Ionic effects of the Alzheimer's disease beta-amyloid precursor protein and its metabolic fragments

Impact of the Cannabinoid System in Alzheimer's Disease

Cannabinoids are compounds isolated from cannabis and are also widely present in both nervous and immune systems of animals. In recent years, with in-depth research on cannabinoids, their clinical medicinal value has been evaluated, and many exciting achievements have been continuously accumulating, especially in the field of neurodegenerative disease. Alzheimer's disease is the most common type of neurodegenerative disease that causes dementia and has become a global health problem that seriously impacts human health today. In this review, we discuss the therapeutic potential of cannabinoids for the treatment of Alzheimer's disease. How cannabinoids act on different endocannabinoid receptor subtypes to regulate Alzheimer's disease and the roles of the endocannabinoid system in Alzheimer's disease are outlined, and the underlying mechanisms are discussed. Finally, we summarize the most relevant opportunities of cannabinoid pharmacology related to Alzheimer's disease and discuss the potential usefulness of cannabinoids in the clinical treatment of Alzheimer's disease.

An Unbalanced Synaptic Transmission: Cause or Consequence of the Amyloid Oligomers Neurotoxicity?

Amyloid-β (Aβ) 1-40 and 1-42 peptides are key mediators of synaptic and cognitive dysfunction in Alzheimer's disease (AD). Whereas in AD, Aβ is found to act as a pro-epileptogenic factor even before plaque formation, amyloid pathology has been detected among patients with epilepsy with increased risk of developing AD. Among Aβ aggregated species, soluble oligomers are suggested to be responsible for most of Aβ's toxic effects. Aβ oligomers exert extracellular and intracellular toxicity through different mechanisms, including interaction with membrane receptors and the formation of ion-permeable channels in cellular membranes. These damages, linked to an unbalance between excitatory and inhibitory neurotransmission, often result in neuronal hyperexcitability and neural circuit dysfunction, which in turn increase Aβ deposition and facilitate neurodegeneration, resulting in an Aβ-driven vicious loop. In this review, we summarize the most representative literature on the effects that oligomeric Aβ induces on synaptic dysfunction and network disorganization.

Superior Synaptogenic Effect of Electrospun PLGA-PEG Nanofibers Versus PLGA Nanofibers on Human Neural SH-SY5Y Cells in a Three-Dimensional Culture System

Synapses are touted as the main structural and functional components of neural cells within in the nervous system, providing tissue connectivity and integration via the formation of perineuronal nets. In the present study, we evaluated the synaptogenic activity of electrospun PLGA and PLGA-PEG nanofibers on human SH-SY5Y cells after 14 days in vitro. Electrospun PLGA and PLGA-PEG nanofibers were fabricated and physicochemical properties were examined using the HNMR technique. The cells were classified into three random groups, i.e., control (laminin-coated surface), PLGA, and PLGA-PEG. Scaffolds' features, cell morphology, attachment, and alignment were monitored by SEM imaging. We performed MTT assay to measure cell survival rate. To evaluate neurite formation and axonal outgrowth, cells were stained with an antibody against β-tubulin III using immunofluorescence imaging. Antibodies against synapsin-1 and synaptophysin were used to explore the impact of PLGA and PLGA-PEG scaffolds on synaptogenesis and functional activity of synapses. According to SEM analysis, the PLGA-PEG scaffold had less thick nanofibers compared with the PLGA scaffold. Cell attachment, expansion, neurite outgrowth, and orientation were promoted in the PLGA-PEG group in comparison with the PLGA substrate (p < 0.05). MTT assay revealed that both scaffolds did not exert any neurotoxic effects on cell viability. Notably, PLGA-PEG surface increased cell viability compared to PLGA by time (p < 0.05). Immunofluorescence staining indicated an increased β-tubulin III level in the PLGA-PEG group days coincided with axonal outgrowth and immature neuron marker after seven compared with the PLGA and control groups (p < 0.05). Based on our data, both synaptogenesis and functional connectivity were induced in cells plated on the PLGA-PEG surface that coincide with the increase of synapsin-1 and synaptophysin in comparsion with the PLGA and control groups (p < 0.05). Taken together, our results imply that the PLGA-PEG nanofibers could provide the desirable microenvironment to develop perineuronal net formation, contributing to efficient synaptogenesis and neuron-to-neuron crosstalk.

Alzheimer's Amyloid-β is an Antimicrobial Peptide: A Review of the Evidence

The amyloid-β (Aβ) peptide has long been considered to be the driving force behind Alzheimer's disease (AD). However, clinical trials that have successfully reduced Aβ burden in the brain have not slowed the cognitive decline, and in some instances, have resulted in adverse outcomes. While these results can be interpreted in different ways, a more nuanced picture of Aβ is emerging that takes into account the facts that the peptide is evolutionarily conserved and is present throughout life in cognitively normal individuals. Recent evidence indicates a role for Aβ as an antimicrobial peptide (AMP), a class of innate immune defense molecule that utilizes fibrillation to protect the host from a wide range of infectious agents. In humans and in animal models, infection of the brain frequently leads to increased amyloidogenic processing of the amyloid-β protein precursor (AβPP) and resultant fibrillary aggregates of Aβ. Evidence from in vitro and in vivo studies demonstrates that Aβ oligomers have potent, broad-spectrum antimicrobial properties by forming fibrils that entrap pathogens and disrupt cell membranes. Importantly, overexpression of Aβ confers increased resistance to infection from both bacteria and viruses. The antimicrobial role of Aβ may explain why increased rates of infection have been observed in some of the AD clinical trials that depleted Aβ. Perhaps progress toward a cure for AD will accelerate once treatment strategies begin to take into account the likely physiological functions of this enigmatic peptide.

Quercetin promotes learning and memory performance concomitantly with neural stem/progenitor cell proliferation and neurogenesis in the adult rat dentate gyrus

The decline in neurogenesis is a very critical problem in Alzheimer disease. Different biological activities have been reported for medicinal application of quercetin. Herein, we investigated the neurogenesis potential of quercetin in a rat model of Alzheimer's disease induced by amyloid-beta injection. Rats were randomly divided into Control, Alzheimer + Saline and Alzheimer + Quercetin groups. Following the administration of Amyloid-beta, rats in the Alzheimer + Quercetin group received 40 mg/kg/day quercetin orally for one month. Our data demonstrated amyloid-β injection could impair learning and memory processing in rats indicated by passive avoidance test evaluation. We noted that one-month quercetin treatment alleviated the detrimental effects of amyloid-β on spatial learning and memory parameters using Morris water maze analysis. Quercetin was found to increase the number of proliferating neural stem/progenitor cells. Notably, quercetin increased the number of DCX-expressing cells, indicating the active dynamic growth of neural progenitor cells in the dentate gyrus of the hippocampus. We further observed that the quercetin improved the number of BrdU/NeuN positive cells contributed to enhanced adult neurogenesis. Based on our results, quercetin had the potential to promote the expression of BDNF, NGF, CREB, and EGR-1 genes involved in regulating neurogenesis. These data suggest that quercetin can play a valuable role in alleviating Alzheimer's disease symptoms by enhancing adult neurogenesis mechanism.

