Amyloid flirting with synaptic failure: towards a comprehensive view of Alzheimer's disease pathogenesis

Synaptic dysfunction in early phases of Alzheimer's Disease

The synapse is the locus of plasticity where short-term alterations in synaptic strength are converted to long-lasting memories. In addition to the presynaptic terminal and the postsynaptic compartment, a more holistic view of the synapse includes the astrocytes and the extracellular matrix to form a tetrapartite synapse. All these four elements contribute to synapse health and are crucial for synaptic plasticity events and, thereby, for learning and memory processes. Synaptic dysfunction is a common pathogenic trait of several brain disorders. In Alzheimer's Disease, the degeneration of synapses can be detected at the early stages of pathology progression before neuronal degeneration, supporting the hypothesis that synaptic failure is a major determinant of the disease. The synapse is the place where amyloid-β peptides are generated and is the target of the toxic amyloid-β oligomers. All the elements constituting the tetrapartite synapse are altered in Alzheimer's Disease and can synergistically contribute to synaptic dysfunction. Moreover, the two main hallmarks of Alzheimer's Disease, i.e., amyloid-β and tau, act in concert to cause synaptic deficits. Deciphering the mechanisms underlying synaptic dysfunction is relevant for the development of the next-generation therapeutic strategies aimed at modifying the disease progression.

Red ginseng (Panax ginseng C.A. Meyer) oil: A comprehensive review of extraction technologies, chemical composition, health benefits, molecular mechanisms, and safety

Red ginseng oil (RGO), rather than the conventional aqueous extract of red ginseng, has been receiving much attention due to accumulating evidence of its functional and pharmacological potential. In this review, we describe the key extraction technologies, chemical composition, potential health benefits, and safety of RGO. This review emphasizes the proposed molecular mechanisms by which RGO is involved in various bioactivities. RGO is mainly produced using organic solvents or supercritical fluid extraction, with the choice of method greatly affecting the yield and quality of the end products. RGO contains a high unsaturated fatty acid levels along with considerable amounts of lipophilic components such as phytosterols, tocopherols, and polyacetylenes. The beneficial health properties of RGO include cellular defense, antioxidation, anti-inflammation, anti-apoptosis, chemoprevention, hair growth promotion, and skin health improvement. We propose several molecular mechanisms and signaling pathways that underlie the bioactivity of RGO. In addition, RGO is regarded as safe and nontoxic. Further studies on RGO must focus on a deeper understanding of the underlying molecular mechanisms, composition–functionality relationship, and verification of the bioactivities of RGO in clinical models. This review may provide useful information in the development of RGO-based products in nutraceuticals, functional foods, and functional cosmetics.

Vanillic acid attenuates amyloid β1-40-induced long-term potentiation deficit in male rats: an in vivo investigation

Objectives: Alzheimer disease (AD) is a neurodegenerative disorderliness that involves deductible progressive cognition function caused by amyloid-beta (Aβ) peptide accumulation in the interstitial space. The increase of Aβ stimulates all kinds of active oxygen and causes oxidative stress and apoptosis. In this investigation, we researched the neuroprotective impacts of vanillic acid (VA) on the Aβ-induced (Aβ1-40) long-term potentiation (LTP) of the hippocampus - a commonly probed synaptic plasticity model that happens at the same time as memory and learning - in the AD rats.Methods: Forty-five male Wistar rats were categorized into five groups (n = 8 rats/group, 200-220 g), and studied as control (standard diet), sham (vehicle), VA (50 mg/kg), Aβ and Aβ + VA (50 mg/kg) groups. In vivo electrophysiological recordings were implemented after the stereotaxic surgery to gauge the excitatory postsynaptic potential (EPSP) slope and population spike (PS) amplitude in the dentate gyrus of the hippocampus. By the stimulation at high-frequency of the perforate pathway, long-term potentiation (LTP) was induced. To assess the plasma levels of malondialdehyde (MDA) and total thiol group (TTG), blood samples were garnered.Results: In the Aβ-injected rats, EPSP slope, and PS amplitude were significantly reduced after the induction of LTP. Thus, the findings demonstrate that VA decreases the impacts of Aβ on LTP; also, the treatments through VA neuroprotective against the negative effects of Aβ on the synaptic plasticity of the hippocampus can decrease the MDA levels and also increase the TTG levels significantly.Discussion: Therefore, based on this experiment on male rats, VA has neuroprotective effects and antioxidants benefits against the Aβ-mediated inhibition of long-term potentiation.

Cyclic Nucleotides Signaling and Phosphodiesterase Inhibition: Defying Alzheimer's Disease

Defects in brain functions associated with aging and neurodegenerative diseases benefit insignificantly from existing options, suggesting that there is a lack of understanding of pathological mechanisms. Alzheimer's disease (AD) is such a nearly untreatable, allied to age neurological deterioration for which only the symptomatic cure is available and the agents able to mould progression of the disease, is still far away. The altered expression of phosphodiesterases (PDE) and deregulated cyclic nucleotide signaling in AD has provoked a new thought of targeting cyclic nucleotide signaling in AD. Targeting cyclic nucleotides as an intracellular messenger seems to be a viable approach for certain biological processes in the brain and controlling substantial. Whereas, the synthesis, execution, and/or degradation of cyclic nucleotides has been closely linked to cognitive deficits. In relation to cognition, the cyclic nucleotides (cAMP and cGMP) have an imperative execution in different phases of memory, including gene transcription, neurogenesis, neuronal circuitry, synaptic plasticity and neuronal survival, etc. AD is witnessed by impairments of these basic processes underlying cognition, suggesting a crucial role of cAMP/cGMP signaling in AD populations. Phosphodiesterase inhibitors are the exclusive set of enzymes to facilitate hydrolysis and degradation of cAMP and cGMP thereby, maintains their optimum levels initiating it as an interesting target to explore. The present work reviews a neuroprotective and substantial influence of PDE inhibition on physiological status, pathological progression and neurobiological markers of AD in consonance with the intensities of cAMP and cGMP.

The protective and therapeutic effects of vinpocetine, a PDE1 inhibitor, on oxidative stress and learning and memory impairment induced by an intracerebroventricular (ICV) injection of amyloid beta (aβ) peptide

Alzheimer's disease (AD) is a neurodegenerative disease leading to cognitive and memory impairment. This study aimed at investigating the therapeutic and preserving effects of vinpocetine on amyloid beta (Aβ)-induced rat model of AD. Sixty male adult Wistar rats were randomly divided into 6 groups (n = 10 per group) as follows: 1; control, 2; sham, 3; Aβ, 4; pre-treatment (vinpocetine + Aβ): oral gavage administration of vinpocetine at 4 mg/kg for 30 days followed by intracerebroventricular (ICV) injection of Aβ, 5; treatment (Aβ + vinpocetine): Aβ ICV injection followed by vinpocetine administration for 30 days, 6; pre-treatment + treatment (vinpocetine + Aβ + vinpocetine): vinpocetine administration for 30 days before and 30 days after AD induction. Following treatments, the animals' learning and memory were investigated using passive avoidance learning (PAL) task, Morris water maze (MWM), and novel object recognition (NOR) tests. The results demonstrated that Aβ significantly enhanced escape latency and the distance traveled in the MWM, decreased step-through latency, and increased time spent in the dark compartment in PAL. Vinpocetine ameliorated the Aβ-infused memory deficits in both MWM and PAL tests. Administration of vinpocetine in the Aβ rats increased the discrimination index of the NOR test. It also significantly diminished the nitric oxide and malondialdehyde levels and restored the reduced glutathione (GSH) levels. Vinpocetine can improve memory and learning impairment following Aβ infusion due to its different properties, including antioxidant effects, which indicates that vinpocetine administration can lead to the amelioration of cognitive dysfunction in AD.

