Single-channel Ca(2+) imaging implicates Aβ1-42 amyloid pores in Alzheimer's disease pathology

Calcium channelopathies and Alzheimer's disease: insight into therapeutic success and failures

Calcium ions are versatile and universal biological signaling factors that regulate numerous cellular processes ranging from cell fertilization, to neuronal plasticity that underlies learning and memory, to cell death. For these functions to be properly executed, calcium signaling requires precise regulation, and failure of this regulation may tip the scales from a signal for life to a signal for death. Disruptions in calcium channel function can generate complex multi-system disorders collectively referred to as "calciumopathies" that can target essentially any cell type or organ. In this review, we focus on the multifaceted involvement of calcium signaling in the pathophysiology of Alzheimer's disease (AD), and summarize the various therapeutic options currently available to combat this disease. Detailing the series of disappointing AD clinical trial results on cognitive outcomes, we emphasize the urgency to design alternative therapeutic strategies if synaptic and memory functions are to be preserved. One such approach is to target early calcium channelopathies centrally linked to AD pathogenesis.

Alzheimer's disease-associated peptide Aβ42 mobilizes ER Ca(2+) via InsP3R-dependent and -independent mechanisms

Dysregulation of Ca²⁺ homeostasis is considered to contribute to the toxic action of the Alzheimer's disease (AD)-associated amyloid-β-peptide (Aβ). Ca²⁺ fluxes across the plasma membrane and release from intracellular stores have both been reported to underlie the Ca²⁺ fluxes induced by Aβ₄₂. Here, we investigated the contribution of Ca²⁺ release from the endoplasmic reticulum (ER) to the effects of Aβ₄₂ upon Ca²⁺ homeostasis and the mechanism by which Aβ₄₂ elicited these effects. Consistent with previous reports, application of soluble oligomeric forms of Aβ₄₂ induced an elevation in intracellular Ca²⁺. The Aβ₄₂-stimulated Ca²⁺ signals persisted in the absence of extracellular Ca²⁺ indicating a significant contribution of Ca²⁺ release from the ER Ca²⁺ store to the generation of these signals. Moreover, inositol 1,4,5-trisphosphate (InsP₃) signaling contributed to Aβ₄₂-stimulated Ca²⁺ release. The Ca²⁺ mobilizing effect of Aβ₄₂ was also observed when applied to permeabilized cells deficient in InsP₃ receptors, revealing an additional direct effect of Aβ₄₂ upon the ER, and a mechanism for induction of toxicity by intracellular Aβ₄₂.

Age-related downregulation of the CaV3.1 T-type calcium channel as a mediator of amyloid beta production

Alzheimer's is a crippling neurodegenerative disease that largely affects aged individuals. Decades of research have highlighted age-related changes in calcium homeostasis that occur before and throughout the duration of the disease, and the contributions of such dysregulation to Alzheimer's disease pathogenesis. We report an age-related decrease in expression of the CaV3.1 T-type calcium channel at the level of messenger RNA and protein in both humans and mice that is exacerbated with the presence of Alzheimer's disease. Downregulating T-type calcium channels in N2a cells and the 3xTg-AD mouse model of Alzheimer's disease, by way of pharmacologic inhibition with NNC-55-0396, results in a rapid increase in amyloid beta production via reductions in non-amyloidogenic processing, whereas genetic overexpression of the channel in human embryonic kidney cells expressing amyloid precursor protein produces complementary effects. The age-related decline in CaV3.1 expression may therefore contribute to a pro-amyloidogenic environment in the aging brain and represents a novel opportunity to intervene in the course of Alzheimer's disease pathogenesis.