Amyloid growth and membrane damage: Current themes and emerging perspectives from theory and experiments on Aβ and hIAPP

Alzheimer's Disease (AD) and Type 2 diabetes mellitus (T2DM) are two incurable diseases both hallmarked by an abnormal deposition of the amyloidogenic peptides Aβ and Islet Amyloid Polypeptide (IAPP) in affected tissues. Epidemiological data demonstrate that patients suffering from diabetes are at high risk of developing AD, thus making the search for factors common to the two pathologies of special interest for the design of new therapies. Accumulating evidence suggests that the toxic properties of both Aβ or IAPP are ascribable to their ability to damage the cell membrane. However, the molecular details describing Aβ or IAPP interaction with membranes are poorly understood. This review focuses on biophysical and in silico studies addressing these topics. Effects of calcium, cholesterol and membrane lipid composition in driving aberrant Aβ or IAPP interaction with the membrane will be specifically considered. The cross correlation of all these factors appears to be a key issue not only to shed light in the countless and often controversial reports relative to this area but also to gain valuable insights into the central events leading to membrane damage caused by amyloidogenic peptides. This article is part of a Special Issue entitled: Protein Aggregation and Misfolding at the Cell Membrane Interface edited by Ayyalusamy Ramamoorthy.

Role of Amyloid-β and Tau Proteins in Alzheimer's Disease: Confuting the Amyloid Cascade

The "Amyloid Cascade Hypothesis" has dominated the Alzheimer's disease (AD) field in the last 25 years. It posits that the increase of amyloid-β (Aβ) is the key event in AD that triggers tau pathology followed by neuronal death and eventually, the disease. However, therapeutic approaches aimed at decreasing Aβ levels have so far failed, and tau-based clinical trials have not yet produced positive findings. This begs the question of whether the hypothesis is correct. Here we have examined literature on the role of Aβ and tau in synaptic dysfunction, memory loss, and seeding and spreading of AD, highlighting important parallelisms between the two proteins in all of these phenomena. We discuss novel findings showing binding of both Aβ and tau oligomers to amyloid-β protein precursor (AβPP), and the requirement for the presence of this protein for both Aβ and tau to enter neurons and induce abnormal synaptic function and memory. Most importantly, we propose a novel view of AD pathogenesis in which extracellular oligomers of Aβ and tau act in parallel and upstream of AβPP. Such a view will call for a reconsideration of therapeutic approaches directed against Aβ and tau, paving the way to an increased interest toward AβPP, both for understanding the pathogenesis of the disease and elaborating new therapeutic strategies.

LTP and memory impairment caused by extracellular Aβ and Tau oligomers is APP-dependent

The concurrent application of subtoxic doses of soluble oligomeric forms of human amyloid-beta (oAβ) and Tau (oTau) proteins impairs memory and its electrophysiological surrogate long-term potentiation (LTP), effects that may be mediated by intra-neuronal oligomers uptake. Intrigued by these findings, we investigated whether oAβ and oTau share a common mechanism when they impair memory and LTP in mice. We found that as already shown for oAβ, also oTau can bind to amyloid precursor protein (APP). Moreover, efficient intra-neuronal uptake of oAβ and oTau requires expression of APP. Finally, the toxic effect of both extracellular oAβ and oTau on memory and LTP is dependent upon APP since APP-KO mice were resistant to oAβ- and oTau-induced defects in spatial/associative memory and LTP. Thus, APP might serve as a common therapeutic target against Alzheimer's Disease (AD) and a host of other neurodegenerative diseases characterized by abnormal levels of Aβ and/or Tau.

DOI:http://dx.doi.org/10.7554/eLife.26991.001

Retro-inverso peptide inhibitor nanoparticles as potent inhibitors of aggregation of the Alzheimer's Aβ peptide

Aggregation of amyloid-β peptide (Aβ) is a key event in the pathogenesis of Alzheimer's disease (AD). We investigated the effects of nanoliposomes decorated with the retro-inverso peptide RI-OR2-TAT (Ac-rGffvlkGrrrrqrrkkrGy-NH2) on the aggregation and toxicity of Aβ. Remarkably low concentrations of these peptide inhibitor nanoparticles (PINPs) were required to inhibit the formation of Aβ oligomers and fibrils in vitro, with 50% inhibition occurring at a molar ratio of ~1:2000 of liposome-bound RI-OR2-TAT to Aβ. PINPs also bound to Aβ with high affinity (Kd=13.2-50 nM), rescued SHSY-5Y cells from the toxic effect of pre-aggregated Aβ, crossed an in vitro blood-brain barrier model (hCMEC/D3 cell monolayer), entered the brains of C57 BL/6 mice, and protected against memory loss in APPSWE transgenic mice in a novel object recognition test. As the most potent aggregation inhibitor that we have tested so far, we propose to develop PINPs as a potential disease-modifying treatment for AD.

Lixisenatide attenuates the detrimental effects of amyloid β protein on spatial working memory and hippocampal neurons in rats

Type 2 diabetes mellitus(T2DM) is a risk factor of Alzheimer's disease (AD), which is most likely linked to impairments of insulin signaling in the brain. Hence, drugs enhancing insulin signaling may have therapeutic potential for AD. Lixisenatide, a novel long-lasting glucagon-like peptide 1 (GLP-1) analogue, facilitates insulin signaling and has neuroprotective properties. We previously reported the protective effects of lixisenatide on memory formation and synaptic plasticity. Here, we describe additional key neuroprotective properties of lixisenatide and its possible molecular and cellular mechanisms against AD-related impairments in rats. The results show that lixisenatide effectively alleviated amyloid β protein (Aβ) 25-35-induced working memory impairment, reversed Aβ25-35-triggered cytotoxicity on hippocampal cell cultures, and prevented against Aβ25-35-induced suppression of the Akt-MEK1/2 signaling pathway. Lixisenatide also reduced the Aβ25-35 acute application induced intracellular calcium overload, which was abolished by U0126, a specific MEK1/2 inhibitor. These results further confirmed the neuroprotective and cytoprotective action of lixisenatide against Aβ-induced impairments, suggesting that the protective effects of lixisenatide may involve the activation of the Akt-MEK1/2 signaling pathway and the regulation of intracellular calcium homeostasis.

Nicotinic Cholinergic Mechanisms in Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative condition characterized by increased accumulation of Aβ and degeneration of cholinergic signaling between basal forebrain and hippocampus. Nicotinic acetylcholine receptors (nAChRs) are important mediators of cholinergic signaling in modulation of learning and memory function. Accumulating lines of evidence indicate that a nAChR subtype, α7 receptor (α7-nAChR), plays an important role in modulations of excitatory neurotransmitter release, improvement of learning and memory ability, and enhancement of cognitive function. Importantly, the expression and function of α7-nAChRs is altered in the brain of AD animal models and AD patients, suggesting that this nAChR subtype participates in AD pathogenesis and may serve as a novel therapeutic target for AD treatment. However, the mechanisms underlying the role of α7-nAChRs in AD pathogenesis are very complex, and either neuroprotective effects or neurotoxic effects may occur through the α7-nAChRs. These effects depend on the levels of α7-nAChR expression and function, disease stages, or the use of α7-nAChR agonists, antagonists, or allosteric modulators. In this chapter, we summarize recent progresses in the roles of α7-nAChRs played in AD pathogenesis and therapy.