AMPA-ergic regulation of amyloid-β levels in an Alzheimer's disease mouse model

BACKGROUND

Extracellular aggregation of the amyloid-β (Aβ) peptide into toxic multimers is a key event in Alzheimer's disease (AD) pathogenesis. Aβ aggregation is concentration-dependent, with higher concentrations of Aβ much more likely to form toxic species. The processes that regulate extracellular levels of Aβ therefore stand to directly affect AD pathology onset. Studies from our lab and others have demonstrated that synaptic activity is a critical regulator of Aβ production through both presynaptic and postsynaptic mechanisms. AMPA receptors (AMPA-Rs), as the most abundant ionotropic glutamate receptors, have the potential to greatly impact Aβ levels.

METHODS

In order to study the role of AMPA-Rs in Aβ regulation, we used in vivo microdialysis in an APP/PS1 mouse model to simultaneously deliver AMPA and other treatments while collecting Aβ from the interstitial fluid (ISF). Changes in Aβ production and clearance along with inflammation were assessed using biochemical approaches. IL-6 deficient mice were utilized to test the role of IL-6 signaling in AMPA-R-mediated regulation of Aβ levels.

RESULTS

We found that AMPA-R activation decreases in ISF Aβ levels in a dose-dependent manner. Moreover, the effect of AMPA treatment involves three distinct pathways. Steady-state activity of AMPA-Rs normally promotes higher ISF Aβ. Evoked AMPA-R activity, however, decreases Aβ levels by both stimulating glutamatergic transmission and activating downstream NMDA receptor (NMDA-R) signaling and, with extended AMPA treatment, acting independently of NMDA-Rs. Surprisingly, we found this latter, direct AMPA pathway of Aβ regulation increases Aβ clearance, while Aβ production appears to be largely unaffected. Furthermore, the AMPA-dependent decrease is not observed in IL-6 deficient mice, indicating a role for IL-6 signaling in AMPA-R-mediated Aβ clearance.

CONCLUSION

Though basal levels of AMPA-R activity promote higher levels of ISF Aβ, evoked AMPA-R signaling decreases Aβ through both NMDA-R-dependent and -independent pathways. We find that evoked AMPA-R signaling increases clearance of extracellular Aβ, at least in part through enhanced IL-6 signaling. These data emphasize that Aβ regulation by synaptic activity involves a number of independent pathways that together determine extracellular Aβ levels. Understanding how these pathways maintain Aβ levels prior to AD pathology may provide insights into disease pathogenesis.

Protective Effects of Red Ginseng Oil against Aβ25-35-Induced Neuronal Apoptosis and Inflammation in PC12 Cells

One of pathological characteristics of Alzheimer's disease (AD), aggregation and deposition of β amyloid (Aβ), has been accepted as a potent activator of neuronal cell death. Red ginseng is well-known for various pharmacological activities, but most studies have been focused on red ginseng water extract (RGW), which has resulted in the conception of the present study of red ginseng oil (RGO) against Aβ_25-35-induced neurotoxicity. Cytotoxicity and apoptosis induction by Aβ were verified and the underlying mechanism by which RGO inhibited neuronal cell death, mitochondria dysfunction and NF-κB pathway related protein markers were evaluated. RGO attenuated Aβ_25-35-induced apoptosis, not only by inhibiting calcium influx, but also by reducing mitochondrial membrane potential loss. RGO significantly decreased Bax, whereas increased Bcl-2 and inactivated of caspase-3 and -9 and PARP-1 stimulated by Aβ_25-35. Anti-neuroinflammatory effect of RGO was demonstrated by downregulating c-Jun N-terminal kinase (JNK) and p38, resulting in inhibiting of the NF-κB pathway and thereby suppressing the expressions of pro-inflammatory mediators such as inducible nitric oxide synthase (iNOS), cyclooxygenase-2 (COX-2), prostaglandin E₂ (PGE₂), nitric oxide (NO) and tumor necrosis factor-α (TNF-α). The present study revealed that RGO is a potential natural resource of the functional foods industry as well as a promising candidate of multi-target neuronal protective agent for the prevention of AD.

Filling the void: a role for exercise-induced BDNF and brain amyloid precursor protein processing

Inactivity, obesity, and insulin resistance are significant risk factors for the development of Alzheimer's disease (AD). Several studies have demonstrated that diet-induced obesity, inactivity, and insulin resistance exacerbate the neuropathological hallmarks of AD. The aggregation of β-amyloid peptides is one of these hallmarks. β-Site amyloid precursor protein-cleaving enzyme 1 (BACE1) is the rate-limiting enzyme in amyloid precursor protein (APP) processing, leading to β-amyloid peptide formation. Understanding how BACE1 content and activity are regulated is essential for establishing therapies aimed at reducing and/or slowing the progression of AD. Exercise training has been proven to reduce the risk of AD as well as decrease β-amyloid production and BACE1 content and/or activity. However, these long-term interventions also result in improvements in adiposity, circulating metabolites, glucose tolerance, and insulin sensitivity making it difficult to determine the direct effects of exercise on brain APP processing. This review highlights this large void in our knowledge and discusses our current understanding of the direct of effect of exercise on β-amyloid production. We have concentrated on the central role that brain-derived neurotrophic factor (BDNF) may play in mediating the direct effects of exercise on reducing brain BACE1 content and activity as well as β-amyloid production. Future studies should aim to generate a greater understanding of how obesity and exercise can directly alter APP processing and AD-related pathologies. This knowledge could provide evidence-based hypotheses for designing therapies to reduce the risk of AD and dementia.

Functional interplay between plasma membrane Ca2+-ATPase, amyloid β-peptide and tau

It is well known that dysregulation of Ca²⁺ homeostasis is involved in Alzheimeŕs disease (AD), a neurodegenerative disorder characterized by the presence of toxic aggregates of amyloid β-peptide (Aβ) and neurofibrillary tangles of tau. Alteration of calcium signaling has been linked to Aβ and tau pathologies, although the understanding of underlying molecular and cellular mechanisms is far from clear. This review summarizes the functional inhibition of plasma membrane Ca²⁺-ATPase (PMCA) by Aβ and tau, and its modulation by calmodulin and the ionic nature of phospholipids. The data obtained until now in our laboratory suggest that PMCA injury linked to Aβ and tau can be significantly involved in the cascade of events leading to intracellular calcium overload associated to AD.

