function. In this brief review of the synaptotoxic effects of Aβ oligomers, we focus on the neuronal properties governing oligomer targeting and toxicity-especially with respect to binding sites and mechanisms of binding. We also discuss ways in which mechanistic insights from other diseases offer clues in the pursuit of the molecular basis of Alzheimer's disease.

Amyloid-beta oligomers increase the localization of prion protein at the cell surface

In Alzheimer's disease, the amyloid-β peptide (Aβ) interacts with distinct proteins at the cell surface to interfere with synaptic communication. Recent data have implicated the prion protein (PrP(C)) as a putative receptor for Aβ. We show here that Aβ oligomers signal in cells in a PrP(C)-dependent manner, as might be expected if Aβ oligomers use PrP(C) as a receptor. Immunofluorescence, flow cytometry and cell surface protein biotinylation experiments indicated that treatment with Aβ oligomers, but not monomers, increased the localization of PrP(C) at the cell surface in cell lines. These results were reproduced in hippocampal neuronal cultures by labeling cell surface PrP(C). In order to understand possible mechanisms involved with this effect of Aβ oligomers, we used live cell confocal and total internal reflection microscopy in cell lines. Aβ oligomers inhibited the constitutive endocytosis of PrP(C), but we also found that after Aβ oligomer-treatment PrP(C) formed more clusters at the cell surface, suggesting the possibility of multiple effects of Aβ oligomers. Our experiments show for the first time that Aβ oligomers signal in a PrP(C)-dependent way and that they can affect PrP(C) trafficking, increasing its localization at the cell surface.