The amyloid hypothesis in Alzheimer disease (AD) considers amyloid β peptide (Aβ) deposition causative in triggering down-stream events like neurofibrillary tangles, cell loss, vascular damage and memory decline. In the past years N-truncated Aβ peptides especially N-truncated pyroglutamate AβpE3-42 have been extensively studied. Together with full-length Aβ1–42 and Aβ1–40, N-truncated AβpE3-42 and Aβ4–42 are major variants in AD brain. Although Aβ4–42 has been known for a much longer time, there is a lack of studies addressing the question whether AβpE3-42 or Aβ4–42 may precede the other in Alzheimer’s disease pathology.
Using different Aβ antibodies specific for the different N-termini of N-truncated Aβ, we discovered that Aβ4-x preceded AβpE3-x intraneuronal accumulation in a transgenic mouse model for AD prior to plaque formation. The novel Aβ4-x immunoreactive antibody NT4X-167 detected high molecular weight aggregates derived from N-truncated Aβ species. While NT4X-167 significantly rescued Aβ4–42 toxicity in vitro no beneficial effect was observed against Aβ1–42 or AβpE3-42 toxicity. Phenylalanine at position four of Aβ was imperative for antibody binding, because its replacement with alanine or proline completely prevented binding. Although amyloid plaques were observed using NT4X-167 in 5XFAD transgenic mice, it barely reacted with plaques in the brain of sporadic AD patients and familial cases with the Arctic, Swedish and the presenilin-1 PS1Δ9 mutation. A consistent staining was observed in blood vessels in all AD cases with cerebral amyloid angiopathy. There was no cross-reactivity with other aggregates typical for other common neurodegenerative diseases showing that NT4X-167 staining is specific for AD.
Aβ4-x precedes AβpE3-x in the well accepted 5XFAD AD mouse model underlining the significance of N-truncated species in AD pathology. NT4X-167 therefore is the first antibody reacting with Aβ4-x and represents a novel tool in Alzheimer research.
Pyroglutamate Abeta; Abeta oligomer; Toxicity; Arctic; Swedish; Presenilin-1; 5XFAD; Transgenic mouse model; Familial Alzheimer’s disease; Sporadic Alzheimer’s disease; Abeta 4-40; Abeta 4–42
The presence of AβpE3 (N-terminal truncated Aβ starting with pyroglutamate) in Alzheimer’s disease (AD) has received considerable attention since the discovery that this peptide represents a dominant fraction of Aβ peptides in senile plaques of AD brains. This was later confirmed by other reports investigating AD and Down’s syndrome postmortem brain tissue. Importantly, AβpE3 has a higher aggregation propensity, and stability, and shows an increased toxicity compared to full-length Aβ. We have recently shown that intraneuronal accumulation of AβpE3 peptides induces a severe neuron loss and an associated neurological phenotype in the TBA2 mouse model for AD. Given the increasing interest in AβpE3, we have generated two novel monoclonal antibodies which were characterized as highly specific for AβpE3 peptides and herein used to analyze plaque deposition in APP/PS1KI mice, an AD model with severe neuron loss and learning deficits. This was compared with the plaque pattern present in brain tissue from sporadic and familial AD cases. Abundant plaques positive for AβpE3 were present in patients with sporadic AD and familial AD including those carrying mutations in APP (arctic and Swedish) and PS1. Interestingly, in APP/PS1KI mice we observed a continuous increase in AβpE3 plaque load with increasing age, while the density for Aβ1-x plaques declined with aging. We therefore assume that, in particular, the peptides starting with position 1 of Aβ are N-truncated as disease progresses, and that, AβpE3 positive plaques are resistant to age-dependent degradation likely due to their high stability and propensity to aggregate.
Transgenic mice; Arctic mutation; Swedish mutation; Presenilin-1 mutation; Sporadic Alzheimer’s disease; Pyroglutamate Abeta; Biacore; Antibody specificity
Deposition of aggregated amyloid beta (Aβ) is a major hallmark of Alzheimer’s disease (AD)—a common age-related neurodegenerative disorder. Typically, Aβ is generated as a peptide of varying lengths. However, a major fraction of Aβ peptides in the brains of AD patients has undergone posttranslational modifications, which often radically change the properties of the peptides. Aβ3(pE)-42 is an N-truncated, pyroglutamate-modified variant that is abundantly present in AD brain and was suggested to play a role early in the pathogenesis. Here we show that intracellular accumulation of oligomeric aggregates of Aβ3(pE)-42 results in loss of lysosomal integrity. Using a novel antibody specific for aggregates of AβpE3, we show that in postmortem human brain tissue, aggregated AβpE3 is predominantly found in the lysosomes of both neurons and glial cells. Our data further demonstrate that AβpE3 is relatively resistant to lysosomal degradation, which may explain its accumulation in the lysosomes. The intracellular AβpE3 aggregates increase in an age-dependent manner. The results presented in this study support a model where Aβ pathology and aging converge, leading to accumulation of the degradation-resistant pE-modified Aβ in the lysosomes, lysosomal dysfunction, and neurodegeneration.