The novel α7β2-nicotinic acetylcholine receptor subtype is expressed in mouse and human basal forebrain: biochemical and pharmacological characterization

We examined α7β2-nicotinic acetylcholine receptor (α7β2-nAChR) expression in mammalian brain and compared pharmacological profiles of homomeric α7-nAChRs and α7β2-nAChRs. α-Bungarotoxin affinity purification or immunoprecipitation with anti-α7 subunit antibodies (Abs) was used to isolate nAChRs containing α7 subunits from mouse or human brain samples. α7β2-nAChRs were detected in forebrain, but not other tested regions, from both species, based on Western blot analysis of isolates using β2 subunit-specific Abs. Ab specificity was confirmed in control studies using subunit-null mutant mice or cell lines heterologously expressing specific human nAChR subtypes and subunits. Functional expression in Xenopus oocytes of concatenated pentameric (α7)5-, (α7)4(β2)1-, and (α7)3(β2)2-nAChRs was confirmed using two-electrode voltage clamp recording of responses to nicotinic ligands. Importantly, pharmacological profiles were indistinguishable for concatenated (α7)5-nAChRs or for homomeric α7-nAChRs constituted from unlinked α7 subunits. Pharmacological profiles were similar for (α7)5-, (α7)4(β2)1-, and (α7)3(β2)2-nAChRs except for diminished efficacy of nicotine (normalized to acetylcholine efficacy) at α7β2- versus α7-nAChRs. This study represents the first direct confirmation of α7β2-nAChR expression in human and mouse forebrain, supporting previous mouse studies that suggested relevance of α7β2-nAChRs in Alzheimer disease etiopathogenesis. These data also indicate that α7β2-nAChR subunit isoforms with different α7/β2 subunit ratios have similar pharmacological profiles to each other and to α7 homopentameric nAChRs. This supports the hypothesis that α7β2-nAChR agonist activation predominantly or entirely reflects binding to α7/α7 subunit interface sites.

Effects of amyloid β-peptide fragment 31-35 on the BK channel-mediated K⁺ current and intracellular free Ca²⁺ concentration of hippocampal CA1 neurons

The present study characterizes the effects of Aβ31-35, a short active fragment of amyloid β-peptide (Aβ), upon the BK channel-mediated K⁺ current and intracellular free Ca²⁺ concentration ([Ca²⁺]i) of freshly dissociated pyramidal cells from rat CA1 hippocampus by using whole-cell patch-clamp recording and single cell Ca²⁺ imaging techniques. The results show that: (1) in the presence of voltage- and ATP-gated K⁺ channel blockers application of 5.0 μM Aβ31-35 significantly diminished transient outward K⁺ current amplitudes at clamped voltages between 0 and 45mV; (2) under the same conditions [Ca²⁺]i was minimally affected by 5.0 μM but significantly increased by 12.5 μM and 25 μM Aβ31-35; and (3) when 25 μM of a larger fragment of the amyloid β-peptide, Aβ25-35, was applied, the results were similar to those obtained with the same concentration of Aβ31-35. These results indicate that Aβ31-35 is likely to be the shortest active fragment of the full Aβ sequence, and can be as effectively as the full-length Aβ peptide in suppressing BK-channel mediated K⁺ currents and significantly elevating [Ca²⁺]i in hippocampal CA1 neurons.

The selective A-type K+ current blocker Tx3-1 isolated from the Phoneutria nigriventer venom enhances memory of naïve and Aβ(25-35)-treated mice

Potassium channels regulate many neuronal functions, including neuronal excitability and synaptic plasticity, contributing, by these means, to mnemonic processes. In particular, A-type K(+) currents (IA) play a key role in hippocampal synaptic plasticity. Therefore, we evaluated the effect of the peptidic toxin Tx3-1, a selective blocker of IA currents, extracted from the venom of the spider Phoneutria nigriventer, on memory of mice. Administration of Tx3-1 (i.c.v., 300 pmol/site) enhanced both short- and long-term memory consolidation of mice tested in the novel object recognition task. In comparison, 4-aminopyridine (4-AP; i.c.v., 30-300 pmol/site), a non-selective K(+) channel blocker did not alter long-term memory and caused toxic side effects such as circling, freezing and tonic-clonic seizures. Moreover, Tx3-1 (i.c.v., 10-100 pmol/site) restored memory of Aβ25-35-injected mice, and exhibited a higher potency to improve memory of Aβ25-35-injected mice when compared to control group. These results show the effect of the selective blocker of IA currents Tx3-1 in both short- and long-term memory retention and in memory impairment caused by Aβ25-35, reinforcing the role of IA in physiological and pathological memory processes.

Cations as switches of amyloid-mediated membrane disruption mechanisms: calcium and IAPP

Disruption of the integrity of the plasma membrane by amyloidogenic proteins is linked to the pathogenesis of a number of common age-related diseases. Although accumulating evidence suggests that adverse environmental stressors such as unbalanced levels of metal ions may trigger amyloid-mediated membrane damage, many features of the molecular mechanisms underlying these events are unknown. Using human islet amyloid polypeptide (hIAPP, aka amylin), an amyloidogenic peptide associated with β-cell death in type 2 diabetes, we demonstrate that the presence of Ca(2+) ions inhibits membrane damage occurring immediately after the interaction of freshly dissolved hIAPP with the membrane, but significantly enhances fiber-dependent membrane disruption. In particular, dye leakage, quartz crystal microbalance, atomic force microscopy, and NMR experiments show that Ca(2+) ions promote a shallow membrane insertion of hIAPP, which leads to the removal of lipids from the bilayer through a detergent-like mechanism triggered by fiber growth. Because both types of membrane-damage mechanisms are common to amyloid toxicity by most amyloidogenic proteins, it is likely that unregulated ion homeostasis, amyloid aggregation, and membrane disruption are all parts of a self-perpetuating cycle that fuels amyloid cytotoxicity.

Alzheimer disease and platelets: how's that relevant

Alzheimer Disease (AD) is the most common neurodegenerative disorder worldwide, and account for 60% to 70% of all cases of progressive cognitive impairment in elderly patients. At the microscopic level distinctive features of AD are neurons and synapses degeneration, together with extensive amounts of senile plaques and neurofibrillars tangles. The degenerative process probably starts 20-30 years before the clinical onset of the disease. Senile plaques are composed of a central core of amyloid β peptide, Aβ, derived from the metabolism of the larger amyloid precursor protein, APP, which is expressed not only in the brain, but even in non neuronal tissues. More than 30 years ago, some studies reported that human platelets express APP and all the enzymatic activities necessary to process this protein through the same pathways described in the brain. Since then a large number of evidence has been accumulated to suggest that platelets may be a good peripheral model to study the metabolism of APP, and the pathophysiology of the onset of AD. In this review, we will summarize the current knowledge on the involvement of platelets in Alzheimer Disease. Although platelets are generally accepted as a suitable model for AD, the current scientific interest on this model is very high, because many concepts still remain debated and controversial. At the same time, however, these still unsolved divergences mirror a difficulty to establish constant parameters to better defined the role of platelets in AD.

Antimicrobial properties of amyloid peptides

More than two dozen clinical syndromes known as amyloid diseases are characterized by the buildup of extended insoluble fibrillar deposits in tissues. These amorphous Congo red staining deposits known as amyloids exhibit a characteristic green birefringence and cross-β structure. Substantial evidence implicates oligomeric intermediates of amyloids as toxic species in the pathogenesis of these chronic disease states. A growing body of data has suggested that these toxic species form ion channels in cellular membranes causing disruption of calcium homeostasis, membrane depolarization, energy drainage, and in some cases apoptosis. Amyloid peptide channels exhibit a number of common biological properties including the universal U-shape β-strand-turn-β-strand structure, irreversible and spontaneous insertion into membranes, production of large heterogeneous single-channel conductances, relatively poor ion selectivity, inhibition by Congo red, and channel blockade by zinc. Recent evidence has suggested that increased amounts of amyloids not only are toxic to its host target cells but also possess antimicrobial activity. Furthermore, at least one human antimicrobial peptide, protegrin-1, which kills microbes by a channel-forming mechanism, has been shown to possess the ability to form extended amyloid fibrils very similar to those of classic disease-forming amyloids. In this paper, we will review the reported antimicrobial properties of amyloids and the implications of these discoveries for our understanding of amyloid structure and function.