Capturing single L-type Ca(2+) channel function with optics

Advances in imaging technology have allowed optical analysis of Ca(2+)-permeable ion channel activity. Here, we briefly review novel developments in optical recording of L-type voltage-dependent Ca(2+) channel (LTCC) function with high spatial and temporal resolution. Underlying principles supporting the use of total internal reflection fluorescence (TIRF) microscopy for optical measurement of channel activity and new functional characteristics of LTCCs revealed by application of this approach are discussed. Visualization of Ca(2+) influx through single LTCCs ("LTCC sparklets") has demonstrated that channel activity is regionally heterogeneous and that clustered channels are capable of operating in a cooperative, or "coupled" manner. In light of these findings, we describe a current molecular model for the local control of LTCC activity and coupled gating in physiological and pathological contexts. This article is part of a Special Issue entitled: 12th European Symposium on Calcium.

Amyloid-β peptide-induced extracellular S100A9 depletion is associated with decrease of antimicrobial peptide activity in human THP-1 monocytes

Background

S100A9 protein (myeloid-related protein MRP14, also referred to as calgranulin B) is a reliable marker of inflammation, an important proinflammatory factor of innate immunity and acts as an additional antimicrobial peptide in the innate immune system. Evidence indicates that S100A9 contributes to Alzheimer’s disease (AD) pathology, although the precise mechanisms are not clear.

Methods

We were interested to study the mechanisms of S100A9 release upon Aβ1-42 stimulation, the potential roles of extracellular S100A9 depletion in Aβ-induced cytotoxicity, and the interaction with innate immune response in THP-1 monocytic cells that have been challenged with mostly Aβ1-42 monomers instead of oligomers. We used protein preparation, Ca²⁺ influx fluorescence imaging, MTT assay, siRNA knockdown, colony forming units (CFUs) assay and western blotting techniques to perform our study.

Results

Aβ1-42 monomers elicited a marked decrease of S100A9 release into the cell culture supernatant in a dose-dependent manner in human THP-1 monocytes. This reduction of S100A9 release was accompanied by an increase of intracellular Ca²⁺ level. Aβ1-42-mediated decrease of S100A9 release was not associated with Aβ1-42-induced cytotoxicity as measured by MTT reduction assay. This observation was confirmed with the recombinant S100A9, which had little effect on Aβ1-42-induced cytotoxicity. Moreover, depletion of S100A9 with siRNA did not significantly evoke the cell toxicity. On the other hand, Aβ1-42-induced extracellular S100A9 depletion resulted in decreased antimicrobial activity of the culture supernatant after Aβ1-42 stimulation. Immunodepletion of S100A9 with anti-S100A9 also decreased the antimicrobial peptide activity of the vehicle treated culture supernatant. Consistently, the recombinant S100A9 clearly elicited the antimicrobial peptide activity in vitro, confirming the observed antimicrobial activity of S100A9 in the culture supernatant.

Conclusion

Collectively, our findings suggest that the mostly monomeric form of Aβ1-42 negatively regulates the innate immune system by down-regulating the secretion of S100A9, which is likely a main mediator of antimicrobial activity in the conditioned media of human THP-1 monocytes.

Cytotoxicity of intracellular aβ42 amyloid oligomers involves Ca2+ release from the endoplasmic reticulum by stimulated production of inositol trisphosphate

Oligomeric forms of β-amyloid (Aβ(42)) peptides associated with Alzheimer's disease (AD) disrupt cellular Ca(2+) regulation by liberating Ca(2+) into the cytosol from both extracellular and intracellular sources. We elucidated the actions of intracellular Aβ by imaging Ca(2+) responses to injections of Aβ oligomers into Xenopus oocytes. Two types of signal were observed: (1) local, "channel-like" transients dependent on extracellular Ca(2+) influx, which resembled signals from amlyoid pores formed by extracellular application of oligomers; and (2) local transients and global Ca(2+) waves, resembling Ca(2+) puffs and waves mediated by inositol trisphosphate (IP(3)). The latter responses were suppressed by antagonists of the IP(3) receptor (caffeine and heparin), pretreatment with the G(i)(o)-protein inhibitor pertussis toxin, and pretreatment with lithium to deplete membrane inositol lipids. We show that G-protein-mediated stimulation of IP(3) production and consequent liberation of Ca(2+) from the endoplasmic reticulum by intracellular Aβ oligomers is cytotoxic, potentially representing a novel pathological mechanism in AD which may be further exacerbated by AD-linked mutations in presenilins to promote opening of IP(3) receptor/channels.