Electronic supplementary material
The online version of this article (doi:10.1007/s11357-012-9403-0) contains supplementary material, which is available to authorized users.
Alzheimer’s disease; Pyroglutamate amyloid beta; Lysosomal pathology
N-terminal truncated amyloid beta (Aβ) derivatives, especially the forms having pyroglutamate at the 3 position (AβpE3) or at the 11 position (AβpE11) have become the topic of considerable study. AβpE3 is known to make up a substantial portion of the Aβ species in senile plaques while AβpE11 has received less attention. We have generated very specific polyclonal antibodies against both species. Each antibody recognizes only the antigen against which it was generated on Western blots and neither recognizes full length Aβ. Both anti-AβpE3 and anti-AβpE11 stain senile plaques specifically in Alzheimer’s disease cerebral cortex and colocalize with Aβ, as shown by confocal microscopy. In a majority of plaques examined, AβpE11 was observed to be the dominant form in the innermost core. These data suggest that AβpE11 may serve as a generating site for senile plaque formation.
The neurotoxicity of β-amyloid protein (AβP) is implicated in the etiology of Alzheimer’s disease. We previously have demonstrated that AβP forms Ca2+-permeable pores on neuronal membranes, causes a marked increase in intracellular calcium level, and leads to neuronal death. Here, we investigated in detail the features of AβP-induced changes in intracellular Ca2+ level in primary cultured rat hippocampal neurons using a multisite Ca2+-imaging system with fura-2 as a fluorescent probe. Only a small fraction of short-term cultured hippocampal neurons (ca 1 week in vitro) exhibited changes in intracellular Ca2+ level after AβP exposure. However, AβP caused an acute increase in intracellular Ca2+ level in long-term cultured neurons (ca 1 month in vitro). The responses to AβP were highly heterogeneous, and immunohistochemical analysis using an antibody to AβP revealed that AβP is deposited on some but not all neurons. Considering that the disruption of Ca2+ homeostasis is the primary event in AβP neurotoxicity, substances that protect neurons from an AβP-induced intracellular Ca2+ level increase may be candidates as therapeutic drugs for Alzheimer’s disease. In line with the search for such protective substances, we found that the preadministration of neurosteroids including dehydroepiandrosterone, dehydroepiandrosterone sulfate, and pregnenolone significantly inhibits the increase in intracellular calcium level induced by AβP. Our results suggest the possible significance of neurosteroids, whose levels are reduced in the elderly, in preventing AβP neurotoxicity.
neurotoxicity; pore; calcium homeostasis; channel; aging
The major amyloid beta (Aβ) peptides found in the brain of familial and late onset Alzheimer’s disease include the full-length Aβ1-42 and N-terminally truncated, pyroglutamylated peptides Aβp3-42 and Aβp11-42. The biophysical properties of Aβ1-42 have been extensively studied, yet little is known about the other modified peptides. We investigated the aggregation kinetics of brain-specific Aβ peptides to better understand their potential roles in plaque formation. Synthetic peptides were analyzed individually and in mixtures representing various ratios found in the brain. Spectrofluorometric analyses using Thioflavin-T showed that the aggregation of Aβ1-42 was faster compared to Aβp3-42; however, Aβp11-42 displayed similar kinetics. Surprisingly, mixtures of full-length Aβ1-42 and Aβp3-42 showed an initial delay in beta-sheet formation from both equimolar and non-equimolar samples. Electron microscopy of peptides individually and in mixtures further supported fluorescence data. These results indicate that Aβ-Aβ peptide interactions involving different forms may play a critical role in senile plaque formation and maintenance of the soluble Aβ pool in the brain.
Alzheimer’s disease; Amyloid-beta peptides; Aggregation; Pyroglutamate
Nicotinic acetylcholine receptors (AChRs), implicated in memory and learning, in subjects affected by Alzheimer's disease result altered. Stimulation of α7-nAChRs inhibits amyloid plaques and increases ACh release. β-amyloid peptide (AβP) forms ion channels in the cell and model phospholipid membranes that are retained responsible in Alzheimer disease. We tested if choline, precursor of ACh, could affect the AβP1-40 channels in oxidized cholesterol (OxCh) and in palmitoyl-oleoyl-phosphatidylcholine (POPC):Ch lipid bilayers.