Membrane pores in the pathogenesis of neurodegenerative disease

The neurodegenerative diseases described in this volume, as well as many nonneurodegenerative diseases, are characterized by deposits known as amyloid. Amyloid has long been associated with these various diseases as a pathological marker and has been implicated directly in the molecular pathogenesis of disease. However, increasing evidence suggests that these proteinaceous Congo red staining deposits may not be toxic or destructive of tissue. Recent studies strongly implicate smaller aggregates of amyloid proteins as the toxic species underlying these neurodegenerative diseases. Despite the outward obvious differences among these clinical syndromes, there are some striking similarities in their molecular pathologies. These include dysregulation of intracellular calcium levels, impairment of mitochondrial function, and the ability of virtually all amyloid peptides to form ion-permeable pores in lipid membranes. Pore formation is enhanced by environmental factors that promote protein aggregation and is inhibited by agents, such as Congo red, which prevent aggregation. Remarkably, the pores formed by a variety of amyloid peptides from neurodegenerative and other diseases share a common set of physiologic properties. These include irreversible insertion of the pores in lipid membranes, formation of heterodisperse pore sizes, inhibition by Congo red of pore formation, blockade of pores by zinc, and a relative lack of ion selectivity and voltage dependence. Although there exists some information about the physical structure of these pores, molecular modeling suggests that 4-6-mer amyloid subunits may assemble into 24-mer pore-forming aggregates. The molecular structure of these pores may resemble the β-barrel structure of the toxics pore formed by bacterial toxins, such as staphylococcal α-hemolysin, anthrax toxin, and Clostridium perfringolysin.

A mitocentric view of Alzheimer's disease suggests multi-faceted treatments

Alzheimer's disease (AD) is defined by senile plaques made of amyloid-beta peptide (Abeta), neurofibrillary tangles made of hyperphosphorylated tau proteins, and memory deficits. Thus, the events initiating the cascade leading to these end points may be more effective therapeutic targets than treating each facet individually. In the small percentage of cases of AD that are genetic (or animal models that reflect this form of AD), the factor initiating AD is clear (e.g., genetic mutations lead to high Abeta1-42 or hyperphosphorylated tau proteins). In the vast majority of AD cases, the cause is unknown. Substantial evidence now suggests that abnormalities in glucose metabolism/mitochondrial function/oxidative stress (GMO) are an invariant feature of AD and occur at an early stage of the disease process in both genetic and non-genetic forms of AD. Indeed, decreases in brain glucose utilization are diagnostic for AD. Changes in calcium homeostasis also precede clinical manifestations of AD. Abnormal GMO can lead to plaques, tangles, and the calcium abnormalities that accompany AD. Abnormalities in GMO diminish the ability of the brain to adapt. Therapies targeting mitochondria may ameliorate abnormalities in plaques, tangles, calcium homeostasis, and cognition that comprise AD.

Inhibition of soluble TNF signaling in a mouse model of Alzheimer's disease prevents pre-plaque amyloid-associated neuropathology

Microglial activation and overproduction of inflammatory mediators in the central nervous system (CNS) have been implicated in Alzheimer's disease (AD). Elevated levels of the pro-inflammatory cytokine tumor necrosis factor (TNF) have been reported in serum and post-mortem brains of patients with AD, but its role in progression of AD is unclear. Using novel engineered dominant negative TNF inhibitors (DN-TNFs) selective for soluble TNF (solTNF), we investigated whether blocking TNF signaling with chronic infusion of the recombinant DN-TNF XENP345 or a single injection of a lentivirus encoding DN-TNF prevented the acceleration of AD-like pathology induced by chronic systemic inflammation in 3xTgAD mice. We found that chronic inhibition of solTNF signaling with either approach decreased the LPS-induced accumulation of 6E10-immunoreactive protein in hippocampus, cortex, and amygdala. Immunohistological and biochemical approaches using a C-terminal APP antibody indicated that a major fraction of the accumulated protein was likely to be C-terminal APP fragments (beta-CTF) while a minor fraction consisted of Av40 and 42. Genetic inactivation of TNFR1-mediated TNF signaling in 3xTgAD mice yielded similar results. Taken together, our studies indicate that soluble TNF is a critical mediator of the effects of neuroinflammation on early (pre-plaque) pathology in 3xTgAD mice. Targeted inhibition of solTNF in the CNS may slow the appearance of amyloid-associated pathology, cognitive deficits, and potentially the progressive loss of neurons in AD.

Translocation of amyloid precursor protein C-terminal fragment(s) to the nucleus precedes neuronal death due to thiamine deficiency-induced mild impairment of oxidative metabolism

Thiamine deficiency (TD) is a model of neurodegeneration induced by mild impairment of oxidative metabolism. TD produces time-dependent glial activation, inflammation, oxidative stress, altered metabolism of amyloid precursor protein (APP), exacerbation of plaque formation from APP, and finally, selective neuron death in specific brain regions. The sub-medial thalamic nucleus (SmTN) is the most sensitive region to TD. Alteration in APP metabolism and nuclear translocation of carboxy-terminal fragments (CTF) of APP has been implicated in neuron death in other models of neurodegeneration. These experiments tested whether TD causes translocation of CTF into the nucleus of neurons in the SmTN that are destined to die after 9 days of TD by examining overlapping immunoreactivity (IR) of antibody APP 369 with either Alz90, 6E10 or 4G8 epitopes in the nuclei of the neurons in the SmTN. TD caused the accumulation of the CTF of APP in nuclei of SmTN neurons within 3 days of TD. These changes did not occur in the cortex which is spared in TD. Western blot analysis of nuclear fractions revealed a significant (61%; P < 0.026) increase in CTF 12 levels in TD SmTN (2.08 +/- 0.56) compared to control SmTN (1.29 +/- 0.41). Although TD increased CTF 15 levels in TD SmTN (1.95 +/- 0.73) compared to control SmTN (0.62 +/- 0.52) by 214%; P < 0.665 and decreased the full-length holo-APP levels in TD SmTN (0.32 +/- 0.30) compared to control SmTN (0.47 +/- 0.18) by 34%; P < 0.753, the differences were statistically insignificant. TD did not alter CTF 15 or CTF 12 levels in cortex. These findings demonstrate that changes in APP metabolism occur in early stages of TD, and they may play an important role in TD-induced selective neuronal loss.

Neurosteroids block the increase in intracellular calcium level induced by Alzheimer’s β-amyloid protein in long-term cultured rat hippocampal neurons

The neurotoxicity of beta-amyloid protein (AbetaP) is implicated in the etiology of Alzheimer's disease. We previously have demonstrated that AbetaP forms Ca(2+)-permeable pores on neuronal membranes, causes a marked increase in intracellular calcium level, and leads to neuronal death. Here, we investigated in detail the features of AbetaP-induced changes in intracellular Ca(2+) level in primary cultured rat hippocampal neurons using a multisite Ca(2+)-imaging system with fura-2 as a fluorescent probe. Only a small fraction of short-term cultured hippocampal neurons (ca 1 week in vitro) exhibited changes in intracellular Ca(2+) level after AbetaP exposure. However, AbetaP caused an acute increase in intracellular Ca(2+) level in long-term cultured neurons (ca 1 month in vitro). The responses to AbetaP were highly heterogeneous, and immunohistochemical analysis using an antibody to AbetaP revealed that AbetaP is deposited on some but not all neurons. Considering that the disruption of Ca(2+) homeostasis is the primary event in AbetaP neurotoxicity, substances that protect neurons from an AbetaP-induced intracellular Ca(2+) level increase may be candidates as therapeutic drugs for Alzheimer's disease. In line with the search for such protective substances, we found that the preadministration of neurosteroids including dehydroepiandrosterone, dehydroepiandrosterone sulfate, and pregnenolone significantly inhibits the increase in intracellular calcium level induced by AbetaP. Our results suggest the possible significance of neurosteroids, whose levels are reduced in the elderly, in preventing AbetaP neurotoxicity.