The progression towards Alzheimer's disease described as a bistable switch arising from the positive loop between amyloids and Ca(2+)

Alzheimer's disease is a progressive neurodegenerative disorder affecting millions of people. It is characterized by the slow deposition of cerebral amyloid-β peptides in the brain and by dysregulations in neuronal Ca(2+) homeostasis. Numerous experimental studies have revealed the existence of a feed-forward loop wherein amyloids-β disturb neuronal Ca(2+) levels, which in turn affect the production of amyloids. Here, we formalize this positive loop in a minimal, qualitative model and show that it exhibits bistability. Thus, a stable steady state characterized by low levels of Ca(2+) and amyloids, corresponding to a healthy situation, coexists with another 'pathological state' where the levels of both compounds are high. The onset of the disease corresponds to the switch from the lower steady state to the higher one induced by a large-enough perturbation in either the metabolism of amyloids or the homeostasis of intracellular Ca(2+). Numerical simulations of the model reproduce a variety of experimental observations about the disease, as its irreversible character, the threshold-like transition to a severe pathology after the slow accumulation of symptoms, the effect of presenilins, the so-called 'prion-like' autocatalytic behaviour of amyloids and the inherent random character of the apparition of the disease that is well known for the sporadic form. The model thus provides a conceptual framework that could be useful when developing therapeutic protocols to slow down the progression of Alzheimer's disease.

Thermodynamically stable amyloid-β monomers have much lower membrane affinity than the small oligomers

Amyloid beta (Aβ) is an extracellular 39–43 residue long peptide present in the mammalian cerebrospinal fluid, whose aggregation is associated with Alzheimer's disease (AD). Small oligomers of Aβ are currently thought to be the key to toxicity. However, it is not clear why the monomers of Aβ are non-toxic, and at what stage of aggregation toxicity emerges. Interactions of Aβ with cell membranes is thought to be the initiator of toxicity, but membrane binding studies with different preparations of monomers and oligomers have not settled this issue. We have earlier found that thermodynamically stable Aβ monomers emerge spontaneously from oligomeric mixtures upon long term incubation in physiological solutions (Nag et al., 2011). Here we show that the membrane-affinity of these stable Aβ monomers is much lower than that of a mixture of monomers and small oligomers (containing dimers to decamers), providing a clue to the emergence of toxicity. Fluorescently labeled Aβ₄₀ monomers show negligible binding to cell membranes of a neuronal cell line (RN46A) at physiological concentrations (250 nM), while oligomers at the same concentrations show strong binding within 30 min of incubation. The increased affinity most likely does not require any specific neuronal receptor, since this difference in membrane-affinity was also observed in a somatic cell-line (HEK 293T). Similar results are also obtained for Aβ₄₂ monomers and oligomers. Minimal amount of cell death is observed at these concentrations even after 36 h of incubation. It is likely that membrane binding precedes subsequent slower toxic events induced by Aβ. Our results (a) provide an explanation for the non-toxic nature of Aβ monomers, (b) suggest that Aβ toxicity emerges at the initial oligomeric phase, and (c) provide a quick assay for monitoring the benign-to-toxic transformation of Aβ.

