Deposition of monomeric, not oligomeric, Abeta mediates growth of Alzheimer's disease amyloid plaques in human brain preparations.

Senile plaques composed of the peptide Abeta contribute to the pathogenesis of Alzheimer's disease (AD), and mechanisms underlying their formation and growth may be exploitable as therapeutic targets. To examine the process of amyloid plaque growth in human brain, we have utilized size exclusion chromatography (SEC), translational diffusion measured by NMR, and in vitro models of Abeta amyloid growth to identify the oligomerization state of Abeta that is competent to add onto an existing amyloid deposit. SEC of radiolabeled and unlabeled Abeta over a concentration range of 10(-)(10)-10(-)(4) M demonstrated that the freshly dissolved peptide eluted as a single low molecular weight species, consistent with monomer or dimer. This low molecular weight Abeta species isolated by SEC was competent to deposit onto preexisting amyloid in preparations of AD cortex, with first-order kinetic dependence on soluble Abeta concentration, establishing that solution-phase oligomerization is not rate limiting. Translational diffusion measurements of the low molecular weight Abeta fraction demonstrate that the form of the peptide active in plaque deposition is a monomer. In deliberately aged (>6 weeks) Abeta solutions, a high molecular weight (>100 000 M(r)) species was detectable in the SEC column void. In contrast to the active monomer, assembled Abeta isolated from the column showed little or no focal association with AD tissue. These studies establish that, at least in vitro, Abeta exists as a monomer at physiological concentrations and that deposition of monomers, rather than of oligomeric Abeta assemblies, mediates the growth of existing amyloid in human brain preparations.     

NF-kappaB factor c-Rel mediates neuroprotection elicited by mGlu5 receptor agonists against amyloid beta-peptide toxicity

Opposite effects of nuclear factor-kappaB (NF-kappaB) on neuron survival rely on activation of diverse NF-kappaB factors. While p65 is necessary for glutamate-induced cell death, c-Rel mediates prosurvival effects of interleukin-1beta. However, it is unknown whether activation of c-Rel-dependent pathways reduces neuron vulnerability to amyloid-beta (Abeta), a peptide implicated in Alzheimer's disease pathogenesis. We show that neuroprotection elicited by activation of metabotropic glutamate receptors type 5 (mGlu5) against Abeta toxicity depends on c-Rel activation. Abeta peptide induced NF-kappaB factors p50 and p65. The mGlu5 agonists activated c-Rel, besides p50 and p65, and the expression of manganese superoxide dismutase (MnSOD) and Bcl-X(L). Targeting c-Rel expression by RNA interference suppressed the induction of both antiapoptotic genes. Targeting c-Rel or Bcl-X(L) prevented the prosurvival effect of mGlu5 agonists. Conversely, c-Rel overexpression or TAT-Bcl-X(L) addition rescued neurons from Abeta toxicity. These data demonstrate that mGlu5 receptor activation promotes a c-Rel-dependent antiapoptotic pathway responsible for neuroprotection against Abeta peptide.     

Mutant presenilin 1 increases the levels of Alzheimer amyloid beta-peptide Abeta42 in late compartments of the constitutive secretory pathway

Mutations in the presenilin 1 (PS1) gene are associated with autosomal dominant, early-onset, familial Alzheimer's disease and result in increased release of the hyperaggregatable 42-amino acid form of the amyloid beta-peptide (A(beta)42). To determine which subcellular compartments are potential source(s) of released Abeta42, we compared the levels and spatial segregation of intracellular A(beta)40 and A(beta)42 peptides between N2a neuroblastoma cells doubly transfected with the "Swedish" familial Alzheimer's disease-linked amyloid precursor protein variant and either wild-type PS1 (PS1(wt)) or familial Alzheimer's disease-linked delta9 mutant PS1 (PS1delta9). As expected, PS1delta9-expressing cells had dramatically higher levels of intracellular Abeta42 than did cells expressing PS1wt. However, the highest levels of A(beta)42 colocalized not with endoplasmic reticulum or Golgi markers but with rab8, a marker for trans-Golgi network (TGN)-to-plasma membrane (PM) transport vesicles. We show that PS1 mutants are capable of causing accumulation of A(beta)42 in late compartments of the secretory pathway, generating there a readily releasable source of A(beta)42. Our findings indicate that PS1 "bioactivity" localizes to the vicinity of the TGN and/or PM and reconcile the apparent discrepancy between the preponderant concentration of PS1 protein in proximal compartments of the secretory pathway and the recent findings that PS1 "bioactivity" can control gamma-secretase-like processing of another transmembrane substrate, Notch, at or near the PM.     