Beta-amyloid enhances intracellular calcium rises mediated by repeated activation of intracellular calcium stores and nicotinic receptors in acutely dissociated rat basal forebrain neurons

Beta-amyloid, a 39-43 amino acid peptide, may exert its biological effects via neuronal nicotinic acetylcholine receptors. Using the ratiometric dye, fura-2, we examined the effect of soluble beta-amyloid(1-42) on the concentration of intracellular Ca(2+) ([Ca(2+)](i)) in acutely dissociated rat basal forebrain neurons. Focal applications of nicotine (0.5-20 mM), evoked dose-dependent increases in intracellular [Ca(2+)](i) that were mediated by the entry of extracellular Ca(2+) via nicotinic acetylcholine receptors, and the release of intracellular Ca(2+) from stores. With repeated nicotine challenges, the nicotinic responses were potentiated by 98 +/- 12% (P < 0.05) while beta-amyloid(1-42)(100 nM) was present for approximately 5 min. This potentiation became larger during the subsequent washout of beta-amyloid(1-42), which was associated with a gradual rise in baseline [Ca(2+)](i). Application of beta-amyloid(1-42)by itself did not alter [Ca(2+)](i), and beta-amyloid(1-42)also had no significant effect on the response to repeated KCl challenges. Therefore, beta-amyloid(1-42) caused neither gross disturbance of cellular Ca(2+) homeostasis nor enhancement of voltage-gated Ca(2+) channels. Interestingly, beta-amyloid(1-42) transiently potentiated the response to repeated caffeine challenges, which was also associated with a transient rise in baseline [Ca(2+)](i). beta-amyloid(1-42) potentiation of nicotine-evoked rises in [Ca(2+)](i) was reversed by the SERCA pump inhibitor, thapsigargin, and the mitochondrial Na(+)/Ca(2+) exchanger inhibitor, CGP-37157. These results suggest that the dysregulation of [Ca(2+)](i) by beta-amyloid(1-42) during multiple challenges with nicotine or caffeine involved the sensitization or overfilling of intracellular stores that are maintained by SERCA pump and Ca(2+) efflux from the mitochondria.

Neuronal nicotinic acetylcholine receptors serve as sensitive targets that mediate beta-amyloid neurotoxicity

Alzheimer's disease (AD) is the most common form of brain dementia characterized by the accumulation of beta-amyloid peptides (Abeta) and loss of forebrain cholinergic neurons. Abeta accumulation and aggregation are thought to contribute to cholinergic neuronal degeneration, in turn causing learning and memory deficits, but the specific targets that mediate Abeta neurotoxicity remain elusive. Recently, accumulating lines of evidence have demonstrated that Abeta directly modulates the function of neuronal nicotinic acetylcholine receptors (nAChRs), which leads to the new hypothesis that neuronal nAChRs may serve as important targets that mediate Abeta neurotoxicity. In this review, we summarize current studies performed in our laboratory and in others to address the question of how Abeta modulates neuronal nAChRs, especially nAChR subunit function.

The role of beta-amyloid protein in synaptic function: implications for Alzheimer's disease therapy

Alzheimer's disease (AD) is a neurodegenerative disorder characterized by progressive and irreversible loss of memory and other cognitive functions. Substantial evidence based on genetic, neuropathological and biochemical data has established the central role of beta-amyloid protein (betaAP) in this pathology. Although the precise etiology of AD is not well understood yet, strong evidence for some of the molecular events that lead to progressive brain dysfunction and neurodegeneration in AD has been afforded by identification of biochemical pathways implicated in the generation of betaAP, development of transgenic models exhibiting progressive disease pathology and by data on the effects of betaAP at the neuronal network level. However, the mechanisms by which betaAP causes cognitive decline have not been determined, nor is it clear if the degree of dementia correlates in time with the degree of neuronal loss. Hence, it is of interest to understand the biochemical processes involved in the mechanisms of betaAP-induced neurotoxicity and the mechanisms involved in electrophysiological effects of this protein on different parameters of synaptic transmission and on neuronal firing properties. In this review we analyze recent evidence suggesting a complex role of betaAP in the molecular events that lead to progressive loss of function and eventually to neurodegeneration in AD as well as the therapeutic implications based on betaAP metabolism inhibition.

BACE inhibitor reduces APP-beta-C-terminal fragment accumulation in axonal swellings of okadaic acid-induced neurodegeneration

Emerging evidence suggests that not only beta-amyloid but also other amyloid precursor protein (APP) fragments, such as the beta-C-terminal fragment (betaCTF), might be involved in Alzheimer's disease (AD). Treatment of neurons with okadaic acid (OA), a protein phosphatase-2A inhibitor, has been used to induce tau phosphorylation and neuronal death to create a research model of AD. In this study, we analyzed axonopathy and APP regulation in cultured rat neurons treated with OA. After OA treatment, the neurons presented with axonal swellings filled with vesicles, microtubule fragments, and transport molecules such as kinesin and synapsin-I. Western blotting showed that intracellular APP levels were increased and immunocytochemistry using antibodies against the APP C-terminus showed that APP accumulated in the axonal swellings. This APP C-terminus immunoreactivity disappeared when neurons were cotreated with a beta-secretase inhibitor, but not with alpha- or gamma-secretase inhibitors, indicating that the accumulation was primarily composed of APP-betaCTF. These findings provide the first evidence that APP-betaCTF can accumulate in the axons of OA-treated neurons, and may suggest that APP-betaCTF is involved in the pathogenesis of AD.

Nuclear factor-κB activation by reactive oxygen species mediates voltage-gated K+ current enhancement by neurotoxic β-amyloid peptides in nerve growth factor-differentiated PC-12 cells and hippocampal neurones

Increased activity of plasma membrane K+ channels, leading to decreased cytoplasmic K+ concentrations, occurs during neuronal cell death. In the present study, we showed that the neurotoxic beta-amyloid peptide Abeta(25-35) caused a dose-dependent (0.1-10 microm) and time-dependent (> 12 h) enhancement of both inactivating and non-inactivating components of voltage-dependent K+ (VGK) currents in nerve growth factor-differentiated rat phaeochromocytoma (PC-12) cells and primary rat hippocampal neurones. Similar effects were exerted by Abeta(1-42), but not by the non-neurotoxic Abeta(35-25) peptide. Abeta(25-35) and Abeta(1-42) caused an early (15-20 min) increase in intracellular Ca(2+) concentration. This led to an increased production of reactive oxygen species (ROS), which peaked at 3 h and lasted for 24 h; ROS production seemed to trigger the VGK current increase as vitamin E (50 microm) blocked both the Abeta(25-35)- and Abeta(1-42)-induced ROS increase and VGK current enhancement. Inhibition of protein synthesis (cycloheximide, 1 microg/mL) and transcription (actinomycin D, 50 ng/mL) blocked Abeta(25-35)-induced VGK current enhancement, suggesting that this potentiation is mediated by transcriptional activation induced by ROS. Interestingly, the specific nuclear factor-kappaB inhibitor SN-50 (5 microm), but not its inactive analogue SN-50M (5 microm), fully counteracted Abeta(1-42)- or Abeta(25-35)-induced enhancement of VGK currents, providing evidence for a role of this family of transcription factors in regulating neuronal K+ channel function during exposure to Abeta.