High affinity binding of amyloid β-peptide to calmodulin: Structural and functional implications

Amyloid β-peptides (Aβ) are a major hallmark of Alzheimer's disease (AD) and their neurotoxicity develop with cytosolic calcium dysregulation. On the other hand, calmodulin (CaM), a protein which plays a major multifunctional role in neuronal calcium signaling, has been shown to be involved in the regulation of non-amyloidogenic processing of amyloid β precursor protein (APP). Using fluorescent 6-bromoacetyl-2-dimethylaminonaphthalene derivatives of CaM, Badan-CaM, and human amyloid β(1-42) HiLyte™-Fluor555, we show in this work that Aβ binds with high affinity to CaM through the neurotoxic Aβ25-35 domain. In addition, the affinity of Aβ for calcium-saturated CaM conformation is approximately 20-fold higher than for CaM conformation in the absence of calcium (apo-CaM). Moreover, the value of K_d of 0.98 ± 0.11 nM obtained for Aβ1-42 dissociation from CaM saturated by calcium points out that CaM is one of the cellular targets with highest affinity for neurotoxic Aβ peptides. A major functional consequence of Aβ-CaM interaction is that it slowdowns Aβ fibrillation. The novel and high affinity interaction between calmodulin and Aβ shown in this work opens a yet-unexplored gateway to further understand the neurotoxic effect of Aβ in different neural cells and also to address the potential of calmodulin and calmodulin-derived peptides as therapeutic agents in AD.

Alzheimer-related decrease in CYFIP2 links amyloid production to tau hyperphosphorylation and memory loss

Characteristic features of Alzheimer's disease are memory loss, plaques resulting from abnormal processing of amyloid precursor protein (APP), and presence of neurofibrillary tangles and dystrophic neurites containing hyperphosphorylated tau. Currently, it is not known what links these abnormalities together. Cytoplasmic FMR1 interacting protein 2 (CYFIP2) has been suggested to regulate mRNA translation at synapses and this may include local synthesis of APP and alpha-calcium/calmodulin-dependent kinase II, a kinase that can phosphorylate tau. Further, CYFIP2 is part of the Wiskott-Aldrich syndrome protein-family verprolin-homologous protein complex, which has been implicated in actin polymerization at synapses, a process thought to be required for memory formation. Our previous studies on p25 dysregulation put forward the hypothesis that CYFIP2 expression is reduced in Alzheimer's disease and that this contributes to memory impairment, abnormal APP processing and tau hyperphosphorylation. Here, we tested this hypothesis. First, in post-mortem tissue CYFIP2 expression was reduced by ∼50% in severe Alzheimer's hippocampus and superior temporal gyrus when normalized to expression of a neuronal or synaptic marker protein. Interestingly, there was also a trend for decreased expression in mild Alzheimer's disease hippocampus. Second, CYFIP2 expression was reduced in old but not in young Tg2576 mice, a model of familial Alzheimer's disease. Finally, we tested the direct impact of reduced CYFIP2 expression in heterozygous null mutant mice. We found that in hippocampus this reduced expression causes an increase in APP and β-site amyloid precursor protein cleaving enzyme 1 (BACE1) protein, but not mRNA expression, and elevates production of amyloid-β42 Reduced CYFIP2 expression also increases alpha-calcium/calmodulin-dependent kinase II protein expression, and this is associated with hyperphosphorylation of tau at serine-214. The reduced expression also impairs spine maturity without affecting spine density in apical dendrites of CA1 pyramidal neurons. Furthermore, the reduced expression prevents retention of spatial memory in the water maze. Taken together, our findings indicate that reduced CYFIP2 expression triggers a cascade of change towards Alzheimer's disease, including amyloid production, tau hyperphosphorylation and memory loss. We therefore suggest that CYFIP2 could be a potential hub for targeting treatment of the disease.

Presenilin 1 and Presenilin 2 Target γ-Secretase Complexes to Distinct Cellular Compartments

γ-Secretase complexes achieve the production of amyloid peptides playing a key role in Alzheimer disease. These proteases have many substrates involved in important physiological functions. They are composed of two constant subunits, nicastrin and PEN2, and two variable ones, presenilin (PS1 or PS2) and APH1 (APH1aL, APH1aS, or APH1b). Whether the composition of a given γ-secretase complex determines a specific cellular targeting remains unsolved. Here we combined a bidirectional inducible promoter and 2A peptide technology to generate constructs for the temporary, stoichiometric co-expression of six different combinations of the four γ-secretase subunits including EGFP-tagged nicastrin. These plasmids allow for the formation of functional γ-secretase complexes displaying specific activities and maturations. We show that PS1-containing γ-secretase complexes were targeted to the plasma membrane, whereas PS2-containing ones were addressed to the trans-Golgi network, to recycling endosomes, and, depending on the APH1-variant, to late endocytic compartments. Overall, these novel constructs unravel a presenilin-dependent subcellular targeting of γ-secretase complexes. These tools should prove useful to determine whether the cellular distribution of γ-secretase complexes contributes to substrate selectivity and to delineate regulations of their trafficking.

Identification of the novel activity-driven interaction between synaptotagmin 1 and presenilin 1 links calcium, synapse, and amyloid beta

Background

Synaptic loss strongly correlates with memory deterioration. Local accumulation of amyloid β (Aβ) peptide, and neurotoxic Aβ42 in particular, due to abnormal neuronal activity may underlie synaptic dysfunction, neurodegeneration, and memory impairments. To gain an insight into molecular events underlying neuronal activity-regulated Aβ production at the synapse, we explored functional outcomes of the newly discovered calcium-dependent interaction between Alzheimer’s disease-associated presenilin 1 (PS1)/γ-secretase and synaptic vesicle proteins.

Results

Mass spectrometry screen of mouse brain lysates identified synaptotagmin 1 (Syt1) as a novel synapse-specific PS1-binding partner that shows Ca²⁺-dependent PS1 binding profiles in vitro and in vivo. We found that Aβ level, and more critically, conformation of the PS1 and the Aβ_42/40 ratio, are affected by Syt1 overexpression or knockdown, indicating that Syt1 and its interaction with PS1 might regulate Aβ production at the synapse. Moreover, β-secretase 1 (BACE1) stability, β- and γ-secretase activity, as well as intracellular compartmentalization of PS1 and BACE1, but not of amyloid precursor protein (APP), nicastrin (Nct), presenilin enhancer 2 (Pen-2), or synaptophysin (Syp) were altered in the absence of Syt1, suggesting a selective effect of Syt1 on PS1 and BACE1 trafficking.

Conclusions

Our findings identify Syt1 as a novel Ca²⁺-sensitive PS1 modulator that could regulate synaptic Aβ, opening avenues for novel and selective synapse targeting therapeutic strategies.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-016-0248-3) contains supplementary material, which is available to authorized users.