Choline concentrations of 5 × 10−11 M–1.5 × 10−8 M added to the cis- or trans-side of membrane quickly increased AβP1-40 ion channel frequency (events/min) and ion conductance in OxCh membranes, but not in POPC:Ch membranes. Circular Dichroism (CD) spectroscopy shows that after 24 and 48 hours of incubation with AβP1-40, choline stabilizes the random coil conformation of the peptide, making it less prone to fibrillate. These actions seem to be specific in that ACh is ineffective either in solution or on AβP1-40 channel incorporated into PLMs.
Oligomerization, conformational changes, and the consequent neurodegeneration of Alzheimer's β-amyloid protein (AβP) play crucial roles in the pathogenesis of Alzheimer's disease (AD). Mounting evidence suggests that oligomeric AβPs cause the disruption of calcium homeostasis, eventually leading to neuronal death. We have demonstrated that oligomeric AβPs directly incorporate into neuronal membranes, form cation-sensitive ion channels (“amyloid channels”), and cause the disruption of calcium homeostasis via the amyloid channels. Other disease-related amyloidogenic proteins, such as prion protein in prion diseases or α-synuclein in dementia with Lewy bodies, exhibit similarities in the incorporation into membranes and the formation of calcium-permeable channels. Here, based on our experimental results and those of numerous other studies, we review the current understanding of the direct binding of AβP into membrane surfaces and the formation of calcium-permeable channels. The implication of composition of membrane lipids and the possible development of new drugs by influencing membrane properties and attenuating amyloid channels for the treatment and prevention of AD is also discussed.
Efforts to discover new drugs for Alzheimer’s disease emphasizing multiple targets was conducted seeking to inhibit amyloid oligomer formation and to prevent radical formation. The tryptoline and tryptamine cores of BACE1 inhibitors previously identified by virtual screening were modified in silico for additional modes of action. These core structures were readily linked to different side chains using 1,2,3-triazole rings as bridges by copper catalyzed azide-alkyne cycloaddition reactions. Three compounds among the sixteen designed compounds exerted multifunctional activities including β-secretase inhibitory action, anti-amyloid aggregation, metal chelating and antioxidant effects at micromolar levels. The neuroprotective effects of the multifunctional compounds 6h, 12c and 12h on Aβ1–42 induced neuronal cell death at 1 μM were significantly greater than those of the potent single target compound, BACE1 inhibitor IV and were comparable to curcumin. The observed synergistic effect resulting from the reduction of the Aβ1–42 neurotoxicity cascade substantiates the validity of our multifunctional strategy in drug discovery for Alzheimer’s disease.
multifunction drugs; BACE1 inhibitor; anti-amyloid aggregation; chelator; antioxidant; neuroprotection
Amyloid-like plaques are characteristic lesions defining the neuropathology of Alzheimer's disease (AD). The size and density of these plaques are closely associated with cognitive decline. To combat this disease, the few therapies that are available rely on drugs that increase neurotransmission; however, this approach has had limited success as it has simply slowed an imminent decline and failed to target the root cause of AD. Amyloid-like deposits result from aggregation of the Aβ peptide, and thus, reducing amyloid burden by preventing Aβ aggregation represents an attractive approach to improve the therapeutic arsenal for AD. Recent studies have shown that the natural product curcumin is capable of crossing the blood-brain barrier in the CNS in sufficient quantities so as to reduce amyloid plaque burden. Based upon this bioactivity, we hypothesized that curcumin presents molecular features that make it an excellent lead compound for the development of more effective inhibitors of Aβ aggregation. To explore this hypothesis, we screened a library of curcumin analogs and identified structural features that contribute to the anti-oligomerization activity of curcumin and its analogs. First, at least one enone group in the spacer between aryl rings is necessary for measureable anti-Aβ aggregation activity. Second, an unsaturated carbon spacer between aryl rings is essential for inhibitory activity, as none of the saturated carbon spacers showed any margin of improvement over that of native curcumin. Third, methoxyl and hydroxyl substitutions in the meta- and para-positions on the aryl rings appear necessary for some measure of improved inhibitory activity. The best lead inhibitors have either their meta- and para-substituted methoxyl and hydroxyl groups reversed from that of curcumin or methoxyl or hydroxyl groups placed in both positions. The simple substitution of the para-hydroxy group on curcumin with a methoxy substitution improved inhibitor function by 6-7-fold over that measured for curcumin.