Biophysical insights into how surfaces, including lipid membranes, modulate protein aggregation related to neurodegeneration

There are a vast number of neurodegenerative diseases, including Alzheimer’s disease (AD), Parkinson’s disease (PD), and Huntington’s disease (HD), associated with the rearrangement of specific proteins to non-native conformations that promotes aggregation and deposition within tissues and/or cellular compartments. These diseases are commonly classified as protein-misfolding or amyloid diseases. The interaction of these proteins with liquid/surface interfaces is a fundamental phenomenon with potential implications for protein-misfolding diseases. Kinetic and thermodynamic studies indicate that significant conformational changes can be induced in proteins encountering surfaces, which can play a critical role in nucleating aggregate formation or stabilizing specific aggregation states. Surfaces of particular interest in neurodegenerative diseases are cellular and subcellular membranes that are predominately comprised of lipid components. The two-dimensional liquid environments provided by lipid bilayers can profoundly alter protein structure and dynamics by both specific and non-specific interactions. Importantly for misfolding diseases, these bilayer properties can not only modulate protein conformation, but also exert influence on aggregation state. A detailed understanding of the influence of (sub)cellular surfaces in driving protein aggregation and/or stabilizing specific aggregate forms could provide new insights into toxic mechanisms associated with these diseases. Here, we review the influence of surfaces in driving and stabilizing protein aggregation with a specific emphasis on lipid membranes.

Nuclear microinjection to assess how heterologously expressed proteins impact Ca2+ signals in Xenopus oocytes

The Xenopus oocyte is frequently used for heterologous expression and for studying the spatiotemporal patterning of Ca(2+) signals. Here, we outline a protocol for nuclear microinjection of the Xenopus oocyte for the purpose of studying how subsequently expressed proteins impact intracellular Ca(2+) signals evoked by inositol trisphosphate (InsP3). Injected oocytes can easily be identified by reporter technologies and the impact of heterologously expressed proteins on the generation and properties of InsP3-evoked Ca(2+) signals can be resolved using caged InsP3 and fluorescent Ca(2+) indicators.

Acetylcholine and antibodies against the acetylcholine receptor protect neurons and astrocytes against beta-amyloid toxicity

Aggregated amyloid-β causes pathological changes in mixed cultures of neurons and astrocytes such as sporadic cytoplasmic intracellular Ca(2+)-signalling, increase in reactive oxygen species production and cell death. Some of the toxic effects of amyloid-β are mediated through the interaction of the peptide with α7-type nicotinic acetylcholine receptors at the cell surface. Here we demonstrated that affinity purified antibodies to synthetic fragment 173-193 of the α7-subunit of the nAChR are able to protect cells from amyloid-β induced cell death. The antibodies had no effect on the amyloid-β induced calcium signal in astrocytes. However, they significantly reduced amyloid-β induced and NADPH oxidase mediated ROS production. Modulation of the NADPH oxidase activity by either the antibodies, the receptor agonist acetylcholine or the antagonist of the α7-type nicotinic acetylcholine receptors α-bungarotoxin was vital in inhibiting both amyloid-β induced ROS production, caspase 3 cleavage as well as cell death. The uncovered details of the mechanism underlying the action of antibodies to α7-type nicotinic acetylcholine receptors gives additional insight into the involvement of this receptor in Alzheimer's disease pathology and provides a new approach to anti-Alzheimer's disease vaccine design.

Stabilizing ER Ca2+ channel function as an early preventative strategy for Alzheimer's disease

Alzheimer’s disease (AD) is a devastating neurodegenerative condition with no known cure. While current therapies target late-stage amyloid formation and cholinergic tone, to date, these strategies have proven ineffective at preventing disease progression. The reasons for this may be varied, and could reflect late intervention, or, that earlier pathogenic mechanisms have been overlooked and permitted to accelerate the disease process. One such example would include synaptic pathology, the disease component strongly associated with cognitive impairment. Dysregulated Ca²⁺ homeostasis may be one of the critical factors driving synaptic dysfunction. One of the earliest pathophysiological indicators in mutant presenilin (PS) AD mice is increased intracellular Ca²⁺ signaling, predominantly through the ER-localized inositol triphosphate (IP₃) and ryanodine receptors (RyR). In particular, the RyR-mediated Ca²⁺ upregulation within synaptic compartments is associated with altered synaptic homeostasis and network depression at early (presymptomatic) AD stages. Here, we offer an alternative approach to AD therapeutics by stabilizing early pathogenic mechanisms associated with synaptic abnormalities. We targeted the RyR as a means to prevent disease progression, and sub-chronically treated AD mouse models (4-weeks) with a novel formulation of the RyR inhibitor, dantrolene. Using 2-photon Ca²⁺ imaging and patch clamp recordings, we demonstrate that dantrolene treatment fully normalizes ER Ca²⁺ signaling within somatic and dendritic compartments in early and later-stage AD mice in hippocampal slices. Additionally, the elevated RyR2 levels in AD mice are restored to control levels with dantrolene treatment, as are synaptic transmission and synaptic plasticity. Aβ deposition within the cortex and hippocampus is also reduced in dantrolene-treated AD mice. In this study, we highlight the pivotal role of Ca²⁺ aberrations in AD, and propose a novel strategy to preserve synaptic function, and thereby cognitive function, in early AD patients.