Modified-peptide inhibitors of amyloid beta-peptide polymerization.

Cellular toxicity resulting from nucleation-dependent polymerization of amyloid beta-peptide (Abeta) is considered to be a major and possibly the primary component of Alzheimer's disease (AD). Inhibition of Abeta polymerization has thus been identified as a target for the development of therapeutic agents for the treatment of AD. The intrinsic affinity of Abeta for itself suggested that Abeta-specific interactions could be adapted to the development of compounds that would bind to Abeta and prevent it from polymerizing. Abeta-derived peptides of fifteen residues were found to be inhibitory of Abeta polymerization. The activity of these peptides was subsequently enhanced through modification of their amino termini with specific organic reagents. Additional series of compounds prepared to probe structural requirements for activity allowed reduction of the size of the inhibitors and optimization of the Abeta-derived peptide portion to afford a lead compound, cholyl-Leu-Val-Phe-Phe-Ala-OH (PPI-368), with potent polymerization inhibitory activity but limited biochemical stability. The corresponding all-D-amino acyl analogue peptide acid (PPI-433) and amide (PPI-457) retained inhibitory activity and were both stable in monkey cerebrospinal fluid for 24 h.     

The binding constant for amyloid Abeta40 peptide interaction with human serum albumin

Human serum albumin (HSA) is the major carrier of Aβ peptides in blood plasma. 1:1 interaction stoichiometries were established in previous indirect antibody-based studies for both Aβ40 and Aβ42, but corresponding binding constants were not provided. In this study we applied direct titrations of HSA with Aβ40 monitored using circular dichroism spectroscopy and obtained a dissociation constant (K_d) of 5 ± 1 μM for a HSA complex with Aβ40. The interaction resulted in an increase of the α-helical contents in the complex, compared to its components, which is quantitatively consistent with the known ability of Aβ40 to adopt a partially α-helical conformation in a hydrophobic environment. The relevance of these findings for the role of HSA in Aβ physiology is discussed.     

Ion channel formation by Alzheimer's disease amyloid beta-peptide (Abeta40) in unilamellar liposomes is determined by anionic phospholipids

Incorporation of Alzheimer's disease amyloid beta-proteins (AbetaPs) across natural and artificial bilayer membranes leads to the formation of cation-selective channels. To study the peptide-membrane interactions involved in channel formation, we used cation reporter dyes to measure AbetaP-induced influx of Na+, Ca2+, and K+ into liposomes formed from phosphatidylserine (PS), phosphatidylinositol (PI) and phosphatidylcholine (PC). We found that Abeta40, but not Abeta40-1 or Abeta28, caused a dose-dependent increase in the concentration of each cation in the lumen of liposomes formed from the acidic phospholipids PS and PI. The Abeta40-induced changes in cation concentration, which we attribute to ion entry through Abeta40 channels, were not observed when using liposomes formed from the neutral phospholipid PC. Using mixtures of phospholipids, the magnitude of the AbetaP40-induced ion entry increased with the acidic phospholipid content of the liposomes, with entry being observed with as little as 5% PS or PI. Thus, while negatively charged phospholipids are required for formation of cation-permeable channels by Abeta40, a small amount is sufficient to support the process. These results have implications for the mechanisms of AbetaP cytotoxicity, suggesting that even a small amount of externalized negative charge could render cells susceptible to the deleterious effects of unregulated ion influx through AbetaP channels.     