Calcium/calmodulin-dependent kinase II and Alzheimer's disease

CaMKII is a remarkably complex protein kinase, known to have a fundamental role in synaptic plasticity and memory formation. Further, CaMKII has also been suggested to be a tau kinase. CaMKII dysregulation may therefore be a modulator of toxicity in Alzheimer's disease, a dementia characterised by aberrant calcium signalling, synapse and neuronal loss, and impaired memory. Here, we first examine the evidence for CaMKII dysregulation in Alzheimer's patients and draw parallels to findings in disease models which recapitulate key aspects of the disease. We then put forward the hypothesis that these changes critically contribute to neurodegeneration and memory impairment in Alzheimer's disease.

Buying time: a rationale for examining the use of circadian rhythm and sleep interventions to delay progression of mild cognitive impairment to Alzheimer's disease

As of 2010, the worldwide economic impact of dementia was estimated at $604 billion USD; and without discovery of a cure or effective interventions to delay disease progression, dementia's annual global economic impact is expected to surpass $1 trillion USD as early as 2030. Alzheimer's disease (AD) is the leading cause of dementia accounting for over 75% of all cases. Toxic accumulation of amyloid beta (Aβ), either by overproduction or some clearance failure, is thought to be an underlying mechanism of the neuronal cell death characteristic of AD-though this amyloid hypothesis has been increasingly challenged in recent years. A compelling alternative hypothesis points to chronic neuroinflammation as a common root in late-life degenerative diseases including AD. Apolipoprotein-E (APOE) genotype is the strongest genetic risk factor for AD: APOE-ε4 is proinflammatory and individuals with this genotype accumulate more Aβ, are at high risk of developing AD, and almost half of all AD patients have at least one ε4 allele. Recent studies suggest a bidirectional relationship exists between sleep and AD pathology. Sleep may play an important role in Aβ clearance, and getting good quality sleep vs. poor quality sleep might reduce the AD risk associated with neuroinflammation and the ε4 allele. Taken together, these findings are particularly important given the sleep disruptions commonly associated with AD and the increased burden disrupted sleep poses for AD caregivers. The current review aims to: (1) identify individuals at high risk for dementia who may benefit most from sleep interventions; (2) explore the role poor sleep quality plays in exacerbating AD type dementia; (3) examine the science of sleep interventions to date; and (4) provide a road map in pursuit of comprehensive sleep interventions, specifically targeted to promote cognitive function and delay progression of dementia.

Tritium-labeled (E,E)-2,5-bis(4'-hydroxy-3'-carboxystyryl)benzene as a probe for β-amyloid fibrils

Accumulation of Aβ in the brains of Alzheimer disease (AD) patients reflects an imbalance between Aβ production and clearance from their brains. Alternative cleavage of amyloid precursor protein (APP) by processing proteases generates soluble APP fragments including the neurotoxic amyloid Aβ40 and Aβ42 peptides that assemble into fibrils and form plaques. Plaque-buildup occurs over an extended time-frame, and the early detection and modulation of plaque formation are areas of active research. Radiolabeled probes for the detection of amyloid plaques and fibrils in living subjects are important for noninvasive evaluation of AD diagnosis, progression, and differentiation of AD from other neurodegenerative diseases and age-related cognitive decline. Tritium-labeled (E,E)-1-[(3)H]-2,5-bis(4'-hydroxy-3'-carbomethoxystyryl)benzene possesses an improved level of chemical stability relative to a previously reported radioiodinated analog for radiometric quantification of Aβ plaque and tau pathology in brain tissue and in vitro studies with synthetic Aβ and tau fibrils.

Alzheimer's therapeutics targeting amyloid beta 1-42 oligomers I: Abeta 42 oligomer binding to specific neuronal receptors is displaced by drug candidates that improve cognitive deficits

Synaptic dysfunction and loss caused by age-dependent accumulation of synaptotoxic beta amyloid (Abeta) 1-42 oligomers is proposed to underlie cognitive decline in Alzheimer's disease (AD). Alterations in membrane trafficking induced by Abeta oligomers mediates reduction in neuronal surface receptor expression that is the basis for inhibition of electrophysiological measures of synaptic plasticity and thus learning and memory. We have utilized phenotypic screens in mature, in vitro cultures of rat brain cells to identify small molecules which block or prevent the binding and effects of Abeta oligomers. Synthetic Abeta oligomers bind saturably to a single site on neuronal synapses and induce deficits in membrane trafficking in neuronal cultures with an EC50 that corresponds to its binding affinity. The therapeutic lead compounds we have found are pharmacological antagonists of Abeta oligomers, reducing the binding of Abeta oligomers to neurons in vitro, preventing spine loss in neurons and preventing and treating oligomer-induced deficits in membrane trafficking. These molecules are highly brain penetrant and prevent and restore cognitive deficits in mouse models of Alzheimer's disease. Counter-screening these compounds against a broad panel of potential CNS targets revealed they are highly potent and specific ligands of the sigma-2/PGRMC1 receptor. Brain concentrations of the compounds corresponding to greater than 80% receptor occupancy at the sigma-2/PGRMC1 receptor restore cognitive function in transgenic hAPP Swe/Ldn mice. These studies demonstrate that synthetic and human-derived Abeta oligomers act as pharmacologically-behaved ligands at neuronal receptors--i.e. they exhibit saturable binding to a target, they exert a functional effect related to their binding and their displacement by small molecule antagonists blocks their functional effect. The first-in-class small molecule receptor antagonists described here restore memory to normal in multiple AD models and sustain improvement long-term, representing a novel mechanism of action for disease-modifying Alzheimer's therapeutics.

Alzheimer's therapeutics targeting amyloid beta 1-42 oligomers II: Sigma-2/PGRMC1 receptors mediate Abeta 42 oligomer binding and synaptotoxicity

Amyloid beta (Abeta) 1-42 oligomers accumulate in brains of patients with Mild Cognitive Impairment (MCI) and disrupt synaptic plasticity processes that underlie memory formation. Synaptic binding of Abeta oligomers to several putative receptor proteins is reported to inhibit long-term potentiation, affect membrane trafficking and induce reversible spine loss in neurons, leading to impaired cognitive performance and ultimately to anterograde amnesia in the early stages of Alzheimer's disease (AD). We have identified a receptor not previously associated with AD that mediates the binding of Abeta oligomers to neurons, and describe novel therapeutic antagonists of this receptor capable of blocking Abeta toxic effects on synapses in vitro and cognitive deficits in vivo. Knockdown of sigma-2/PGRMC1 (progesterone receptor membrane component 1) protein expression in vitro using siRNA results in a highly correlated reduction in binding of exogenous Abeta oligomers to neurons of more than 90%. Expression of sigma-2/PGRMC1 is upregulated in vitro by treatment with Abeta oligomers, and is dysregulated in Alzheimer's disease patients' brain compared to age-matched, normal individuals. Specific, high affinity small molecule receptor antagonists and antibodies raised against specific regions on this receptor can displace synthetic Abeta oligomer binding to synaptic puncta in vitro and displace endogenous human AD patient oligomers from brain tissue sections in a dose-dependent manner. These receptor antagonists prevent and reverse the effects of Abeta oligomers on membrane trafficking and synapse loss in vitro and cognitive deficits in AD mouse models. These findings suggest sigma-2/PGRMC1 receptors mediate saturable oligomer binding to synaptic puncta on neurons and that brain penetrant, small molecules can displace endogenous and synthetic oligomers and improve cognitive deficits in AD models. We propose that sigma-2/PGRMC1 is a key mediator of the pathological effects of Abeta oligomers in AD and is a tractable target for small molecule disease-modifying therapeutics.