Direct three-dimensional visualization of membrane disruption by amyloid fibrils

Protein misfolding and aggregation cause serious degenerative conditions such as Alzheimer's, Parkinson, and prion diseases. Damage to membranes is thought to be one of the mechanisms underlying cellular toxicity of a range of amyloid assemblies. Previous studies have indicated that amyloid fibrils can cause membrane leakage and elicit cellular damage, and these effects are enhanced by fragmentation of the fibrils. Here we report direct 3D visualization of membrane damage by specific interactions of a lipid bilayer with amyloid-like fibrils formed in vitro from β(2)-microglobulin (β(2)m). Using cryoelectron tomography, we demonstrate that fragmented β(2)m amyloid fibrils interact strongly with liposomes and cause distortions to the membranes. The normally spherical liposomes form pointed teardrop-like shapes with the fibril ends seen in proximity to the pointed regions on the membranes. Moreover, the tomograms indicated that the fibrils extract lipid from the membranes at these points of distortion by removal or blebbing of the outer membrane leaflet. Tiny (15-25 nm) vesicles, presumably formed from the extracted lipids, were observed to be decorating the fibrils. The findings highlight a potential role of fibrils, and particularly fibril ends, in amyloid pathology, and report a previously undescribed class of lipid-protein interactions in membrane remodelling.

Ryanodine receptor blockade reduces amyloid-β load and memory impairments in Tg2576 mouse model of Alzheimer disease

In Alzheimer disease (AD), the perturbation of the endoplasmic reticulum (ER) calcium (Ca²⁺) homeostasis has been linked to presenilins, the catalytic core in γ-secretase complexes cleaving the amyloid precursor protein (APP), thereby generating amyloid-β (Aβ) peptides. Here we investigate whether APP contributes to ER Ca²⁺ homeostasis and whether ER Ca²⁺ could in turn influence Aβ production. We show that overexpression of wild-type human APP (APP(695)), or APP harboring the Swedish double mutation (APP(swe)) triggers increased ryanodine receptor (RyR) expression and enhances RyR-mediated ER Ca²⁺ release in SH-SY5Y neuroblastoma cells and in APP(swe)-expressing (Tg2576) mice. Interestingly, dantrolene-induced lowering of RyR-mediated Ca²⁺ release leads to the reduction of both intracellular and extracellular Aβ load in neuroblastoma cells as well as in primary cultured neurons derived from Tg2576 mice. This Aβ reduction can be accounted for by decreased Thr-668-dependent APP phosphorylation and β- and γ-secretases activities. Importantly, dantrolene diminishes Aβ load, reduces Aβ-related histological lesions, and slows down learning and memory deficits in Tg2576 mice. Overall, our data document a key role of RyR in Aβ production and learning and memory performances, and delineate RyR-mediated control of Ca²⁺ homeostasis as a physiological paradigm that could be targeted for innovative therapeutic approaches.