Somatostatin regulates brain amyloid beta peptide Abeta42 through modulation of proteolytic degradation

Expression of somatostatin in the brain declines during aging in various mammals including apes and humans. A prominent decrease in this neuropeptide also represents a pathological characteristic of Alzheimer disease. Using in vitro and in vivo paradigms, we show that somatostatin regulates the metabolism of amyloid beta peptide (Abeta), the primary pathogenic agent of Alzheimer disease, in the brain through modulating proteolytic degradation catalyzed by neprilysin. Among various effector candidates, only somatostatin upregulated neprilysin activity in primary cortical neurons. A genetic deficiency of somatostatin altered hippocampal neprilysin activity and localization, and increased the quantity of a hydrophobic 42-mer form of Abeta, Abeta(42), in a manner similar to presenilin gene mutations that cause familial Alzheimer disease. These results indicate that the aging-induced downregulation of somatostatin expression may be a trigger for Abeta accumulation leading to late-onset sporadic Alzheimer disease, and suggest that somatostatin receptors may be pharmacological-target candidates for prevention and treatment of Alzheimer disease.     

Protective effects of S-nitrosoglutathione against amyloid beta-peptide neurotoxicity

Amyloid beta-peptide (Abeta) is a major constituent of senile plaques in the brains of Alzheimer's disease (AD) patients. We have previously demonstrated ceramide production secondary to Abeta-induced activation of neutral sphingomyelinase (nSMase) in cerebral endothelial cells and oligodendrocytes, which may contribute to cellular injury during progression of AD. In this study, we first established the "Abeta --> nSMase --> ceramide --> free radical --> cell death" pathway in primary cultures of fetal rat cortical neurons. We also provided experimental evidence showing that S-nitrosoglutathione (GSNO), a potent endogenous antioxidant derived from the interaction between nitric oxide (NO) and glutathione, caused dose-dependent protective effects against Abeta/ceramide neurotoxicity via inhibition of caspase activation and production of reactive oxygen species (ROS). This GSNO-mediated neuroprotection appeared to involve activation of cGMP-dependent protein kinase (PKG), phosphatidylinositol 3-kinase (PI3K), and extracellular signal-regulated kinase (ERK). Activation of the cGMP/PKG pathway induced expression of thioredoxin and Bcl-2 that were beneficial to cortical neurons in antagonizing Abeta/ceramide toxicity. Consistently, exogenous application of thioredoxin exerted remarkable neuroprotective efficacy in our experimental paradigm. Results derived from the present study establish a neuroprotective role of GSNO, an endogenous NO carrier, against Abeta toxicity via multiple signaling pathways.     

Evidence of oxidative damage in Alzheimer's disease brain: central role for amyloid beta-peptide.

Amyloid beta-peptide (Abeta) is heavily deposited in the brains of Alzheimer's disease (AD) patients. Free-radical oxidative stress, particularly of neuronal lipids, proteins and DNA, is extensive in those AD brain areas in which Abeta is abundant. Recent research suggests that these observations might be linked, and it is postulated that Abeta-induced oxidative stress leads to neurodegeneration in AD brain. Consonant with this postulate, Abeta leads to neuronal lipid peroxidation, protein oxidation and DNA oxidation by means that are inhibited by free-radical antioxidants. Here, we summarize current research on phospholipid peroxidation, as well as protein and DNA oxidation, in AD brain, and discuss the potential role of Abeta in this oxidative stress.     

