Ion channel formation by N-terminally truncated Aß (4-42): relevance for the pathogenesis of Alzheimer's disease.

Aß deposition is a pathological hallmark of Alzheimer's disease (AD). Besides the full-length amyloid forming peptides (Aß1-40 and Aß1-42), biochemical analyses of brain deposits have identified a variety of N- and C-terminally truncated Aß variants in sporadic and familial AD patients. However, their relevance for AD pathogenesis remains largely understudied. We demonstrate that Aß4-42 exhibits a high tendency to form ß-sheet structures leading to fast self-aggregation and formation of oligomeric assemblies. Atomic force microscopy and electrophysiological studies reveal that Aß4-42 forms highly stable ion channels in lipid membranes. These channels that are blocked by monoclonal antibodies specifically recognizing the N-terminus of Aß4-42. An Aß variant with a double truncation at phenylalanine-4 and leucine 34, (Aß4-34), exhibits unstable channel formation capability. Taken together the results presented herein highlight the potential benefit of C-terminal proteolytic cleavage and further support an important pathogenic role for N-truncated Aß species in AD pathophysiology.

Amyloid ß Ion Channels in a Membrane Comprising Brain Total Lipid Extracts.

Amyloid ß (Aß) oligomers are the predominant toxic species in the pathology of Alzheimer's disease. The prevailing mechanism for toxicity by Aß oligomers includes ionic homeostasis destabilization in neuronal cells by forming ion channels. These channel structures have been previously studied in model lipid bilayers. In order to gain further insight into the interaction of Aß oligomers with natural membrane compositions, we have examined the structures and conductivities of Aß oligomers in a membrane composed of brain total lipid extract (BTLE). We utilized two complementary techniques: atomic force microscopy (AFM) and black lipid membrane (BLM) electrical recording. Our results indicate that Aß1-42 forms ion channel structures in BTLE membranes, accompanied by a heterogeneous population of ionic current fluctuations. Notably, the observed current events generated by Aß1-42 peptides in BTLE membranes possess different characteristics compared to current events generated by the presence of Aß1-42 in model membranes comprising a 1:1 mixture of DOPS and POPE lipids. Oligomers of the truncated Aß fragment Aß17-42 (p3) exhibited similar ion conductivity behavior as Aß1-42 in BTLE membranes. However, the observed macroscopic ion flux across the BTLE membranes induced by Aß1-42 pores was larger than for p3 pores. Our analysis of structure and conductance of oligomeric Aß pores in a natural lipid membrane closely mimics the in vivo cellular environment suggesting that Aß pores could potentially accelerate the loss of ionic homeostasis and cellular abnormalities. Hence, these pore structures may serve as a target for drug development and therapeutic strategies for AD treatment.

Computational Methods for Structural and Functional Studies of Alzheimer's Amyloid Ion Channels.

Aggregation can be studied by a range of methods, experimental and computational. Aggregates form in solution, across solid surfaces, and on and in the membrane, where they may assemble into unregulated leaking ion channels. Experimental probes of ion channel conformations and dynamics are challenging. Atomistic molecular dynamics (MD) simulations are capable of providing insight into structural details of amyloid ion channels in the membrane at a resolution not achievable experimentally. Since data suggest that late stage Alzheimer's disease involves formation of toxic ion channels, MD simulations have been used aiming to gain insight into the channel shapes, morphologies, pore dimensions, conformational heterogeneity, and activity. These can be exploited for drug discovery. Here we describe computational methods to model amyloid ion channels containing the ß-sheet motif at atomic scale and to calculate toxic pore activity in the membrane.

Ion Channel Formation by Tau Protein: Implications for Alzheimer's Disease and Tauopathies.

Tau is a microtubule associated protein implicated in the pathogenesis of several neurodegenerative diseases. Because of the channel forming properties of other amyloid peptides, we employed planar lipid bilayers and atomic force microscopy to test tau for its ability to form ion permeable channels. Our results demonstrate that tau can form such channels, but only under acidic conditions. The channels formed are remarkably similar to amyloid peptide channels in their appearance, physical and electrical size, permanence, lack of ion selectivity, and multiple channel conductances. These channels differ from amyloid channels in their voltage dependence and resistance to blockade by zinc ion. These channels could explain tau's pathologic role in disease by lowering membrane potential, dysregulating calcium, depolarizing mitochondria, or depleting energy stores. Tau might also combine with amyloid beta peptides to form toxic channels.