A new concept: Aβ1-42 generates a hyperfunctional proteolytic NCX3 fragment that delays caspase-12 activation and neuronal death

Although the amyloid-β(1-42) (Aβ(1-42)) peptide involved in Alzheimer's disease is known to cause a dysregulation of intracellular Ca(2+) homeostasis, its molecular mechanisms still remain unclear. We report that the extracellular-dependent early increase (30 min) in intracellular calcium concentration ([Ca(2+)](i)), following Aβ(1-42) exposure, caused the activation of calpain that in turn elicited a cleavage of the Na(+)/Ca(2+) exchanger isoform NCX3. This cleavage generated a hyperfunctional form of the antiporter and increased NCX currents (I(NCX)) in the reverse mode of operation. Interestingly, this NCX3 calpain-dependent cleavage was essential for the Aβ(1-42)-dependent I(NCX) increase. Indeed, the calpain inhibitor calpeptin and the removal of the calpain-cleavage recognition sequence, via site-directed mutagenesis, abolished this effect. Moreover, the enhanced NCX3 activity was paralleled by an increased Ca(2+) content in the endoplasmic reticulum (ER) stores. Remarkably, the silencing in PC-12 cells or the knocking-out in mice of the ncx3 gene prevented the enhancement of both I(NCX) and Ca(2+) content in ER stores, suggesting that NCX3 was involved in the increase of ER Ca(2+) content stimulated by Aβ(1-42). By contrast, in the late phase (72 h), when the NCX3 proteolytic cleavage abruptly ceased, the occurrence of a parallel reduction in ER Ca(2+) content triggered ER stress, as revealed by caspase-12 activation. Concomitantly, the late increase in [Ca(2+)](i) coincided with neuronal death. Interestingly, NCX3 silencing caused an earlier activation of Aβ(1-42)-induced caspase-12. Indeed, in NCX3-silenced neurons, Aβ(1-42) exposure hastened caspase-dependent apoptosis, thus reinforcing neuronal cell death. These results suggest that Aβ(1-42), through Ca(2+)-dependent calpain activation, generates a hyperfunctional form of NCX3 that, by increasing Ca(2+) content into ER, delays caspase-12 activation and thus neuronal death.

Multivariate analyses of amyloid-beta oligomer populations indicate a connection between pore formation and cytotoxicity

Aggregates of amyloid-beta (Aβ) peptides are thought to be involved in the development of Alzheimer's disease because they can change synaptic plasticity and induce neuronal cell death by inflammation, oxidative damage, and transmembrane pore formation. Exactly which oligomeric species underlie these cytotoxic effects remains unclear. The work presented here established well-controlled aggregation conditions of Aβ₁₋₄₀ or Aβ₁₋₄₂ peptides over a 20-day period and characterized these preparations with regard to their β-sheet content, degree of fibril formation, relative abundance of various oligomer sizes, and propensity to induce membrane pore formation and cytotoxicity. Using this multivariate data set, a systematic and inherently unbiased partial least squares (PLS) approach showed that for both peptides the abundance of oligomers in the tetramer to 13-mer range contributed positively to both pore formation and cytotoxicity, while monomers, dimers, trimers, and the largest oligomers (>210 kDa) were negatively correlated to both phenomena. Multivariate PLS analysis is ideally suited to handle complex data sets and interdependent variables such as relative oligomer concentrations, making it possible to elucidate structure function relationships in complex mixtures. This approach, therefore, introduces an enabling tool to the field of amyloid research, in which it is often difficult to interpret the activity of individual species within a complex mixture of bioactive species.

Distinct annular oligomers captured along the assembly and disassembly pathways of transthyretin amyloid protofibrils

Background

Defects in protein folding may lead to severe degenerative diseases characterized by the appearance of amyloid fibril deposits. Cytotoxicity in amyloidoses has been linked to poration of the cell membrane that may involve interactions with amyloid intermediates of annular shape. Although annular oligomers have been detected in many amyloidogenic systems, their universality, function and molecular mechanisms of appearance are debated.