Synapse stability in the precuneus early in the progression of Alzheimer's disease

Amnestic mild cognitive impairment (aMCI) is considered to be one of the early stages in the progression from no cognitive impairment (NCI) to Alzheimer's disease (AD). Individuals with aMCI have increased levels of AD-type neuropathology in multiple regions of the neocortex and hippocampus and demonstrate a loss of synaptic connectivity. Recent neuroimaging studies have reported increased levels of 11C-PiB (Pittsburgh, compound B) in regions of the neocortex including the precuneus region of the medial parietal lobe. This cortical region has been implicated in episodic memory, which is disrupted early in the progression of AD. In this study, unbiased stereology coupled with electron microscopy was used to quantify total synaptic numbers in lamina 3 of the precuneus from short postmortem autopsy tissue harvested from subjects who died at different cognitive stages during the progression of AD. Individuals with aMCI did not reveal a statistically significant decline in total synapses compared to the NCI cohort while the AD group did show a modest but significant decline. Synaptic numbers failed to correlate with several different cognitive tasks including the Mini-Mental State Examination scores and episodic memory scores. Although levels of [3H]PiB binding were elevated in both the aMCI and AD groups, it did not strongly correlate with synaptic counts. These results support the idea that despite increased amyloid load, the precuneus region does not show early changes in synaptic decline during the progression of AD.

Roles of brain-derived neurotrophic factor/tropomyosin-related kinase B (BDNF/TrkB) signalling in Alzheimer's disease

Alzheimer's disease (AD) is one of the most common causes of dementia in the elderly. It is characterized by extracellular deposition of the neurotoxic peptide, amyloid-beta (Aβ) peptide fibrils, and is accompanied by extensive loss of neurons in the brains of affected individuals. However, the pathogenesis of AD is not fully understood. The aim of this review is to discuss the possible role of brain-derived neurotrophic factor (BDNF)/tropomyosin-related kinase B (TrkB) signalling in the development of AD, focusing on BDNF/TrkB signalling in the production of Aβ, tau hyperphosphorylation and cognition decline, and exploring new possibilities for AD intervention.

Inhibition of microglial activation protects hippocampal neurogenesis and improves cognitive deficits in a transgenic mouse model for Alzheimer's disease

BACKGROUND

Activated microglia with macrophage-like functions invade and surround β-amyloid (Aβ) plaques in Alzheimer's disease (AD), possibly contributing to the turnover of Aβ, but they can also secrete proinflammatory factors that may be involved in the pathogenesis of AD. Microglia are known to modulate adult hippocampal neurogenesis.

OBJECTIVES/METHODS

To determine the role of microglia on neurogenesis in brains with Aβ pathology, we inhibited microglial activation with the tetracycline derivative minocycline in doubly transgenic mice expressing mutant human amyloid precursor protein (APP) and mutant human presenilin-1 (PS1).

RESULTS

Minocycline increased the survival of new dentate granule cells in APP/PS1 mice indicated by more BrdU+/NeuN+ cells as compared to vehicle-treated transgenic littermates, accompanied by improved behavioral performance in a hippocampus-dependent learning task. Both brain levels of Aβ and Aβ-related morphological deficits in the new neurons labeled with GFP-expressing retrovirus were unaffected in minocycline-treated mice.

CONCLUSIONS

These results suggest a role for microglia in Aβ-related functional deficits and in suppressing the survival of new neurons, and show that modulation of microglial function with minocycline can protect hippocampal neurogenesis in the presence of Aβ pathology.

The complex world of post-transcriptional mechanisms: is their deregulation a common link for diseases? Focus on ELAV-like RNA-binding proteins

Post-transcriptional mechanisms are key determinants in the modulation of the expression of final gene products. Within this context, fundamental players are RNA-binding proteins (RBPs), and among them ELAV-like proteins. RBPs are able to affect every aspect in the processing of transcripts, from alternative splicing, polyadenylation, and nuclear export to cytoplasmic localization, stability, and translation. Of interest, more than one RBP can bind simultaneously the same mRNA; therefore, since each RBP is endowed with different properties, the balance of these interactions dictates the ultimate fate of the transcript, especially in terms of both stability and rate of translation. Besides RBPs, microRNAs are also important contributors to the post-transcriptional control of gene expression. Within this general context, the present review focuses on ELAV-like proteins describing their roles in the nucleus and in the cytoplasm, also highlighting some examples of interactions with other RBPs and with microRNAs. We also examine the putative role and the observed changes of ELAV-like proteins and of their interactions with other regulatory elements in Alzheimer's disease, cancer, and inflammation. The changes in the expression of proteins involved in these diseases are examples of how a derangement in the mRNA stabilization process may be associated with disease development and contribute to pathology. Overall, we hope that the topics handled in the present manuscript provide a hint to look at ELAV-like-mediated mRNA stabilization as a mechanism relevant to disease as well as a novel putative drug target.

PET amyloid-beta imaging in preclinical Alzheimer's disease

Alzheimer's disease (AD) is the leading cause of dementia, accounting for 60-70% of all cases [Hebert et al., 2003, 1]. The need for effective therapies for AD is great. Current approaches, including cholinesterase inhibitors and N-methyl-d-aspartate (NMDA) receptor antagonists, are symptomatic treatments for AD but do not prevent disease progression. Many diagnostic and therapeutic approaches to AD are currently changing due to the knowledge that underlying pathology starts 10 to 20 years before clinical signs of dementia appear [Holtzman et al., 2011, 2]. New therapies which focus on prevention or delay of the onset or cognitive symptoms are needed. Recent advances in the identification of AD biomarkers now make it possible to detect AD pathology in the preclinical stage of the disease, in cognitively normal (CN) individuals; this biomarker data should be used in the selection of high-risk populations for clinical trials. In vivo visualization of AD neuropathology and biological, biochemical or physiological confirmation of the effects of treatment likely will substantially improve development of novel pharmaceuticals. Positron emission tomography (PET) is the leading neuroimaging tool to detect and provide quantitative measures of AD amyloid pathology in vivo at the early stages and follow its course longitudinally. This article is part of a Special Issue entitled: Imaging Brain Aging and Neurodegenerative disease.