Activity and architecture of pyroglutamate-modified amyloid-ß (AßpE3-42) pores.

Among the family of Aß peptides, pyroglutamate-modified Aß (AßpE) peptides are particularly associated with cytotoxicity in Alzheimer's disease (AD). They represent the dominant fraction of Aß oligomers in the brains of AD patients, but their accumulation in the brains of elderly individuals with normal cognition is significantly lower. Accumulation of AßpE plaques precedes the formation of plaques of full-length Aß (Aß1-40/42). Most of these properties appear to be associated with the higher hydrophobicity of AßpE as well as an increased resistance to enzymatic degradation. However, the important question of whether AßpE peptides induce pore activity in lipid membranes and their potential toxicity compared with other Aß pores is still open. Here we examine the activity of AßpE pores in anionic membranes using planar bilayer electrical recording and provide their structures using molecular dynamics simulations. We find that AßpE pores spontaneously induce ionic current across the membrane and have some similar properties to the other previously studied pores of the Aß family. However, there are also some significant differences. The onset of AßpE3-42 pore activity is generally delayed compared with Aß1-42 pores. However, once formed, AßpE3-42 pores produce increased ion permeability of the membrane, as indicated by a greater occurrence of higher conductance electrical events. Structurally, the lactam ring of AßpE peptides induces a change in the conformation of the N-terminal strands of the AßpE3-42 pores. While the N-termini of wild-type Aß1-42 peptides normally reside in the bulk water region, the N-termini of AßpE3-42 peptides tend to reside in the hydrophobic lipid core. These studies provide a first step to an understanding of the enhanced toxicity attributed to AßpE peptides.

Role of the fast kinetics of pyroglutamate-modified amyloid-ß oligomers in membrane binding and membrane permeability.

Membrane permeability to ions and small molecules is believed to be a critical step in the pathology of Alzheimer's disease (AD). Interactions of oligomers formed by amyloid-ß (Aß) peptides with the plasma cell membrane are believed to play a fundamental role in the processes leading to membrane permeability. Among the family of Aßs, pyroglutamate (pE)-modified Aß peptides constitute the most abundant oligomeric species in the brains of AD patients. Although membrane permeability mechanisms have been studied for full-length Aß1-40/42 peptides, these have not been sufficiently characterized for the more abundant AßpE3-42 fragment. Here we have compared the adsorbed and membrane-inserted oligomeric species of AßpE3-42 and Aß1-42 peptides. We find lower concentrations and larger dimensions for both species of membrane-associated AßpE3-42 oligomers. The larger dimensions are attributed to the faster self-assembly kinetics of AßpE3-42, and the lower concentrations are attributed to weaker interactions with zwitterionic lipid headgroups. While adsorbed oligomers produced little or no significant membrane structural damage, increased membrane permeabilization to ionic species is understood in terms of enlarged membrane-inserted oligomers. Membrane-inserted AßpE3-42 oligomers were also found to modify the mechanical properties of the membrane. Taken together, our results suggest that membrane-inserted oligomers are the primary species responsible for membrane permeability.

Disordered amyloidogenic peptides may insert into the membrane and assemble into common cyclic structural motifs.