Methodology/Principal Findings

We investigated with high-resolution in situ atomic force microscopy the assembly and disassembly of transthyretin (TTR) amyloid protofibrils formed of the native protein by pH shift. Annular oligomers were the first morphologically distinct intermediates observed in the TTR aggregation pathway. Morphological analysis suggests that they can assemble into a double-stack of octameric rings with a 16±2 nm diameter, and displaying the tendency to form linear structures. According to light scattering data coupled to AFM imaging, annular oligomers appeared to undergo a collapse type of structural transition into spheroid oligomers containing 8–16 monomers. Disassembly of TTR amyloid protofibrils also resulted in the rapid appearance of annular oligomers but with a morphology quite distinct from that observed in the assembly pathway.

Conclusions/Significance

Our observations indicate that annular oligomers are key dynamic intermediates not only in the assembly but also in the disassembly of TTR protofibrils. The balance between annular and more compact forms of aggregation could be relevant for cytotoxicity in amyloidogenic disorders.

Dysregulation of neural calcium signaling in Alzheimer disease, bipolar disorder and schizophrenia

Neurons have highly developed Ca(2+) signaling systems responsible for regulating a large number of neural functions such as the control of brain rhythms, information processing and the changes in synaptic plasticity that underpin learning and memory. The tonic excitatory drive, which is activated by the ascending arousal system, is particularly important for processes such as sensory perception, cognition and consciousness. The Ca(2+) signaling pathway is a key component of this arousal system that regulates the neuronal excitability responsible for controlling the neural brain rhythms required for information processing and cognition. Dysregulation of the Ca(2+) signaling pathway responsible for many of these neuronal processes has been implicated in the development of some of the major neural diseases in man such as Alzheimer disease, bipolar disorder and schizophrenia. Various treatments, which are known to act by reducing the activity of Ca(2+) signaling, have proved successful in alleviating the symptoms of some of these neural diseases.

Pore formation by human stefin B in its native and oligomeric states and the consequent amyloid induced toxicity

It is well documented that amyloid forming peptides and proteins interact with membranes and that this correlates with cytotoxicity. To introduce the theme we give a brief description of some amyloidogenic proteins and note their similarities with pore forming toxins (PFTs) and cell penetrating peptides. Human stefin B, a member of the family of cystatins, is an amyloidogenic protein in vitro. This review describes our studies of the interaction of stefin B oligomers and prefibrillar aggregates with model membranes leading to pore formation. We have studied the interaction between human stefin B and artificial membranes of various compositions. We also have prepared distinct sizes and morphologies of stefin B prefibrillar states and assessed their toxicity. Furthermore, we have measured electrical currents through pores formed by stefin B prefibrillar oligomers in a planar lipid bilayer setup. We finally discuss the possible functional and pathological significance of such pores formed by human stefin B.

Microglial KCa3.1 Channels as a Potential Therapeutic Target for Alzheimer's Disease

There exists an urgent need for new target discovery to treat Alzheimer's disease (AD); however, recent clinical trials based on anti-Aβ and anti-inflammatory strategies have yielded disappointing results. To expedite new drug discovery, we propose reposition targets which have been previously pursued by both industry and academia for indications other than AD. One such target is the calcium-activated potassium channel KCa3.1 (KCNN4), which in the brain is primarily expressed in microglia and is significantly upregulated when microglia are activated. We here review the existing evidence supporting that KCa3.1 inhibition could block microglial neurotoxicity without affecting their neuroprotective phagocytosis activity and without being broadly immunosuppressive. The anti-inflammatory and neuroprotective effects of KCa3.1 blockade would be suitable for treating AD as well as cerebrovascular and traumatic brain injuries, two well-known risk factors contributing to the dementia in AD patients presenting with mixed pathologies. Importantly, the pharmacokinetics and pharmacodynamics of several KCa3.1 blockers are well known, and a KCa3.1 blocker has been proven safe in clinical trials. It is therefore promising to reposition old or new KCa3.1 blockers for AD preclinical and clinical trials.