Immunohistochemical investigation of amyloid beta-protein (Abeta) in the brain of aged cats

To clarify the immunohistochemical features of amyloid deposits and cerebral amyloid angiopathy (CAA), the distribution of the amyloid beta-protein subtypes Abeta40, Abeta42, Abeta43 and Abeta precursor protein (APP) were examined in the brains of fourteen aged cats (7.5-21 year-old). Two types of plaques were detected. The first type was characterized by Ass positive antigenic material and detected in the cortical layers of the frontal and parietal lobes of all examined cats. The second type was characterized by diffuse positive immune staining representing diffuse plaques, which were detected only in the very aged cats (17-21 years old) and distributed throughout the cortical layers of the parietal lobes. Vascular amyloid and the amyloid deposits were strongly positive-stained with the antibody Abeta42. APP was exhibited in neurons and axons while the staining was stronger in the very aged cats (17-21 years old). Our findings suggest that the feline forms a spontaneous model for understanding the early changes of normal brain aging and the early stage of amyloid beta-protein deposition.     

Physicochemical interactions of amyloid beta-peptide with lipid bilayers

The aggregation and deposition onto neuronal cells of amyloid beta-peptide (Abeta) is central to the pathogenesis of Alzheimer's disease. Accumulating evidence suggests that membranes play a catalytic role in the aggregation of Abeta. This article summarizes the structures and properties of Abeta in solution and the physicochemical interaction of Abeta with lipid bilayers of various compositions. Reasons for discrepancies between results by different research groups are discussed. The importance of ganglioside clusters in the aggregation of Abeta is emphasized. Finally, a hypothetical physicochemical cascade in the pathogenesis of the disease is proposed.     

Apical sorting of beta-secretase limits amyloid beta-peptide production.

Polarized cells such as neurons and endothelial cells appear to be involved in two invariant pathological features of Alzheimer's disease pathology, namely the formation of senile plaques and cerebral amyloid angiopathy. This implicates polarized sorting mechanisms in the production and accumulation of amyloid beta-peptide (Abeta). We have now studied polarized sorting of beta-secretase (BACE) in Madin-Darby canine kidney (MDCK) cells. The majority of BACE is sorted to the apical surface of MDCK cells where very little beta-amyloid precursor protein (betaAPP) is observed, because betaAPP undergoes basolateral sorting. Consistent with the usage of similar mechanisms for polarized sorting, BACE was also found to be targeted to axons of hippocampal neurons. The remaining basolaterally sorted BACE competes with the highly polarized basolateral alpha-secretase activity. Therefore, substantial amounts of BACE are targeted away from betaAPP to a non-amyloidogenic compartment, a cellular mechanism that limits Abeta generation. In addition, no alpha-secretase activity was observed on the apical side whereas gamma-secretase activity is observed on the basolateral and the apical side. Consistent with this finding, substantial amounts of Abeta can be produced apically upon missorting of betaAPP to the apical surface. These data demonstrate that Abeta production is limited in polarized cells by differential targeting of BACE and its substrate betaAPP. Moreover, our findings suggest that betaAPP may not be a major physiological substrate of BACE.     

A molecular model of Alzheimer amyloid beta-peptide fibril formation

Polymerization of the amyloid beta (Abeta) peptide into protease-resistant fibrils is a significant step in the pathogenesis of Alzheimer's disease. It has not been possible to obtain detailed structural information about this process with conventional techniques because the peptide has limited solubility and does not form crystals. In this work, we present experimental results leading to a molecular level model for fibril formation. Systematically selected Abeta-fragments containing the Abeta16-20 sequence, previously shown essential for Abeta-Abeta binding, were incubated in a physiological buffer. Electron microscopy revealed that the shortest fibril-forming sequence was Abeta14-23. Substitutions in this decapeptide impaired fibril formation and deletion of the decapeptide from Abeta1-42 inhibited fibril formation completely. All studied peptides that formed fibrils also formed stable dimers and/or tetramers. Molecular modeling of Abeta14-23 oligomers in an antiparallel beta-sheet conformation displayed favorable hydrophobic interactions stabilized by salt bridges between all charged residues. We propose that this decapeptide sequence forms the core of Abeta-fibrils, with the hydrophobic C terminus folding over this core. The identification of this fundamental sequence and the implied molecular model could facilitate the design of potential inhibitors of amyloidogenesis.     