Neurodegenerative diseases result in the loss of functional neurons and synapses. Although future stem cell therapies offer some hope, current treatments for most of these diseases are less than adequate and our best hope is to prevent these devastating diseases. Neuroprotective approaches work best prior to the initiation of damage, suggesting that some safe and effective prophylaxis would be highly desirable. Curcumin has an outstanding safety profile and a number of pleiotropic actions with potential for neuroprotective efficacy, including anti-inflammatory, antioxidant, and anti-protein-aggregate activities. These can be achieved at sub-micromolar levels. Curcumin’s dose–response curves are strongly dose dependent and often biphasic so that in vitro data need to be cautiously interpreted; many effects might not be achievable in target tissues in vivo with oral dosing. However, despite concerns about poor oral bioavailability, curcumin has at least 10 known neuroprotective actions and many of these might be realized in vivo. Indeed, accumulating cell culture and animal model data show that dietary curcumin is a strong candidate for use in the prevention or treatment of major disabling age-related neurodegenerative diseases like Alzheimer’s, Parkinson’s, and stroke. Promising results have already led to ongoing pilot clinical trials.
The mechanisms underlying prion-linked neurodegeneration remain to be elucidated, despite several recent advances in this field. Herein, we show that soluble, low molecular weight oligomers of the full-length prion protein (PrP), which possess characteristics of PrP to PrPsc conversion intermediates such as partial protease resistance, are neurotoxic in vitro on primary cultures of neurons and in vivo after subcortical stereotaxic injection. Monomeric PrP was not toxic. Insoluble, fibrillar forms of PrP exhibited no toxicity in vitro and were less toxic than their oligomeric counterparts in vivo. The toxicity was independent of PrP expression in the neurons both in vitro and in vivo for the PrP oligomers and in vivo for the PrP fibrils. Rescue experiments with antibodies showed that the exposure of the hydrophobic stretch of PrP at the oligomeric surface was necessary for toxicity. This study identifies toxic PrP species in vivo. It shows that PrP-induced neurodegeneration shares common mechanisms with other brain amyloidoses like Alzheimer disease and opens new avenues for neuroprotective intervention strategies of prion diseases targeting PrP oligomers.
Prion diseases are transmissible neurodegenerative diseases caused by an infectious agent thought to be composed mainly of a host protein, the prion protein (PrP). The mechanisms of neurodegeneration prevailing in these diseases are not well understood. In the present study, we demonstrate that small PrP aggregates, called oligomers, cause the death of neurons in culture and after injection in vivo. On the contrary, larger PrP aggregates, visualized as fibrils by electron microscopy, do not cause the death of cultured neurons and are much less toxic than PrP oligomers in vivo. We propose that the PrP oligomers exert their toxicity by disturbing neuronal membranes, as well as by an excessive intracellular concentration leading to the generation of death signals (also called apoptotic signals) by the cell. Moreover, the use of antibodies recognizing a certain portion of the PrP oligomers could prevent neuronal death. This study assigns prion diseases to the same group of diseases as Alzheimer disease, in which protein oligomers constitute the major trigger of the neurodegenerative process, and suggests new possible neuroprotective approaches for therapeutic strategies.
This study was designed to investigate whether telomerase was involved in the neuroprotective effect of curcumin and Cur1. Alzheimer's disease is a consequence of an imbalance between the generation and clearance of amyloid-beta peptide in the brain. In this study, we used Aβ1-42 (10 µg/ml) to establish a damaged cell model, and curcumin and Cur1 were used in treatment groups. We measured cell survival and cell growth, intracellular oxidative stress and hTERT expression. After RNA interference, the effects of curcumin and Cur1 on cells were verified. Exposure to Aβ1–42 resulted in significant oxidative stress and cell toxicity, and the expression of hTERT was significantly decreased. Curcumin and Cur1 both protected SK-N-SH cells from Aβ1–42 and up-regulated the expression of hTERT. Furthermore, Cur1 demonstrated stronger protective effects than curcumin. However, when telomerase was inhibited by TERT siRNA, the neuroprotection by curcumin and Cur1 were ceased. Our study indicated that the neuroprotective effects of curcumin and Cur1 depend on telomerase, and thus telomerase may be a target for therapeutic effects of curcumin and Cur1.