Increases in β-amyloid protein in the hippocampus caused by diabetic metabolic disorder are blocked by minocycline through inhibition of NF-κB pathway activation

Activation of the NF-κB pathway plays an important role in the pathophysiology of Alzheimer's disease (AD), and blocking NF-κB pathway activation has been shown to attenuate cognitive impairment. Diabetic metabolic disorder contributes to β-amyloid protein (Aβ) generation. The goal of this study was to determine the effect of minocycline on Aβ generation and the NF-κB pathway in the hippocampus of diabetic rats and to elucidate the neuroprotective mechanisms of minocycline for the treatment of diabetic metabolic disorder. The diabetic rat model was established using a high-fat diet and an intraperitoneal injection of streptozocin (STZ). Behavioral tests showed that the capacity of learning and memory was significantly lower in diabetic rats. The levels of NF-κB, COX-2, iNOS, IL-1β and TNF-α after the STZ injection were significantly increased in the hippocampus. Significant increases in Aβ, BACE1, NF-κB, COX-2, iNOS, IL-1β and TNF-α were found in diabetic rats. The levels of Aβ, NF-κB, COX-2, iNOS, IL-1β and TNF-α were significantly decreased after minocycline administration; however, minocycline had no effect on BACE1 expression. In sum, diabetes contributes to the activation of the NF-κB pathway and upregulates BACE1 and Aβ. Minocycline downregulates Aβ in the hippocampus by inhibiting NF-κB pathway activation.

[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.

The selective and competitive N-methyl-D-aspartate receptor antagonist, (-)-6-phosphonomethyl-deca-hydroisoquinoline-3-carboxylic acid, prevents synaptic toxicity induced by amyloid-β in mice

The toxicity of amyloid β (Aβ) is highly associated with Alzheimer's disease (AD), which has a high incidence in elderly people worldwide. While the current treatment for moderate and severe AD includes blockage of the N-methyl-d-aspartate receptor (NMDAR), the molecular mechanisms of its effect are still poorly understood. Herein, we report that a single i.p. administration of the selective and competitive (NMDAR) antagonist LY235959 reduced Aβ neurotoxicity by preventing the down-regulation of glial glutamate transporters (glutamate-aspartate transporter (GLAST) and glutamate transporter-1 (GLT-1)), the decrease in glutamate uptake, and the production of reactive oxygen species (ROS) induced by Aβ(1-40). Importantly, the blockage of NMDAR restored the Aβ(1-40)-induced synaptic dysfunction and cognitive impairment. However, LY235959 failed to prevent the inflammatory response associated with Aβ(1-40) treatment. Altogether, our data indicate that the acute administration of Aβ promotes oxidative stress, a decrease in glutamate transporter expression, and neurotoxicity. Our results reinforce the idea that NMDAR plays a critical regulatory action in Aβ toxicity and they provide further pre-clinical evidence for the potential role of the selective and competitive NMDAR antagonists in the treatment of AD.

Post-synaptic scaffolding protein interactions with glutamate receptors in synaptic dysfunction and Alzheimer's disease

Alzheimer's disease (AD) is characterized clinically by an insidious decline in cognition. Much attention has been focused on proposed pathogenic mechanisms that relate Aβ plaque and neurofibrillary tangle pathology to cognitive symptoms, but compelling evidence now identifies early synaptic loss and dysfunction, which precede plaque and tangle formation, as the more probable initiators of cognitive impairment. Glutamate-mediated transmission is severely altered in AD. Glutamate receptor expression is most markedly altered in regions of the AD brain that show the greatest pathological changes. Signaling via glutamate receptors controls synaptic strength and plasticity, and changes in these parameters are likely to contribute to memory and cognitive deficits in AD. Glutamate receptor expression and activity are modulated by interactions with post-synaptic scaffolding proteins that augment the strength and direction of signal cascades initiated by glutamate receptor activity. Scaffold proteins offer promising targets for more focused and effective drug therapy. In consequence, interest is developing into the roles these proteins play in neurological disease. In this review we discuss disruptions to excitatory neurotransmission at the level of glutamate receptor-post-synaptic scaffolding protein interactions that may contribute to synaptic dysfunction in AD.

TrkB Isoforms Differentially Affect AICD Production through Their Intracellular Functional Domains

We report that NTRK2, the gene encoding for the TrkB receptor, can regulate APP metabolism, specifically AICD levels. Using the human neuroblastoma cell line SH-SY5Y, we characterized the effect of three TrkB isoforms (FL, SHC, T) on APP metabolism by knockdown and overexpression. We found that TrkB FL increases AICD-mediated transcription and APP levels while it decreases sAPP levels. These effects were mainly mediated by the tyrosine kinase activity of the receptor and partially by the PLC-γ- and SHC-binding sites. The TrkB T truncated isoform did not have significant effects on APP metabolism when transfected by itself, while the TrkB SHC decreased AICD-mediated transcription. TrkB T abolished TrkB FL effects on APP metabolism when cotransfected with it while TrkB SHC cotransfected with TrkB FL still showed increased APP levels. In conclusion, we demonstrated that TrkB isoforms have differential effects on APP metabolism.

Therapeutic utility of phosphodiesterase type I inhibitors in neurological conditions

Neuronal plasticity is an essential property of the brain that is impaired in different neurological conditions. Phosphodiesterase type 1 (PDE1) inhibitors can enhance levels of the second messengers cAMP/cGMP leading to the expression of neuronal plasticity-related genes, neurotrophic factors, and neuroprotective molecules. These neuronal plasticity enhancement properties make PDE1 inhibitors good candidates as therapeutic agents in many neurological conditions. However, the lack of specificity of the drugs currently available poses a challenge to the systematic evaluation of the beneficial effect of these agents. The development of more specific drugs may pave the way for the use of PDE1 inhibitors as therapeutic agents in cases of neurodevelopmental conditions such as fetal alcohol spectrum disorders and in degenerative disorders such as Alzheimer's and Parkinson's.

Protein kinase C-regulated aβ production and clearance

Alzheimer's disease (AD) is the most common form of dementia among the elderly population. AD, which is characterized as a disease of cognitive deficits, is mainly associated with an increase of amyloid β-peptide (Aβ) in the brain. A growing body of recent studies suggests that protein kinase C (PKC) promotes the production of the secretory form of amyloid precursor protein (sAPPα) via the activation of α-secretase activity, which reduces the accumulation of pathogenic Aβ levels in the brain. Moreover, activation of PKCα and mitogen-activated protein kinase (MAPK) is known to increase sAPPα. A novel type of PKC, PKCε, activates the Aβ degrading activity of endothelin converting enzyme type 1 (ECE-1), which might be mediated via the MAPK pathway as well. Furthermore, dysregulation of PKC-MAPK signaling is known to increase Aβ levels in the brain, which results in AD phenotypes. Here, we discuss roles of PKC in Aβ production and clearance and its implication in AD.