Detection of prion protein oligomers by single molecule fluorescence imaging

The degree of polymerization of PrP has a close relationship with the pathological mechanisms of prion diseases. We examined, at the molecular level, the polymerization state of PrP in lysates of prion-infected cells using total internal reflection fluorescence microscopy (TIRFM). The crude lysates were fractionated by gel-filtration spin columns according to their molecular size. Both the oligomer-rich and the monomer-rich fractions were probed with fluorescein-labeled anti-PrP antibodies (mAb SAF70 and mAb 8G8). Fluorescent spots of varying intensity were detected, with the ratio of intense fluorescent spots being greater in the oligomer fraction samples with mAb SAF70 than those with 8G8, the specific epitope of which is thought to be buried in abnormal PrP molecules. The results indicated that PrP oligomers could be specifically detected and conformational changes of abnormal PrP molecules observed. Imaging by TIRFM may aid in determining the polymerization state and properties of PrP oligomers in pathological processes.

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

Alzheimer's disease (AD) is a misfolded protein disease characterized by the accumulation of β-amyloid (Aβ) 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 Aβ 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 Aβ(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 β-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 Aβ 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 β-sheet-based channel structure.

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

Alzheimer's disease (AD) is the most common type of senile dementia in aging populations. Amyloid β (Aβ)-mediated dysregulation of ionic homeostasis is the prevailing underlying mechanism leading to synaptic degeneration and neuronal death. Aβ-dependent ionic dysregulation most likely occurs either directly via unregulated ionic transport through the membrane or indirectly via Aβ 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 Aβ peptide enantiomer, whose entire sequence consists of d-amino acids, can form ion-conducting channels; these channels can directly mediate Aβ 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-Aβ isomer exhibits ion conductance behavior in the bilayer indistinguishable from that described earlier for the l-Aβ isomer. The d isomer forms channel-like pores with heterogeneous ionic conductance similar to the l-Aβ isomer channels, and the d-isomer channel conductance is blocked by Zn(2+), a known blocker of l-Aβ isomer channels. MD simulations further verify formation of β-barrel-like Aβ channels with d- and l-isomers, illustrating that both d- and l-Aβ 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 Aβ toxicity is mediated through a receptor-independent, nonstereoselective mechanism.

Atomic force microscopy and MD simulations reveal pore-like structures of all-D-enantiomer of Alzheimer's β-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 β-amyloid (Aβ) peptide as senile plaques, uncontrolled neurodegeneration, and memory loss. AD pathology is linked to the destabilization of cellular ionic homeostasis and involves Aβ peptide-plasma membrane interactions. In principle, there are two possible ways through which disturbance of the ionic homeostasis can take place: directly, where the Aβ peptide either inserts into the membrane and creates ion-conductive pores or destabilizes the membrane organization, or, indirectly, where the Aβ peptide interacts with existing cell membrane receptors. To distinguish between these two possible types of Aβ-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 Aβ(1-42) when reconstituted in lipid bilayers. AFM imaging shows that both L- and D-Aβ isomers form similar channel-like structures. Molecular dynamics (MD) simulations support the AFM imaged 3D structures. Previously, we have shown that D-Aβ(1-42) channels conduct ions similarly to their L- counterparts. Taken together, our results support the direct mechanism of Aβ ion channel-mediated destabilization of ionic homeostasis rather than the indirect mechanism through Aβ interaction with membrane receptors.

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

A current hypothesis for the pathology of Alzheimer's disease (AD) proposes that amyloid-β (Aβ) peptides induce uncontrolled, neurotoxic ion flux across cellular membranes. The mechanism of ion flux is not fully understood because no experiment-based Aβ 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 Aβ channel is an assembly of loosely associated mobile β-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 Aβ(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 β-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 β-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 Aβ-dependent ion fluxes.

Imaging β amyloid's pore performance

Study visualizes Alzheimer's disease–related peptides forming toxic calcium channels in the plasma membrane.