Amyloid peptide channels

At least 16 distinct clinical syndromes including Alzheimer's disease (AD), Parkinson's disease (PD), rheumatoid arthritis, type II diabetes mellitus (DM), and spongiform encephelopathies (prion diseases), are characterized by the deposition of amorphous, Congo red-staining deposits known as amyloid. These "misfolded" proteins adopt beta-sheet structures and aggregate spontaneously into similar extended fibrils despite their widely divergent primary sequences. Many, if not all, of these peptides are capable of forming ion-permeable channels in vitro and possibly in vivo. Common channel properties include irreversible, spontaneous insertion into membranes, relatively large, heterogeneous single-channel conductances, inhibition of channel formation by Congo red, and blockade of inserted channels by Zn2+. Physiologic effects of amyloid, including Ca2+ dysregulation, membrane depolarization, mitochondrial dysfunction, inhibition of long-term potentiation (LTP), and cytotoxicity, suggest that channel formation in plasma and intracellular membranes may play a key role in the pathophysiology of the amyloidoses.

Pathophysiological roles of amyloidogenic carboxy-terminal fragments of the beta-amyloid precursor protein in Alzheimer's disease

Several lines of evidence suggest that some of the neurotoxicity in Alzheimer's disease (AD) is attributed to proteolytic fragments of amyloid precursor protein (APP) and beta-amyloid (Abeta) may not be the sole active component involved in the pathogenesis of AD. The potential effects of other cleavage products of APP need to be explored. The CTFs, carboxy-terminal fragments of APP, have been found in AD patients' brain and reported to exhibit much higher neurotoxicity in a variety of preparations than Abeta. Furthermore CTFs are known to impair calcium homeostasis and learning and memory through blocking LTP, triggering a strong inflammatory reaction through MAPKs- and NF-kappaB-dependent astrocytosis and iNOS induction. Recently, it was reported that CTF translocated into the nucleus, binding with Fe65 and CP2, and in turn, affected transcription of genes including glycogen synthase kinase-3beta, which results in the induction of tau-rich neurofibrillary tangles and subsequently cell death. Spatial memory of transgenic (Tg) mice overexpressing CT100 was significantly impaired and CTFs were detected in the neurons as well as in plaques of the Tg mice and double Tg mice carrying CT100 and mutant tau. In this review, we summarize observations indicating that both CTF and Abeta may participate in the neuronal degeneration in the progress of AD by differential mechanisms.

Specific functions of Drosophila amyloid precursor-like protein in the development of nervous system and nonneural tissues

Drosophila amyloid precursor-like protein (APPL) is expressed extensively in the nervous system soon after neuronal differentiation. By utilizing different transgenic flies, we studied the physiological function of two APPL protein forms, membrane-bound form (mAPPL) and secreted form (sAPPL), in neural development. We found that neither deletion nor overexpression of APPL protein altered the gross structure of mushroom bodies in the adult brain. No changes were detected in cell types and their relative ration in embryo-derived cultures from all APPL mutants. However, the neurite length was significantly increased in mutants overexpressing mAPPL. In addition, mutants lacking sAPPL had numerous neurite branches with abnormal lamellate membrane structures (LMSs) and blebs, while no apoptosis was detected in these neurons. The abnormal neurite morphology was most likely due to the disorganization of the cytoskeleton, as shown by double staining of actin filaments and microtubules. Electrophysiologically, A-type K+ current was significantly enhanced, and spontaneous excitatory postsynaptic potentials (sEPSPs) were greatly increased in APPL mutants lacking sAPPL. Moreover, panneural overexpression of different forms of APPL protein generated different defects of wings and cuticle in adult flies. Taken together, our results suggest that both mAPPL and sAPPL play essential roles in the development of the central nervous system and nonneural tissues.

Antagonist of the amylin receptor blocks beta-amyloid toxicity in rat cholinergic basal forebrain neurons

Salvage of cholinergic neurons in the brain through a blockade of the neurotoxic effects of amyloidbeta protein (Abeta) is one of the major, but still elusive, therapeutic goals of current research in Alzheimer's disease (AD). To date, no receptor has been unequivocally identified for Abeta. Human amylin, which acts via a receptor composed of the calcitonin receptor-like receptor and a receptor-associated membrane protein, possesses amyloidogenic properties and has a profile of neurotoxicity that is strikingly similar to Abeta. In this study, using primary cultures of rat cholinergic basal forebrain neurons, we show that acetyl-[Asn30, Tyr32] sCT(8-37) (AC187), an amylin receptor antagonist, blocks Abeta-induced neurotoxicity. Treatment of cultures with AC187 before exposure to Abeta results in significantly improved neuronal survival as judged by MTT and live-dead cell assays. Quantitative measures of Abeta-evoked apoptotic cell death, using Hoechst and phosphotidylserine staining, confirm neuroprotective effects of AC187. We also demonstrate that AC187 attenuates the activation of initiator and effector caspases that mediate Abeta-induced apoptotic cell death. These data are the first to show that expression of Abeta toxicity may occur through the amylin receptor and suggest a novel therapeutic target for the treatment of AD.

Apolipoprotein E isoforms and apolipoprotein AI protect from amyloid precursor protein carboxy terminal fragment-associated cytotoxicity

Inheritance of the apolipoprotein (APO) E gene epsilon4 or epsilon2 allele alters the risk of developing Alzheimer disease (AD), while increased alpha-tocopherol (AT) intake appears to lower the risk of AD. As APOE is a major apolipoprotein in the CNS and AT in vivo is transported in lipoproteins, we tested the hypothesis that CNS lipoproteins, as modeled by relevant concentrations of high density lipoprotein (HDL), and AT would interact to suppress neurotoxicity in a cell culture model of amyloid beta (Abeta)- related toxicity. These cells conditionally express C99-derived peptides, proposed to be a key step in AD pathogenesis; this expression is closely associated with subsequent cell death. We found that physiologic concentrations of lipoproteins present in the CNS protected from C99-associated toxicity and provided evidence for two mechanisms of protection. The first was AT-independent, APOE isoform-dependent, and most potent for the APOE2 isoform. The second was a synergistic protection afforded by a combination of APOAI, or less so APOE, and AT. These data provide a novel explanation for the apparent AD-protective effect of inheriting an epsilon2 APOE allele, and suggest that optimizing AT enrichment of CNS lipoproteins or devising APOAI mimetics may augment AT efficacy in treating AD.

Alzheimer's amyloid peptides mediate hypoxic up-regulation of L-type Ca2+ channels

We examined the effects of chronic hypoxia on recombinant human L-type Ca2+ channel alpha1C subunits stably expressed in HEK 293 cells, using whole-cell patch-clamp recordings. Current density was dramatically increased following 24 h exposure to chronic hypoxia (CH), and membrane channel protein levels were enhanced. CH also increased the levels of Alzheimer's amyloid beta peptides (AbetaPs), determined immunocytochemically. Pharmacological prevention of AbetaP production (via exposure to inhibitors of secretase enzymes that are required to cleave AbetaP from its precursor protein) prevented hypoxic augmentation of currents, as did inhibition of vesicular trafficking with bafilomycin A1. The enhancing effect of AbetaPs or CH were abolished following incubation with the monoclonal 3D6 antibody, raised against the extracellular N' terminus of AbetaP. Immunolocalization and immunoprecipitation studies provided compelling evidence that AbetaPs physically associated with the alpha1C subunit, and this association was promoted by hypoxia. These data suggest an important role for AbetaPs in mediating the increase in Ca2+ channel activity following CH and show that AbetaPs act post-transcriptionally to promote alpha1C subunit insertion into (and/or retention within) the plasma membrane. Such an action will likely contribute to the Ca2+ dyshomeostasis of Alzheimer's disease and may contribute to the mechanisms underlying the known increased incidence of this neurodegenerative disease following hypoxic episodes.