Aggregation of disordered amyloidogenic peptides into oligomers is the causative agent of amyloid-related diseases. In solution, disordered protein states are characterized by heterogeneous ensembles. Among these, ß-rich conformers self-assemble via a conformational selection mechanism to form energetically-favored cross-ß structures, regardless of their precise sequences. These disordered peptides can also penetrate the membrane, and electrophysiological data indicate that they form ion-conducting channels. Based on these and additional data, including imaging and molecular dynamics simulations of a range of amyloid peptides, Alzheimer's amyloid-ß (Aß) peptide, its disease-related variants with point mutations and N-terminal truncated species, other amyloidogenic peptides, as well as a cytolytic peptide and a synthetic gel-forming peptide, we suggest that disordered amyloidogenic peptides can also present a common motif in the membrane. The motif consists of curved, moon-like ß-rich oligomers associated into annular organizations. The motif is favored in the lipid bilayer since it permits hydrophobic side chains to face and interact with the membrane and the charged/polar residues to face the solvated channel pores. Such channels are toxic since their pores allow uncontrolled leakage of ions into/out of the cell, destabilizing cellular ionic homeostasis. Here we detail Aß, whose aggregation is associated with Alzheimer's disease (AD) and for which there are the most abundant data. AD is a protein misfolding disease characterized by a build-up of Aß peptide as senile plaques, neurodegeneration, and memory loss. Excessively produced Aß peptides may directly induce cellular toxicity, even without the involvement of membrane receptors through Aß peptide-plasma membrane interactions.

Familial Alzheimer's disease Osaka mutant (ΔE22) ß-barrels suggest an explanation for the different Aß1-40/42 preferred conformational states observed by experiment.

An unusual ΔE693 mutation in the amyloid precursor protein (APP) producing a ß-amyloid (Aß) peptide lacking glutamic acid at position 22 (Glu22) was recently discovered, and dabbed the Osaka mutant (ΔE22). Previously, several point mutations in the Aß peptide involving Glu22 substitutions were identified and implicated in the early onset of familial Alzheimer's disease (FAD). Despite the absence of Glu22, the Osaka mutant is also associated with FAD, showing a recessive inheritance in families affected by the disease. To see whether this aggregation-prone Aß mutant could directly relate to the Aß ion channel-mediated mechanism as observed for the wild type (WT) Aß peptide in AD pathology, we modeled Osaka mutant ß-barrels in a lipid bilayer. Using molecular dynamics (MD) simulations, two conformer ΔE22 barrels with the U-shaped monomer conformation derived from NMR-based WT Aß fibrils were simulated in explicit lipid environment. Here, we show that the ΔE22 barrels obtain the lipid-relaxed ß-sheet channel topology, indistinguishable from the WT Aß1-42 barrels, as do the outer and pore dimensions of octadecameric (18-mer) ΔE22 barrels. Although the ΔE22 barrels lose the cationic binding site in the pore which is normally provided by the negatively charged Glu22 side chains, the mutant pores gain a new cationic binding site by Glu11 at the lower bilayer leaflet, and exhibit ion fluctuations similar to the WT barrels. Of particular interest, this deletion mutant suggests that toxic WT Aß1-42 would preferentially adopt a less C-terminal turn similar to that observed for Aß17-42, and explains why the solid state NMR data for Aß1-40 point to a more C-terminal turn conformation. The observed ΔE22 barrels conformational preferences also suggest an explanation for the lower neurotoxicity in rat primary neurons as compared to WT Aß1-42.

Alzheimer's disease: which type of amyloid-preventing drug agents to employ?

The current paradigm in the amyloid hypothesis brands small ß-amyloid (Aß) oligomers as the toxic species in Alzheimer's disease (AD). These oligomers are fibril-like; contain ß-sheet structure, and present exposed hydrophobic surface. Oligomers with this motif are capable of penetrating the cell membrane, gathering to form toxic ion channels. Current agents suppressing precursor Aß cleavage have only met partial success; and to date, those targeting the peptides and their assemblies in the aqueous environment of the extracellular space largely fail in clinical trials. One possible reason is failure to reach membrane-embedded targets of disease-'infected' cells. Here we provide an overview, point to the need to account for the lipid environment when aiming to prevent the formation of toxic channels, and propose a combination therapy to target the species spectrum.

Mechanisms for the Insertion of Toxic, Fibril-like ß-Amyloid Oligomers into the Membrane.