The rational design of amyloid oligomer inhibitors is yet an unmet drug development need. Previous studies have identified the role of tryptophan in amyloid recognition, association and inhibition. Furthermore, tryptophan was ranked as the residue with highest amyloidogenic propensity. Other studies have demonstrated that quinones, specifically anthraquinones, can serve as aggregation inhibitors probably due to the dipole interaction of the quinonic ring with aromatic recognition sites within the amyloidogenic proteins. Here, using in vitro, in vivo and in silico tools we describe the synthesis and functional characterization of a rationally designed inhibitor of the Alzheimer's disease-associated β-amyloid. This compound, 1,4-naphthoquinon-2-yl-L-tryptophan (NQTrp), combines the recognition capacities of both quinone and tryptophan moieties and completely inhibited Aβ oligomerization and fibrillization, as well as the cytotoxic effect of Aβ oligomers towards cultured neuronal cell line. Furthermore, when fed to transgenic Alzheimer's disease Drosophila model it prolonged their life span and completely abolished their defective locomotion. Analysis of the brains of these flies showed a significant reduction in oligomeric species of Aβ while immuno-staining of the 3rd instar larval brains showed a significant reduction in Aβ accumulation. Computational studies, as well as NMR and CD spectroscopy provide mechanistic insight into the activity of the compound which is most likely mediated by clamping of the aromatic recognition interface in the central segment of Aβ. Our results demonstrate that interfering with the aromatic core of amyloidogenic peptides is a promising approach for inhibiting various pathogenic species associated with amyloidogenic diseases. The compound NQTrp can serve as a lead for developing a new class of disease modifying drugs for Alzheimer's disease.
disease is characterized by pathological aggregation
of protein tau and amyloid-β peptides, both of which are considered
to be toxic to neurons. Naturally occurring dietary flavonoids have
received considerable attention as alternative candidates for Alzheimer’s
therapy taking into account their antiamyloidogenic, antioxidative,
and anti-inflammatory properties. Experimental evidence supports the
hypothesis that certain flavonoids may protect against Alzheimer’s
disease in part by interfering with the generation and assembly of
amyloid-β peptides into neurotoxic oligomeric aggregates and
also by reducing tau aggregation. Several mechanisms have been proposed
for the ability of flavonoids to prevent the onset or to slow the
progression of the disease. Some mechanisms include their interaction
with important signaling pathways in the brain like the phosphatidylinositol
3-kinase/Akt and mitogen-activated protein kinase pathways that regulate
prosurvival transcription factors and gene expression. Other processes
include the disruption of amyloid-β aggregation and alterations
in amyloid precursor protein processing through the inhibition of
β-secretase and/or activation of α-secretase, and inhibiting
cyclin-dependent kinase-5 and glycogen synthase kinase-3β activation,
preventing abnormal tau phosphorylation. The interaction of flavonoids
with different signaling pathways put forward their therapeutic potential
to prevent the onset and progression of Alzheimer’s disease
and to promote cognitive performance. Nevertheless, further studies
are needed to give additional insight into the specific mechanisms
by which flavonoids exert their potential neuroprotective actions
in the brain of Alzheimer’s disease patients.
Flavonoids; Alzheimer’s disease; amyloid
precursor protein; amyloid beta; BACE-1; tau; signaling
Protein aggregation into amyloid fibrils and protofibrillar aggregates is associated with a number of the most common neurodegenerative diseases. We have established, using a computational approach, that knowledge of the primary sequences of proteins is sufficient to predict their in vitro aggregation propensities. Here we demonstrate, using rational mutagenesis of the Aβ42 peptide based on such computational predictions of aggregation propensity, the existence of a strong correlation between the propensity of Aβ42 to form protofibrils and its effect on neuronal dysfunction and degeneration in a Drosophila model of Alzheimer disease. Our findings provide a quantitative description of the molecular basis for the pathogenicity of Aβ and link directly and systematically the intrinsic properties of biomolecules, predicted in silico and confirmed in vitro, to pathogenic events taking place in a living organism.
A wide range of diseases, including diabetes and common brain diseases of old age, are characterised by the deposition of protein in the affected tissues. Alzheimer disease, the most common neurodegenerative disorder, is caused by the aggregation and deposition of a peptide called Aβ in the brain. We have previously developed a computational procedure that predicts a particular peptide or protein's speed of aggregation in the test tube. Our goal was to test whether the speed of aggregate formation that we observe in the test tube is directly linked to the brain toxicity that we see in our fruit fly model of Alzheimer disease. We made flies that produce each of 17 variant forms of Aβ and show that the toxicity of each variant is closely linked to the tendency of each variant to form small soluble aggregates. Our computational procedure has previously been shown to be applicable to a wide range of different proteins and diseases, and so this demonstration that it can predict toxicity in an animal model system has implications for many areas of disease-related research.
A systematic analysis of Alzheimer disease amyloid β peptide variants inDrosophila brain demonstrates that their predicted propensity to form protofibrillar aggregates correlates best with toxicity.