Neurotoxicity of acetylcholinesterase amyloid beta-peptide aggregates is dependent on the type of Abeta peptide and the AChE concentration present in the complexes.

Alzheimer's disease (AD) is a neurodegenerative disorder whose hallmark is the presence of senile plaques and neurofibrillary tangles. Senile plaques are mainly composed of amyloid beta-peptide (Abeta) fibrils and several proteins including acetylcholinesterase (AChE). AChE has been previously shown to stimulate the aggregation of Abeta1-40 into amyloid fibrils. In the present work, the neurotoxicity of different amyloid aggregates formed in the absence or presence of AChE was evaluated in rat pheochromocytoma PC12 cells. Stable AChE-Abeta complexes were found to be more toxic than those formed without the enzyme, for Abeta1-40 and Abeta1-42, but not for amyloid fibrils formed with AbetaVal18-Ala, a synthetic variant of the Abeta1-40 peptide. Of all the AChE-Abeta complexes tested the one containing the Abeta1-40 peptide was the most toxic. When increasing concentrations of AChE were used to aggregate the Abeta1-40 peptide, the neurotoxicity of the complexes increased as a function of the amount of enzyme bound to each complex. Our results show that AChE-Abeta1-40 aggregates are more toxic than those of AChE-Abeta1-42 and that the neurotoxicity depends on the amount of AChE bound to the complexes, suggesting that AChE may play a key role in the neurodegeneration observed in Alzheimer brain.     

Temperature-induced dissociation of Abeta monomers from amyloid fibril

Using all-atom molecular dynamics, we study the temperature-induced dissociation of Aβ monomers from the fibril protofilament. To accelerate conformational sampling, simulations are performed at elevated temperatures and peptide concentrations. By computing free energy disconnectivity graphs we mapped the free energy landscape of monomers on the surface of Aβ fibril. We found that Aβ monomers sample diverse sets of low free energy states with different degrees of association with the fibril. Some of these states have residual amounts of fibril interactions, whereas others lack fibril-like content. Generally, Aβ monomers with partially formed fibril-like interactions have the lowest free energies, but their backbone conformations may differ considerably from those in the fibril interior. Overall, Aβ amyloid protofilaments seem to be highly resistant to thermal dissociation. Monomer dissociation from the fibril edge proceeds via multiple stages and pathways. Our simulation findings are discussed in the context of recent experimental results.     

Acceleration of amyloid beta-peptide aggregation by physiological concentrations of calcium

Alzheimer disease is characterized by the accumulation of aggregated amyloid beta-peptide (Abeta) in the brain. The physiological mechanisms and factors that predispose to Abeta aggregation and deposition are not well understood. In this report, we show that calcium can predispose to Abeta aggregation and fibril formation. Calcium increased the aggregation of early forming protofibrillar structures and markedly increased conversion of protofibrils to mature amyloid fibrils. This occurred at levels 20-fold below the calcium concentration in the extracellular space of the brain, the site at which amyloid plaque deposition occurs. In the absence of calcium, protofibrils can remain stable in vitro for several days. Using this approach, we directly compared the neurotoxicity of protofibrils and mature amyloid fibrils and demonstrate that both species are inherently toxic to neurons in culture. Thus, calcium may be an important predisposing factor for Abeta aggregation and toxicity. The high extracellular concentration of calcium in the brain, together with impaired intraneuronal calcium regulation in the aging brain and Alzheimer disease, may play an important role in the onset of amyloid-related pathology.     