Amyloid-ß (Aß) oligomers destabilize cellular ionic homeostasis, mediating Alzheimer's disease (AD). It is still unclear whether the mechanism (i) is mediated by cell surface receptors; (ii) is direct, with Aß oligomers interacting with membrane lipids; or (iii) both mechanisms take place. Recent studies indicate that Aß oligomers may act by either of the last two. Little is known about the oligomers' structures and how they spontaneously insert into the membrane. Using explicit solvent molecular dynamics (MD) simulations, we show that fibril-like Aß(17-42) (p3) oligomer is capable of penetrating the membrane. Insertion is similar to that observed for protegrin-1 (PG-1), a cytolytic ß-sheet-rich antimicrobial peptide (AMP). Both Aß and PG-1 favor the amphipathic interface of the lipid bilayer in the early stage of interaction with the membrane. U-shaped Aß oligomers are observed in solution and in the membrane, suggesting that the preformed seeds can be shared by amyloid fibrils in the growth phase and membrane toxicity. Here we provide sequential events in possible Aß oligomer membrane-insertion pathways. We speculate that for the U-shaped motif, a trimer is the minimal oligomer size to insert effectively. We propose that monomers and dimers may insert in (apparently on-pathway) aggregation-intermediate ß-hairpin state, and may (or may not) convert to a U-shape in the bilayer. Together with earlier observations, our results point to a non-specific, broadly heterogeneous landscape of membrane-inserting oligomer conformations, pathways, and membrane-mediated toxicity of ß-rich oligomers.

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 ß-barrel structure of the toxics pore formed by bacterial toxins, such as staphylococcal α-hemolysin, anthrax toxin, and Clostridium perfringolysin.

All-d-Enantiomer of ß-Amyloid Peptide Forms Ion Channels in Lipid Bilayers.

Alzheimer's disease (AD) is the most common type of senile dementia in aging populations. Amyloid ß (Aß)-mediated dysregulation of ionic homeostasis is the prevailing underlying mechanism leading to synaptic degeneration and neuronal death. Aß-dependent ionic dysregulation most likely occurs either directly via unregulated ionic transport through the membrane or indirectly via Aß binding to cell membrane receptors and subsequent opening of existing ion channels or transporters. Receptor binding is expected to involve a high degree of stereospecificity. Here, we investigated whether an Aß peptide enantiomer, whose entire sequence consists of d-amino acids, can form ion-conducting channels; these channels can directly mediate Aß effects even in the absence of receptor-peptide interactions. Using complementary approaches of planar lipid bilayer (PLB) electrophysiological recordings and molecular dynamics (MD) simulations, we show that the d-Aß isomer exhibits ion conductance behavior in the bilayer indistinguishable from that described earlier for the l-Aß isomer. The d isomer forms channel-like pores with heterogeneous ionic conductance similar to the l-Aß isomer channels, and the d-isomer channel conductance is blocked by Zn(2+), a known blocker of l-Aß isomer channels. MD simulations further verify formation of ß-barrel-like Aß channels with d- and l-isomers, illustrating that both d- and l-Aß barrels can conduct cations. The calculated values of the single-channel conductance are approximately in the range of the experimental values. These findings are in agreement with amyloids forming Ca(2+) leaking, unregulated channels in AD, and suggest that Aß toxicity is mediated through a receptor-independent, nonstereoselective mechanism.

Effects of point substitutions on the structure of toxic Alzheimer's ß-amyloid channels: atomic force microscopy and molecular dynamics simulations.

Alzheimer's disease (AD) is a misfolded protein disease characterized by the accumulation of ß-amyloid (Aß) peptide as senile plaques, progressive neurodegeneration, and memory loss. Recent evidence suggests that AD pathology is linked to the destabilization of cellular ionic homeostasis mediated by toxic pores made of Aß peptides. Understanding the exact nature by which these pores conduct electrical and molecular signals could aid in identifying potential therapeutic targets for the prevention and treatment of AD. Here using atomic force microscopy (AFM) and molecular dynamics (MD) simulations, we compared the imaged pore structures with models to predict channel conformations as a function of amino acid sequence. Site-specific amino acid (AA) substitutions in the wild-type Aß(1-42) peptide yield information regarding the location and significance of individual AA residues to its characteristic structure-activity relationship. We selected two AAs that our MD simulation predicted to inhibit or permit pore conductance. The substitution of Phe19 with Pro has previously been shown to eliminate conductance in the planar lipid bilayer system. Our MD simulations predict a channel-like shape with a collapsed pore, which is supported by the AFM channel images. We suggest that proline, a known ß-sheet breaker, creates a kink in the center of the pore and prevents conductance via blockage. This residue may be a viable target for drug development studies aiming to inhibit Aß from inducing ionic destabilization toxicity. The substitution of Phe20 with Cys exhibits pore structures indistinguishable from the wild type in AFM images. MD simulations predict site 20 to face the solvated pore. Overall, the mutations support the previously predicted ß-sheet-based channel structure.