Nitrosative stress has recently been demonstrated as a causal in a select sporadic variant of Parkinson’s (PD) and Alzheimer’s (AD) diseases. Specifically, elevated levels of NO disrupt the redox activity of protein disulfide isomerase, a key endoplasmic reticulum-resident chaperone by S-nitroso modification of its redox-active cysteines. This leads to accumulation of misfolded AD- and PD-specific protein debris. We have recently demonstrated in vitro that polyphenolic phytochemicals, curcumin and masoprocol, can rescue S-nitroso-PDI formation by scavenging NOx. In this study, using dopaminergic SHSY-5Y cells, we have monitored the aggregation of green-fluorescent protein (GFP)-tagged synphilin-1 (a known constituent of PD Lewy neurites) as a function of rotenone-induced nitrosative stress. Importantly, we demonstrate a marked decrease in synphilin-1 aggregation when the cell line is previously incubated with 3,5-bis(2-flurobenzylidene) piperidin-4-one (EF-24), a curcumin analogue, prior to rotenone insult. Furthermore, our data also reveal that rotenone attenuates PDI expression in the same cell line, a phenomenon that can be mitigated through EF-24 intervention. Together, these results suggest that EF-24 can exert neuroprotective effects by ameliorating nitrosative stress-linked damage to PDI and the associated onset of PD and AD. Essentially, EF-24 can serve as a scaffold for the design and development of PD and AD specific prophylactics.
protein disulfide isomerase; nitrosative stress; neurodegenerative disorders; synphilin-1; EF-24; Lewy body
Aggregation of the amyloid β
protein (Aβ) peptide with 40 or 42 residues is one key feature
in Alzheimer’s disease (AD). The 1,4-naphthoquinon-2-yl-l-tryptophan (NQTrp) molecule was reported to alter Aβ
self-assembly and reduce toxicity. Though nuclear magnetic resonance
experiments and various simulations provided atomic information about
the interaction of NQTrp with Aβ peptides spanning the regions
of residues 12–28 and 17–42, none of these studies were
conducted on the full-length Aβ1–42 peptide. To this
end, we performed extensive atomistic replica exchange molecular dynamics
simulations of Aβ1–42 dimer with two NQTrp molecules
in explicit solvent, by using a force field known to fold diverse
proteins correctly. The interactions between NQTrp and Aβ1–42,
which change the Aβ interface by reducing most of the intermolecular
contacts, are found to be very dynamic and multiple, leading to many
transient binding sites. The most favorable binding residues are Arg5,
Asp7, Tyr10, His13, Lys16, Lys18, Phe19/Phe20, and Leu34/Met35, providing
therefore a completely different picture from in vitro and in silico experiments with NQTrp with shorter
Aβ fragments. Importantly, the 10 hot residues that we identified
explain the beneficial effect of NQTrp in reducing both the level
of Aβ1–42 aggregation and toxicity. Our results also
indicate that there is room to design more efficient drugs targeting
Aβ1–42 dimer against AD.
Alzheimer’s disease; amyloid β dimer; NQTrp; replica exchange
molecular dynamics simulation
The oligomer cascade hypothesis, which states that oligomers are the initiating pathologic agents in Alzheimer’s disease, has all but supplanted the amyloid cascade hypothesis, which suggested that fibers were the key etiologic agents in Alzheimer’s disease. We review here the results of in vivo, in vitro and in silico studies of amyloid β-protein oligomers, and discuss important caveats that should be considered in the evaluation of these results. This article is divided into four sections that mirror the main approaches used in the field to better understand oligomers: (1) attempts to locate and examine oligomers in vivo in situ; that is, without removing these species from their environment; (2) studies involving oligomers extracted from human or animal tissues and the subsequent characterization of their properties ex vivo; (3) studies of oligomers that have been produced synthetically and studied using a reductionist approach in relatively simple in vitro biophysical systems; and (4) computational studies of oligomers in silico. These multiple orthogonal approaches have revealed much about the molecular and cell biology of amyloid β-protein. However, as informative as these approaches have been, the amyloid β-protein oligomer system remains enigmatic.
Amyloid protein aggregates are associated with dozens of devastating diseases including Alzheimer’s, Parkinson’s, ALS, and diabetes type 2. While structure-based discovery of compounds has been effective in combating numerous infectious and metabolic diseases, ignorance of amyloid structure has hindered similar approaches to amyloid disease. Here we show that knowledge of the atomic structure of one of the adhesive, steric-zipper segments of the amyloid-beta (Aβ) protein of Alzheimer’s disease, when coupled with computational methods, identifies eight diverse but mainly flat compounds and three compound derivatives that reduce Aβ cytotoxicity against mammalian cells by up to 90%. Although these compounds bind to Aβ fibers, they do not reduce fiber formation of Aβ. Structure-activity relationship studies of the fiber-binding compounds and their derivatives suggest that compound binding increases fiber stability and decreases fiber toxicity, perhaps by shifting the equilibrium of Aβ from oligomers to fibers.