In-silico interaction studies suggest RND efflux pump mediates polymyxin resistance in Acinetobacter baumannii

Bacterial efflux pumps have emerged as antibiotic resistance determinants and confers multi-drug resistance to a broad range of antimicrobials as well as non-antibiotic substances. A study about translocation of antibiotic molecules through the efflux transporter, will contribute in determining substrate specificity. In the present study, we have explored RND family efflux pump extensively found in Acinetobacter baumannii i.e. AdeABC. Besides, another well studied RND efflux pump, AcrAB-TolC together with a non-RND efflux pump, NorM was investigated for comparative analysis. We employed a series of computational techniques ranging from molecular docking to binding free energy estimation and molecular dynamics simulations to determine the binding affinity for different classes of drugs, namely aminoglycosides, polymyxins, β-lactams, tetracyclines, glycylcyclines, quinolones and metronidazole with AdeB, AcrB, and NorM efflux proteins. Our results revealed that class polymyxins has the highest binding affinity with the RND efflux pumps i.e. AcrAB-TolC and AdeABC as well as non-RND efflux pump, NorM. The experimental validation study demonstrated bigger zone of inhibition in presence of efflux pump inhibitor than polymyxin alone thus unveiling its specificity toward efflux pump. The reported experimental data comprising of minimum inhibitory concentration of antibiotics toward these efflux pumps also support our finding based on in silico approach. To recapitulate the outcome, polymyxins shows maximum specificity toward RND as well as non-RND efflux pump and may unlatch the way to rationally develop new potential antibacterial agents as well as efflux pump inhibitors in order to combat resistance.     

Inhibitors of amyloid toxicity based on beta-sheet packing of Abeta40 and Abeta42

Amyloid fibrils associated with Alzheimer's disease and a wide range of other neurodegenerative diseases have a cross beta-sheet structure, where main chain hydrogen bonding occurs between beta-strands in the direction of the fibril axis. The surface of the beta-sheet has pronounced ridges and grooves when the individual beta-strands have a parallel orientation and the amino acids are in-register with one another. Here we show that in Abeta amyloid fibrils, Met35 packs against Gly33 in the C-terminus of Abeta40 and against Gly37 in the C-terminus of Abeta42. These packing interactions suggest that the protofilament subunits are displaced relative to one another in the Abeta40 and Abeta42 fibril structures. We take advantage of this corrugated structure to design a new class of inhibitors that prevent fibril formation by placing alternating glycine and aromatic residues on one face of a beta-strand. We show that peptide inhibitors based on a GxFxGxF framework disrupt sheet-to-sheet packing and inhibit the formation of mature Abeta fibrils as assayed by thioflavin T fluorescence, electron microscopy, and solid-state NMR spectroscopy. The alternating large and small amino acids in the GxFxGxF sequence are complementary to the corresponding amino acids in the IxGxMxG motif found in the C-terminal sequence of Abeta40 and Abeta42. Importantly, the designed peptide inhibitors significantly reduce the toxicity induced by Abeta42 on cultured rat cortical neurons.     

Clioquinol mediates copper uptake and counteracts copper efflux activities of the amyloid precursor protein of Alzheimer's disease

The key protein in Alzheimer's disease, the amyloid precursor protein (APP), is a ubiquitously expressed copper-binding glycoprotein that gives rise to the Abeta amyloid peptide. Whereas overexpression of APP results in significantly reduced brain copper levels in three different lines of transgenic mice, knock-out animals revealed increased copper levels. A provoked rise in peripheral levels of copper reduced concentrations of soluble amyloid peptides and resulted in fewer pathogenic Abeta plaques. Contradictory evidence has been provided by the efficacy of copper chelation treatment with the drug clioquinol. Using a yeast model system, we show that adding clioquinol to the yeast culture medium drastically increased the intracellular copper concentration but there was no significant effect observed on zinc levels. This finding suggests that clioquinol can act therapeutically by changing the distribution of copper or facilitating copper uptake rather than by decreasing copper levels. The overexpression of the human APP or APLP2 extracellular domains but not the extracellular domain of APLP1 decreased intracellular copper levels. The expression of a mutant APP deficient for copper binding increased intracellular copper levels several-fold. These data uncover a novel biological function for APP and APLP2 in copper efflux and provide a new conceptual framework for the formerly diverging theories of copper supplementation and chelation in the treatment of Alzheimer's disease.