Atomic force microscopy and MD simulations reveal pore-like structures of all-D-enantiomer of Alzheimer's ß-amyloid peptide: relevance to the ion channel mechanism of AD pathology.

Alzheimer's disease (AD) is a protein misfolding disease characterized by a buildup of ß-amyloid (Aß) peptide as senile plaques, uncontrolled neurodegeneration, and memory loss. AD pathology is linked to the destabilization of cellular ionic homeostasis and involves Aß peptide-plasma membrane interactions. In principle, there are two possible ways through which disturbance of the ionic homeostasis can take place: directly, where the Aß peptide either inserts into the membrane and creates ion-conductive pores or destabilizes the membrane organization, or, indirectly, where the Aß peptide interacts with existing cell membrane receptors. To distinguish between these two possible types of Aß-membrane interactions, we took advantage of the biochemical tenet that ligand-receptor interactions are stereospecific; L-amino acid peptides, but not their D-counterparts, bind to cell membrane receptors. However, with respect to the ion channel-mediated mechanism, like L-amino acids, D-amino acid peptides will also form ion channel-like structures. Using atomic force microscopy (AFM), we imaged the structures of both D- and L-enantiomers of the full length Aß(1-42) when reconstituted in lipid bilayers. AFM imaging shows that both L- and D-Aß isomers form similar channel-like structures. Molecular dynamics (MD) simulations support the AFM imaged 3D structures. Previously, we have shown that D-Aß(1-42) channels conduct ions similarly to their L- counterparts. Taken together, our results support the direct mechanism of Aß ion channel-mediated destabilization of ionic homeostasis rather than the indirect mechanism through Aß interaction with membrane receptors.

Probing structural features of Alzheimer's amyloid-ß pores in bilayers using site-specific amino acid substitutions.

A current hypothesis for the pathology of Alzheimer's disease (AD) proposes that amyloid-ß (Aß) peptides induce uncontrolled, neurotoxic ion flux across cellular membranes. The mechanism of ion flux is not fully understood because no experiment-based Aß channel structures at atomic resolution are currently available (only a few polymorphic states have been predicted by computational models). Structural models and experimental evidence lend support to the view that the Aß channel is an assembly of loosely associated mobile ß-sheet subunits. Here, using planar lipid bilayers and molecular dynamics (MD) simulations, we show that amino acid substitutions can be used to infer which residues are essential for channel structure. We created two Aß(1-42) peptides with point mutations: F19P and F20C. The substitution of Phe19 with Pro inhibited channel conductance. MD simulation suggests a collapsed pore of F19P channels at the lower bilayer leaflet. The kinks at the Pro residues in the pore-lining ß-strands induce blockage of the solvated pore by the N-termini of the chains. The cysteine mutant is capable of forming channels, and the conductance behavior of F20C channels is similar to that of the wild type. Overall, the mutational analysis of the channel activity performed in this work tests the proposition that the channels consist of a ß-sheet rich organization, with the charged/polar central strand containing the mutation sites lining the pore, and the C-terminal strands facing the hydrophobic lipid tails. A detailed understanding of channel formation and its structure should aid studies of drug design aiming to control unregulated Aß-dependent ion fluxes.

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-ß 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 ß-strand-turn-ß-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.

Amyloid peptide pores and the beta sheet conformation.