Reduced Alzheimer's disease ß-amyloid deposition in transgenic mice expressing S-palmitoylation-deficient APH1aL and nicastrin

Sequential cleavage of amyloid precursor protein by β- and γ-secretases generates β-amyloid peptides (Aβ), which accumulate in the brains of patients with Alzheimer's disease. We recently identified S-palmitoylation of two γ-secretase subunits, APH1 and nicastrin. S-Palmitoylation is an essential posttranslational modification for the proper trafficking and function of many neuronal proteins. In cultured cell lines, lack of S-palmitoylation causes instability of nascent APH1 and nicastrin but does not affect γ-secretase processing of amyloid precursor protein. To determine the importance of γ-secretase S-palmitoylation for Aβ deposition in the brain, we generated transgenic mice coexpressing human wild-type or S-palmitoylation-deficient APH1aL and nicastrin in neurons in the forebrain. We found that lack of S-palmitoylation did not impair the ability of APH1aL and nicastrin to form enzymatically active protein complexes with endogenous presenilin 1 and PEN2 or affect the localization of γ-secretase subunits in dendrites and axons of cortical neurons. When we crossed these mice with 85Dbo transgenic mice, which coexpress familial Alzheimer's disease-causing amyloid precursor protein and presenilin 1 variants, we found that coexpression of wild-type or mutant APH1aL and nicastrin led to marked stabilization of transgenic presenilin 1 in the brains of double-transgenic mice. Interestingly, we observed a moderate, but significant, reduction in amyloid deposits in the forebrain of mice expressing S-palmitoylation-deficient γ-secretase subunits compared with mice overexpressing wild-type subunits, as well as a reduction in the levels of insoluble Aβ(40-42). These results indicate that γ-secretase S-palmitoylation modulates Aβ deposition in the brain.

Activation of extrasynaptic, but not synaptic, NMDA receptors modifies amyloid precursor protein expression pattern and increases amyloid-ß production

Calcium is a key mediator controlling essential neuronal functions depending on electrical activity. Altered neuronal calcium homeostasis affects metabolism of amyloid precursor protein (APP), leading to increased production of β-amyloid (Aβ), and contributing to the initiation of Alzheimer's disease (AD). A linkage between excessive glutamate receptor activation and neuronal Aβ release was established, and recent reports suggest that synaptic and extrasynaptic NMDA receptor (NMDAR) activation may have distinct consequences in plasticity, gene regulation, and neuronal death. Here, we report for the first time that prolonged activation of extrasynaptic NMDAR, but not synaptic NMDAR, dramatically increased the neuronal production of Aβ. This effect was preceded by a shift from APP695 to Kunitz protease inhibitory domain (KPI) containing APPs (KPI-APPs), isoforms exhibiting an important amyloidogenic potential. Conversely, after synaptic NMDAR activation, we failed to detect any KPI-APP expression and neuronal Aβ production was not modified. Calcium imaging data showed that intracellular calcium concentration after extrasynaptic NMDAR stimulation was lower than after synaptic activation. This suggests distinct signaling pathways for each pool of receptors. We found that modification of neuronal APP expression pattern triggered by extrasynaptic NMDAR activation was regulated at an alternative splicing level involving calcium-/calmodulin-dependent protein kinase IV, but overall APP expression remained identical. Finally, memantine dose-dependently inhibited extrasynaptic NMDAR-induced KPI-APPs expression as well as neuronal Aβ release. Altogether, these data suggest that a chronic activation of extrasynaptic NMDAR promotes amyloidogenic KPI-APP expression leading to neuronal Aβ release, representing a causal risk factor for developing AD.

Oxidative stress in the progression of Alzheimer disease in the frontal cortex

We investigated oxidative stress in human postmortem frontal cortexfrom individuals characterized as mild cognitive impairment (n= 8), mild/moderate Alzheimer disease (n = 4), and late-stage Alzheimer disease (n = 9). Samples from subjects with no cognitive impairment (n = 10) that were age- and postmortem interval-matched with these cases were used as controls. The short postmortem intervalbrain samples were processed for postmitochondrial supernatant, nonsynaptic mitochondria, and synaptosome fractions. Samples were analyzed for several antioxidants (glutathione, glutathione peroxidase, glutathione reductase, glutathione-S-transferase, glucose-6-phosphate dehydrogenase, superoxide dismutase, catalase) and the oxidative marker, thiobarbituric acid reactive substances. The tissue was also analyzed for possible changes in protein damage using neurochemical markers for protein carbonyls, 3-nitrotyrosine, 4-hydroxynonenal, andacrolein. All 3 neuropil fractions (postmitochondrial supernatant, mitochondrial, and synaptosomal) demonstrated significant disease-dependent increases in oxidative markers. The highest changes were observed in the synaptosomal fraction. Both mitochondrial and synaptosomal fractions had significant declines in antioxidants (glutathione, glutathione peroxidase, glutathione-S-transferase, and superoxide dismutase). Levels of oxidative markers significantly correlated with Mini-Mental Status Examination scores. Oxidative stress was more localized to the synapses, with levels increasing in a disease-dependent fashion. These correlations implicate an involvement of oxidative stress in Alzheimer disease-related synaptic loss.

Reduction of beta-amyloid levels by novel protein kinase C(epsilon) activators

Isoform-specific protein kinase C (PKC) activators may be useful as therapeutic agents for the treatment of Alzheimer disease. Three new epsilon-specific PKC activators, made by cyclopropanation of polyunsaturated fatty acids, have been developed. These activators, AA-CP4, EPA-CP5, and DHA-CP6, activate PKCepsilon in a dose-dependent manner. Unlike PKC activators that bind to the 1,2-diacylglycerol-binding site, such as bryostatin and phorbol esters, which produce prolonged down-regulation, the new activators produced sustained activation of PKC. When applied to cells expressing human APPSwe/PS1delta, which produce large quantities of beta-amyloid peptide (Abeta), DCP-LA and DHA-CP6 reduced the intracellular and secreted levels of Abeta by 60-70%. In contrast to the marked activation of alpha-secretase produced by PKC activators in fibroblasts, the PKC activators produced only a moderate and transient activation of alpha-secretase in neuronal cells. However, they activated endothelin-converting enzyme to 180% of control levels, suggesting that the Abeta-lowering ability of these PKCepsilon activators is caused by increasing the rate of Abeta degradation by endothelin-converting enzyme and not by activating nonamyloidogenic amyloid precursor protein metabolism.