Alzheimer’s disease is the most common form of dementia, estimated to affect roughly five million people in the United States, and its incidence is steadily increasing as the population ages. A pathological hallmark of Alzheimer’s disease is the presence in the brain of aggregates of two proteins: tangles of a protein called tau; and fibers and smaller units (oligomers) of a peptide called amyloid beta.
Many attempts have been made to screen libraries of natural and synthetic compounds to identify substances that might prevent the aggregation and toxicity of amyloid. Such studies revealed that polyphenols found in green tea and in the spice turmeric can inhibit the formation of amyloid fibrils. Moreover, a number of dyes reduce the toxic effects of amyloid on cells, although significant side effects prevent these from being used as drugs.
Structure-based drug design, in which the structure of a target protein is used to help identify compounds that will interact with it, has been used to generate therapeutic agents for a number of diseases. Here, Jiang et al. report the first application of this technique in the hunt for compounds that inhibit the cytotoxicity of amyloid beta. Using the known atomic structure of the protein in complex with a dye, Jiang et al. performed a computational screen of 18,000 compounds in search of those that are likely to bind effectively.
The compounds that showed the strongest predicted binding were then tested for their ability to interfere with the aggregation of amyloid beta and to protect cells grown in culture from its toxic effects. Compounds that reduced toxicity did not reduce the abundance of protein aggregates, but they appear to increase the stability of fibrils. This is consistent with other evidence suggesting that small, soluble forms (oligomers) of amyloid beta that break free from the fibrils may be the toxic agent in Alzheimer’s disease, rather than the fibrils themselves.
In addition to uncovering compounds with therapeutic potential in Alzheimer’s disease, this work presents a new approach for identifying proteins that bind to amyloid fibrils. Given that amyloid accumulation is a feature of many other diseases, including Parkinson’s disease, Huntington’s disease and type 2 diabetes, the approach could have broad therapeutic applications.
amyloid fiber; computational biology; drug discovery; Alzheimer's disease; ligand docking; Other
The pathogenic aggregation of the amyloid β-peptide
is considered a hallmark of the progression of Alzheimer’s
disease, the leading cause of senile dementia in the elderly and one
of the principal causes of death in the United States. In the absence
of effective therapeutics, the incidence and economic burden associated
with the disease are expected to rise dramatically in the coming decades.
Targeting Aβ aggregation is an attractive therapeutic approach,
though structural insights into the nature of Aβ aggregates
from traditional experiments are elusive, making drug design difficult.
Theoretical methods have been used for several years to augment experimental
work and drive progress forward in Alzheimer’s
drug design. In this Review, we will describe how two common techniques,
molecular docking and molecular dynamics simulations, are being applied
in developing small molecules as effective therapeutics against monomeric,
oligomeric, and fibrillated forms of Aβ. Recent successes and
important limitations will be discussed, and we conclude by providing
a perspective on the future of this field by citing recent examples
of sophisticated approaches used to better characterize interactions
of small molecules with Aβ and other amyloidogenic proteins.
Modeling; molecular dynamics; docking; therapeutics; amyloid
It has recently been determined that not only Aβ oligomers, but also other Aβ species and amyloidogenic peptides are neurotoxic in Alzheimer disease (AD) and play a pivotal role in AD pathogenesis. In the present study, we attempted to develop new DNA vaccines targeting a wide range of Aβ species. For this purpose, we first performed in vitro assays with newly developed vaccines to evaluate Aβ production and Aβ secretion abilities and then chose an IgL-Aβx4-Fc-IL-4 vaccine (designated YM3711) for further studies. YM3711 was vaccinated to mice, rabbits and monkeys to evaluate anti-Aβ species antibody-producing ability and Aβ reduction effects. It was found that YM3711 vaccination induced significantly higher levels of antibodies not only to Aβ1-42 but also to AD-related molecules including AβpE3-42, Aβ oligomers and Aβ fibrils. Importantly, YM3711 significantly reduced these Aβ species in the brain of model mice. Binding and competition assays using translated YM3711 protein products clearly demonstrated that a large part of antibodies induced by YM3711 vaccination are directed at conformational epitopes of the Aβ complex and oligomers. Taken together, we demonstrate that YM3711 is a powerful DNA vaccine targeting a wide range of AD-related molecules and is worth examining in preclinical and clinical trials.