Over 20 clinical syndromes have been described as amyloid diseases. Pathologically, these illnesses are characterized by the deposition in various tissues of amorphous, Congo red stainingdeposits, referred to as amyloid. Under polarizing light microscopy, these deposits exhibit characteristic green birefringence. X-ray diffraction reveals cross-beta structure of extended amyloid fibrils. Although there is always a major protein in amyloid deposits, the predominant protein differs in each ofthe clinical syndromes. All the proteins exhibit the characteristic nonnative beta-sheet state. These proteins aggregate spontaneously into extended fibrils and precipitate out of solution. At least a dozen of these peptides have been demonstrated to be capable of channel formation in lipid bilayers and it has been proposed that this represents a pathogenic mechanism. Remarkably, the channels formed by these various peptides exhibit a number of common properties including irreversible, spontaneous insertion into membranes, production oflarge, heterogeneous single-channel conductances, relatively poor ion selectivity, inhibition of channel formation by Congo red and related dyes and blockade of inserted channels by zinc. In vivo amyloid peptides have been shown to disrupt intracellular calcium regulation, plasma membrane potential, mitochondrial membrane potential and function and long-term potentiation in neurons. Amyloid peptides also cause cytotoxicity. Formation of the beta sheet conformation from native protein structures can be induced by high protein concentrations, metal binding, acidic pH, amino acid mutation and interaction with lipid membranes. Most amyloid peptides interact strongly with membranes and this interaction is enhanced by conditions which favor beta-sheet formation. Formation of pores in these illnesses appears to be a spontaneous process and available evidence suggests several steps are critical. First, destabilization of the native structure and formation of the beta-sheet conformation must occur. This may occur in solution or may be facilitated by contact with lipid membranes. Oligomerization of the amyloid protein is then mediated by the beta strands. Amyloid monomers and extended fibrils appear to have little potential for toxicity whereas there is much evidence implicating amyloid oligomers of intermediate size in the pathogenesis of amyloid disease. Insertion of the oligomer appears to take place spontaneously although there may be a contribution of acidic pH and/or membrane potential. Very little is known about the structure of amyloid pores, but given that the amyloid peptides must acquire beta-sheet conformation to aggregate and polymerize, it has been hypothesized that amyloid pores may in fact be beta-sheet barrels similar to the pores formed by alpha-latrotoxin, Staphylococcal alpha-hemolysin, anthrax toxin and clostridial perfringolysin.

Truncated beta-amyloid peptide channels provide an alternative mechanism for Alzheimer's Disease and Down syndrome.

Full-length amyloid beta peptides (Abeta(1-40/42)) form neuritic amyloid plaques in Alzheimer's disease (AD) patients and are implicated in AD pathology. However, recent transgenic animal models cast doubt on their direct role in AD pathology. Nonamyloidogenic truncated amyloid-beta fragments (Abeta(11-42) and Abeta(17-42)) are also found in amyloid plaques of AD and in the preamyloid lesions of Down syndrome, a model system for early-onset AD study. Very little is known about the structure and activity of these smaller peptides, although they could be the primary AD and Down syndrome pathological agents. Using complementary techniques of molecular dynamics simulations, atomic force microscopy, channel conductance measurements, calcium imaging, neuritic degeneration, and cell death assays, we show that nonamyloidogenic Abeta(9-42) and Abeta(17-42) peptides form ion channels with loosely attached subunits and elicit single-channel conductances. The subunits appear mobile, suggesting insertion of small oligomers, followed by dynamic channel assembly and dissociation. These channels allow calcium uptake in amyloid precursor protein-deficient cells. The channel mediated calcium uptake induces neurite degeneration in human cortical neurons. Channel conductance, calcium uptake, and neurite degeneration are selectively inhibited by zinc, a blocker of amyloid ion channel activity. Thus, truncated Abeta fragments could account for undefined roles played by full length Abetas and provide a unique mechanism of AD and Down syndrome pathologies. The toxicity of nonamyloidogenic peptides via an ion channel mechanism necessitates a reevaluation of the current therapeutic approaches targeting the nonamyloidogenic pathway as avenue for AD treatment.