Abeta immunotherapy protects morphology and survival of adult-born neurons in doubly transgenic APP/PS1 mice

The hippocampus is heavily affected by progressive neurodegeneration and beta-amyloid pathology in Alzheimer's disease (AD). The hippocampus is also one of the few brain regions that generate new neurons throughout adulthood. Because hippocampal neurogenesis is regulated by both endogenous and environmental factors, we determined whether it benefits from therapeutic reduction of beta-amyloid peptide (Abeta)-related toxicity induced by passive Abeta immunotherapy. Abeta immunotherapy of 8-9-month-old mice expressing familial AD-causing mutations in the amyloid precursor protein and presenilin-1 genes with an antibody against Abeta decreased compact beta-amyloid plaque burden and promoted survival of newly born neurons in the hippocampal dentate gyrus. As these neurons matured, they exhibited longer dendrites with more complex arborization compared with newly born neurons in control-treated transgenic littermates. The newly born neurons showed signs of functional integration indicated by expression of the immediate-early gene Zif268 in response to exposure to a novel object. Abeta immunotherapy was associated with higher numbers of synaptophysin-positive synaptic boutons. Labeling dividing progenitor cells with a retroviral vector encoding green fluorescent protein (GFP) showed that Abeta immunotherapy restored the impaired dendritic branching, as well as the density of dendritic spines in new mature neurons. The presence of cellular prion protein (PrP(c)) on the dendrites of the GFP(+) newly born neurons is compatible with a putative role of PrP(c) in mediating Abeta-related toxicity in these cells. In addition, passive Abeta immunotherapy was accompanied by increased angiogenesis. Our data establish that passive Abeta immunotherapy can restore the morphological maturation of the newly formed neurons in the adult hippocampus and promote angiogenesis. These findings provide evidence for a role of Abeta immunotherapy in stimulating neurogenesis and angiogenesis in transgenic mouse models of AD, and they suggest the possibility that Abeta immunotherapy can recover neuronal and vascular functions in brains with beta-amyloidosis.

Luteolin promotes long-term potentiation and improves cognitive functions in chronic cerebral hypoperfused rats

Processes of synaptic plasticity, such as long-term potentiation (LTP), has been considered a cellular correlate of learning and memory and many neurological disorders accompanied by cognitive deficits exhibit abnormal synaptic function. This emerging concept is exemplified by Alzheimer's disease. Mounting evidence suggests that Alzheimer's disease begins with subtle alterations of hippocampal synaptic efficacy prior to frank neuronal degeneration, which make it critical to identify LTP enhancers to slow down or stop the progression of Alzheimer's disease. In this study, we found flavonoid luteolin could enhance basal synaptic transmission and facilitate the induction of LTP by high frequency stimulation in the dental gyrus of rat hippocampus. Furthermore, we investigated the effects of luteolin on chronic cerebral hypoperfusion-induced spatial learning dysfunction and LTP impairment in rat. The results showed chronic cerebral hypoperfusion produced by 2-vessel occlusion significantly impaired spatial learning and memory, and luteolin reversed the learning and memory deficit. 2-vessel occlusion resulted in dramatic inhibition of LTP formation in the hippocampus and luteolin significantly rescued the LTP impairment. These results demonstrate that luteolin not only directly modulates LTP formation, but also protects synapses from the detrimental effects of chronic cerebral hypoperfusion on LTP formation, which may contribute to the protective effects of luteolin on learning and memory. By immunoblotting, we found the effects of luteolin on LTP and memory may due to the activation of cAMP response element-binding protein (CREB). Therefore, flavonoid luteolin shows great potential as a novel treatment agent for protecting synaptic function and enhancing memory in neurodegenerative disorders.

Nicotinic receptors, amyloid-beta, and synaptic failure in Alzheimer's disease

Dysfunctional cholinergic transmission is thought to underlie, at least in part, memory impairment and cognitive deficits in Alzheimer's disease (AD). However, it is still unclear whether this is a consequence of the loss of cholinergic neurons and elimination of nicotinic acetycholine receptors (nAChRs) in AD brain or of a direct impact of molecular interactions of the amyloid-beta (Abeta) peptide with nAChRs, leading to dysregulation of receptor function. This review examines recent progress in our understanding of the roles of nicotinic receptors in mechanisms of synaptic plasticity, molecular interactions of Abeta with nAChRs, and how Abeta-induced dysregulation of nicotinic receptor function may underlie synaptic failure in AD.

Synaptic NMDA receptor activation stimulates alpha-secretase amyloid precursor protein processing and inhibits amyloid-beta production

Altered amyloid precursor protein (APP) processing leading to increased production and oligomerization of Abeta may contribute to Alzheimer's disease (AD). Understanding how APP processing is regulated under physiological conditions may provide new insights into AD pathogenesis. Recent reports demonstrate that excitatory neural activity regulates APP metabolism and Abeta levels, although understanding of the molecular mechanisms involved is incomplete. We have investigated whether NMDA receptor activity regulates APP metabolism in primary cultured cortical neurons. We report that a pool of APP is localized to the postsynaptic compartment in cortical neurons and observed partial overlap of APP with both NR1 and PSD-95. NMDA receptor stimulation increased nonamyloidogenic alpha-secretase-mediated APP processing, as measured by a 2.5-fold increase in cellular alpha-C-terminal fragment (C83) levels after glutamate or NMDA treatment. This increase was blocked by the NMDA receptor antagonists d-AP5 and MK801 but not by the AMPA receptor antagonist CNQX or the L-type calcium channel blocker nifedipine, was prevented by chelation of extracellular calcium, and was blocked by the alpha-secretase inhibitor TAPI-1. Cotreatment of cortical neurons with bicuculline and 4-AP, which stimulates glutamate release and activates synaptic NMDA receptors, evoked an MK801-sensitive increase in C83 levels. Furthermore, NMDA receptor stimulation caused a twofold increase in the amount of soluble APP detected in the neuronal culture medium. Finally, NMDA receptor activity inhibited both Abeta1-40 release and Gal4-dependent luciferase activity induced by beta-gamma-secretase-mediated cleavage of an APP-Gal4 fusion protein. Altogether, these data suggest that calcium influx through synaptic NMDA receptors promotes nonamyloidogenic alpha-secretase-mediated APP processing.

Senescence accelerated mouse strain is sensitive to neurodegeneration induced by mild impairment of oxidative metabolism

Neuronal loss and impairment of oxidative metabolism are frequently observed in aging associated neurodegenerative diseases. Thiamine deficiency (TD) induces the region selective neuronal loss in the brain, which has been used to model neurodegeneration, accompanied by mild impairment of oxidative metabolism. C57BL/6 mice were commonly used animals for TD experiments; however, the individual variations among C57BL/6 mice in response to TD limited the consistence of brain pathology. The senescence accelerated prone 8 (SAMP8) mouse strain exhibits age-related morphological changes in the brain and deficits in learning and memory. In this study, we compared the effects of TD on SAMP8 mice, senescence accelerated resistant 1 (SAMR1) mice and C57BL/6 mice. TD-induced body weight loss in SAMP8 mice was much greater than in SAMR1 and C57BL/6 mice. In addition, earlier and more severe loss of neurons in the submedial thalamic nucleus (SmTN) of the thalamus was detected in the SAMP8 mice. After 8 days of TD (TD8), the loss of NeuN-positive neurons in the SmTN of SAMP8, SAMR1 and C57BL/6 mice was 65%, 50%, and 36%, respectively. TD also caused accumulation of amyloid precursor protein (APP) in the thalamus. After TD10, APP immunoreactivity in the thalamus of SAMP8 was much more intense than that of SAMR1 and C57BL/6 mice. These results suggest that SAMP8 mice are sensitive to TD and therefore offer a useful model for studying aging related neurodegeneration caused by the impairment of oxidative metabolism.