A slowly formed transient conformer of Abeta(1-40) is toxic to inward channels of dissociated hippocampal and cortical neurons of rats

The mechanism by which amyloid peptide (Abeta(1-40)) produces effects on neurotransmission is currently unresolved. In initial experiments, using the patch-clamp technique, we found that 11.5 microM of preaggregated Abeta(1-40) altered the hippocampal neuron resting membrane potential and inhibited action potential firing. To identify the toxic species, the effects of Abeta(1-40) on sodium (I(Na)), calcium (I(Ca)), and potassium (I(K)) currents in hippocampal neurons were examined as a function of peptide aggregation state in a specially designed miniature recording chamber. Aggregation reactions were induced by constant shaking, starting with 50 microM monomeric peptide. At 10- to 30-min intervals, the ionic currents were examined on a single neuron suspended in control saline and then in a 100-microl sample of the aggregating peptide. We found that samples of the peptide taken 60-120 min into the aggregation process contained species that exhibited maximal inhibitory effects over a broad potential range in the rank ordering of I(Na) > I(Ca). I(K) was inhibited only slightly at depolarized potentials. Inhibition of APF through blockade of these channels would inhibit normal neuronal activity and directly contribute to cognitive dysfunction. In previous studies on SH-EP cells, we showed that neither monomeric nor fibrillar peptide had significant effect on cell viability except during exposure to the 60-120 minute aggregation product when cell death was recorded. Our kinetic model demonstrated that the toxic species was a slowly formed transient conformer (activated monomer), which was the only aggregating species that passed through a maximum concentration during aggregation. This species amounted to only a small fraction of the total amount of aggregating peptide. We conclude that, for both the native neurons in the present study as well as SH-EP1 cells, the activated monomeric conformer of the peptide is the toxic species.

Messenger RNA and protein expression analysis of voltage-gated potassium channels in the brain of A?25-35-treated rats

Potassium channel dysfunction has been implicated in Alzheimer's disease. In the present study, the expression of voltage-gated potassium channel (Kv) subunits in rat brain was measured after a single intracerebroventricular injection of beta-amyloid peptide 25-35 (Abeta(25-35)). After injection of Abeta, the spatial memory of rats was significantly impaired in the Morris water maze. Expression of five main Kv channel subunits (Kv1.5, Kv2.1, Kv1.4, Kv4.2, and Kv4.3) in mRNA level was assessed by using reverse transcription-polymerase chain reaction (RT-PCR). The mRNA levels of Kv2.1 and Kv1.4 were increased by 72% and 67%, respectively, in hippocampus, and Kv4.2 mRNA was increased by 58% in cortex. No other significant mRNA expression changes were found in Abeta-treated rats. The protein expression of Kv2.1, Kv1.4, and Kv4.2 was detected by using Western blotting. Kv2.1 and Kv1.4 protein levels were increased by 48% and 50%, respectively, in hippocampus of Abeta-treated rats, and Kv4.2 protein was increased by 42% in cerebral cortex. This study indicates that the expression up-regulation of Kv1.4, Kv2.1, and Kv4.2 in Abeta-induced cognitive impairment might play an important role in the pathogenesis of Alzheimer's disease.

Amyloid precursor protein carboxy-terminal fragments modulate G-proteins and adenylate cyclase activity in Alzheimer's disease brain

The influence of three C-terminal sequences and of transmembrane domain from amyloid precursor protein (APP) on the activity of G-proteins and of the coupled cAMP-signalling system in the postmortem Alzheimer's disease (AD) and age-matched control brains was compared. 10 microM APP(639-648)-APP(657-676) (PEP1) causes a fivefold stimulation in the [35S]GTPgammaS-binding to control hippocampal G-proteins. APP(657-676) (PEP2) and APP(639-648) (PEP4) showed less pronounced stimulation whereas cytosolic APP(649-669) (PEP3) showed no regulatory activity in the [35S]GTPgammaS-binding. PEP1 also showed 1.4-fold stimulatory effect of on the high-affinity GTPase and adenylate cyclase activity in control membranes, whereas in AD hippocampal membranes the stimulatory effect of PEP1 was substantially weaker. The PEP1 stimulation of the [35S]GTPgammaS-binding to the control membranes was significantly reduced by 1.5 mM glutathione, 0.5 mM antioxidant N-acetylcysteine and, in the greatest extent, by 0.01 mM of desferrioxamine. In AD hippocampus these antioxidants revealed no remarkable reducing effect on PEP1-induced stimulation. Our results suggest that C-terminal and transmembrane APP sequences possess receptor-like G-protein activating function in human hippocampus and that abnormalities of this function contribute to AD progression. The stimulatory action of these sequences on G-protein mediated signalling suggests the region-specific formation of reactive species.

Beta-amyloid peptide activates non-alpha7 nicotinic acetylcholine receptors in rat basal forebrain neurons

Alzheimer's disease (AD) is a progressive neurodegenerative condition characterized by profound deficits in memory and cognitive function. Neuropathological hallmarks of the disease include a loss of basal forebrain cholinergic neurons and the deposition of beta-amyloid peptide (Abeta) in neuritic plaques. At a cellular level, considerable attention has focused on a study of Abeta interactions with the neuronal nicotinic acetylcholine receptor (nAChR) subtypes. In this study, using cell-attached and outside-out single channel recordings from acutely dissociated rat basal forebrain neurons, we report that Abeta and nicotine activate nAChRs with two distinct levels of single-channel conductance. Whole cell recordings from these neurons reveal Abeta and nicotine, in a concentration-dependent and reversible manner, evoke brisk depolarizing responses and an inward current. The effects of Abeta on both single channel and whole cell are blocked by the noncompetitive nAChR antagonist mecamylamine and competitive nAChR antagonist dihydro-beta-erythroidine, but not the specific alpha7-selective nAChR antagonist methyllycaconitine, indicating that Abeta activated non-alpha7 nAChRs on basal forebrain neurons. In addition, the non-alpha7 nAChR agonists UB-165, epibatidine, and cytisine, but not the selective alpha7 agonist AR-R17779, induced similar responses as Abeta and nicotine. Thus non-alpha7 nAChRs may also represent a novel target in mediating the effects of Abeta in AD.

Activation of muscarinic receptors inhibits beta-amyloid peptide-induced signaling in cortical slices

Deposition of fibrillar aggregates of the beta-amyloid peptide (Abeta) is a key pathologic feature during the early stage of Alzheimer's disease. The initial neuronal responses to Abeta in cortical circuits and the regulation of Abeta-induced signaling remain unclear. In this study, we found that exposure of cortical slices to Abeta(1-42) or Abeta(25-35) induced a marked increase in the activation of protein kinase C (PKC) and Ca(2+)/calmodulin-dependent kinase II (CaMKII), two enzymes critically involved in a variety of cellular functions. Activation of M1 muscarinic receptors, but not nicotinic receptors, significantly inhibited the Abeta activation of PKC and CaMKII. Increasing inhibitory transmission mimicked the M1 effect on Abeta, whereas blocking GABA(A) receptors eliminated the M1 action. Moreover, electrophysiological evidence shows that application of Abeta to cortical slices induced action potential firing and enhanced excitatory postsynaptic currents, whereas muscarinic agonists potently increased inhibitory postsynaptic currents. These results suggest that Abeta activates PKC and CaMKII through enhancing excitatory activity in glutamatergic synaptic networks. Activation of M1 receptors inhibits Abeta signaling by enhancing the counteracting GABA(ergic) inhibitory transmission. Thus the muscarinic reversal of the Abeta-induced biochemical and physiological changes provides a potential mechanism for the treatment of Alzheimer's disease with cholinergic enhancers.

Amyloid ?-protein fragment 31-35 suppresses delayed rectifying potassium channels in membrane patches excised from hippocampal neurons in rats

To clarify the early initial mechanism underlying the neurotoxicity of amyloid beta-protein (AbetaP) and the shorter essential active sequence in native AbetaP molecules, the effects of AbetaP31-35 and AbetaP25-35 on delayed rectifier K+ current (Ik) were investigated in the inside-out membrane patches excised from hippocampal neurons of rats. The results showed that: 1) After application of AbetaP31-35 (5 microM) to the inside of patches, the average open frequency and open probability of Ik channels reversibly decreased by 70.45 +/- 35.75% and 86.9 +/- 11.13%, respectively; the mean open time decreased by 47.1 +/- 38.8%, while the mean current amplitude of Ik channels was not significantly affected. 2) Application of AbetaP25-35 at the same concentration showed similar effects as did the AbetaP31-35 application. It has generally been accepted that AbetaP25-35 acts as a full-length AbetaP molecule does, so our findings suggest that the neurotoxicity of AbetaP may be initiated by the functional suppression of Ik channels and the sequence of 31-35 might be the shorter active sequence in AbetaP responsible for its neurotoxicity.

Novel cognitive improving and neuroprotective activities of Polygala tenuifolia Willdenow extract, BT-11

We carried out this study to search a new active constituent that had cognitive enhancing activity and low side effects from natural source. We found that the extract of dried root of Polygala tenuifolia Willdenow (BT-11, 10 mg/kg, i.p.) could significantly reverse scopolamine-induced cognitive impairments in rat, using a passive avoidance and a water maze test. We also investigated the effects of BT-11 on neurotoxicity induced by glutamate (Glu) and toxic metabolites of amyloid precursor protein (APP) such as amyloid beta protein (A beta) and C-terminal fragment of APP (CT) in primary cultured neurons of rat. The pretreatment of BT-11 (0.5, 3, and 5 micro g/ml) significantly reduced cell death induced by Glu (1 mM), A beta (10 micro M) and CT105 (10 micro M) in a dose-dependent manner. In addition, BT-11 inhibited acetylcholinesterase (AChE) activity in a dose-dependent and non-competitive manner (IC(50) value; 263.7 micro g/ml). Our novel findings suggest the possibility that this extract may have some protective effects against neuronal death and cognitive impairments in Alzheimer's disease (AD), or other neurodegenerative diseases related to excitotoxicity and central cholinergic dysfunction.

The channel hypothesis of Alzheimer's disease: current status

The channel hypothesis of Alzheimer's disease (AD) proposes that the beta-amyloid (Abeta) peptides which accumulate in plaques in the brain actually damage and/or kill neurons by forming ion channels. Evidence from a number of laboratories has demonstrated that Abeta peptides can form ion channels in lipid bilayers, liposomes, neurons, oocyctes, and endothelial cells. These channels possess distinct physiologic characteristics that would be consistent with their toxic properties. Abeta channels are heterogeneous in size, selectivity, blockade, and gating. They are generally large, voltage-independent, and relatively poorly selective amongst physiologic ions, admitting calcium ion (Ca(2+)), Na(+), K(+), Cs(+), Li(+), and possibly Cl(-). They are reversibly blocked by zinc ion (Zn(2+)), and tromethamine (tris), and irreversibly by aluminum ion (Al(3+)). Congo red inhibits channel formation, but does not block inserted channels. Although much evidence implicates Abeta peptides in the neurotoxicity of AD, no other toxic mechanism has been demonstrated to be the underlying etiology of AD. Channel formation by several other amyloid peptides lends credence to the notion that this is a critical mechanism of cytotoxicity.

Annexin 5 and apolipoprotein E2 protect against Alzheimer's amyloid-beta-peptide cytotoxicity by competitive inhibition at a common phosphatidylserine interaction site

Amyloid-beta-protein (betaA/4, AbetaP) accumulates in the brains of patients with Alzheimer's disease (AD), regardless of genetic etiology, and is thought to be the toxic principle responsible for neuronal cell death. The varepsilon4 allele of apoE has been linked closely to earlier onset of AD and increased deposition of the amyloid peptide, regardless of the clinical status of AD, while the apoE varepsilon2 allele is generally protective. We have previously hypothesized that the cell target for amyloid peptide might be the apoptotic signal molecule phosphatidylserine (PS). We report here that annexin 5, a specific ligand for PS, not only blocks amyloid peptide AbetaP[1-40] cytotoxicity, but competitively inhibits AbetaP[1-40]-dependent aggregation of PS liposomes. In addition, we find that apoE2, but not apoE4, can not only perform the same protective effect on cells exposed to AbetaP[1-40], but can also competitively inhibit PS liposome aggregation and fusion by the amyloid peptide. Altogether, the in vivo and in vitro results reported here provide fundamental insight to the process by which amyloid targets specific neurons for destruction, and suggest that PS may be a surface "receptor" site for AbetaP binding. These results also provide a biochemical mechanism by which the apoE varepsilon2 allele, but not apoE varepsilon4, can be protective towards the incidence and progression of Alzheimer's disease.

Hypoxia potentiates exocytosis and Ca2+ channels in PC12 cells via increased amyloid beta peptide formation and reactive oxygen species generation

Exposure of PC12 cells to chronic hypoxia (CH; 10 % O(2), 24 h) augments catecholamine secretion via formation of a Cd2+-resistant Ca2+ influx pathway, and up-regulates native L-type Ca2+ channels. These effects are mimicked by exposure of cells to Alzheimer's disease-associated amyloid beta peptides (AbetaPs). Since pathological effects of AbetaPs have been associated with increased levels of reactive oxygen species (ROS), the involvement of ROS in hypoxia-mediated up-regulation of exocytosis and Ca2+ channel activity was examined. Both melatonin and ascorbic acid (two structurally unrelated antioxidants) fully blocked the enhancement of catecholamine secretion caused by CH (as determined amperometrically). Enhanced immunofluorescence, observed in chronically hypoxic cells using a primary monoclonal antibody raised against the N-terminus of AbetaP, was also suppressed by melatonin. Ascorbic acid, melatonin and ebselen (an additional antioxidant) also fully prevented augmentation of whole-cell Ca2+ currents caused by CH (as monitored using whole-cell patch-clamp recordings). Exposure of normoxic cells to H(2)O(2) (40 microM, 24 h), like hypoxia, caused Ca2+ channel up-regulation. Importantly, AbetaP formation appeared to be an absolute requirement for the effects of hypoxia, since the ability of CH to augment exocytosis and Ca2+ channel activity was blocked by two novel inhibitors of gamma secretase, an enzyme complex required for AbetaP formation. Our results indicate that the effects of hypoxia require ROS generation from AbetaPs, and suggest that elevated levels of ROS mediate hypoxic and AbetaP-mediated pathological remodelling of Ca2+ homeostasis.