Alzheimer's disease (AD) involves a complex pathological cascade thought to be initially triggered by the accumulation of β-amyloid (Aβ) peptide aggregates or aberrant amyloid precursor protein (APP) processing. Much is known of the factors initiating the disease process decades prior to the onset of cognitive deficits, but an unclear understanding of events immediately preceding and precipitating cognitive decline is a major factor limiting the rapid development of adequate prevention and treatment strategies. Multiple pathways are known to contribute to cognitive deficits by disruption of neuronal signal transduction pathways involved in memory. These pathways are altered by aberrant signaling, inflammation, oxidative damage, tau pathology, neuron loss, and synapse loss. We need to develop stage-specific interventions that not only block causal events in pathogenesis (aberrant tau phosphorylation, Aβ production and accumulation, and oxidative damage), but also address damage from these pathways that will not be reversed by targeting prodromal pathways. This approach would not only focus on blocking early events in pathogenesis, but also adequately correct for loss of synapses, substrates for neuroprotective pathways (e.g., docosahexaenoic acid), defects in energy metabolism, and adverse consequences of inappropriate compensatory responses (aberrant sprouting). Monotherapy targeting early single steps in this complicated cascade may explain disappointments in trials with agents inhibiting production, clearance, or aggregation of the initiating Aβ peptide or its aggregates. Both plaque and tangle pathogenesis have already reached AD levels in the more vulnerable brain regions during the “prodromal” period prior to conversion to “mild cognitive impairment (MCI).” Furthermore, many of the pathological events are no longer proceeding in series, but are going on in parallel. By the MCI stage, we stand a greater chance of success by considering pleiotropic drugs or cocktails that can independently limit the parallel steps of the AD cascade at all stages, but that do not completely inhibit the constitutive normal functions of these pathways. Based on this hypothesis, efforts in our laboratories have focused on the pleiotropic activities of omega-3 fatty acids and the anti-inflammatory, antioxidant, and anti-amyloid activity of curcumin in multiple models that cover many steps of the AD pathogenic cascade (Cole and Frautschy, Alzheimers Dement 2:284–286, 2006).
Antioxidants; Curcumin; Inflammation; Non-steroidal anti-inflammatory drugs; Tau; Tau kinases; β-Amyloid
Histone deacetylase (HDAC) inhibitors have been demonstrated to be beneficial in animal models of neurodegenerative diseases. Such results were mainly associated with the epigenetic modulation caused by HDACs, especially those from class I, via chromatin deacetylation. However, other mechanisms may contribute to the neuroprotective effect of HDAC inhibitors, since each HDAC may present distinct specific functions within the neurodegenerative cascades. Such an example is HDAC6 for which the role in neurodegeneration has been partially elucidated so far. The strategy to be adopted in promising therapeutics targeting HDAC6 is still controversial. Specific inhibitors exert neuroprotection by increasing the acetylation levels of α-tubulin with subsequent improvement of the axonal transport, which is usually impaired in neurodegenerative disorders. On the other hand, an induction of HDAC6 would theoretically contribute to the degradation of protein aggregates which characterize various neurodegenerative disorders, including Alzheimer’s, Parkinson’s and Hutington’s diseases. This review describes the specific role of HDAC6 compared to the other HDACs in the context of neurodegeneration, by collecting in silico, in vitro and in vivo results regarding the inhibition and/or knockdown of HDAC6 and other HDACs. Moreover, structure, function, subcellular localization, as well as the level of HDAC6 expression within brain regions are reviewed and compared to the other HDAC isoforms. In various neurodegenerative diseases, the mechanisms underlying HDAC6 interaction with other proteins seem to be a promising approach in understanding the modulation of HDAC6 activity.
Histone deacetylase; HDAC6; Neurodegenerative diseases
Degeneration of synaptic and axonal compartments of neurons is an early event contributing to the pathogenesis of many neurodegenerative diseases, but the underlying molecular mechanisms remain unclear. Here, we demonstrate the effectiveness of a novel “top-down” approach for identifying proteins and functional pathways regulating neurodegeneration in distal compartments of neurons. A series of comparative quantitative proteomic screens on synapse-enriched fractions isolated from the mouse brain following injury identified dynamic perturbations occurring within the proteome during both initiation and onset phases of degeneration. In silico analyses highlighted significant clustering of proteins contributing to functional pathways regulating synaptic transmission and neurite development. Molecular markers of degeneration were conserved in injury and disease, with comparable responses observed in synapse-enriched fractions isolated from mouse models of Huntington's disease (HD) and spinocerebellar ataxia type 5. An initial screen targeting thirteen degeneration-associated proteins using mutant Drosophila lines revealed six potential regulators of synaptic and axonal degeneration in vivo. Mutations in CALB2, ROCK2, DNAJC5/CSP, and HIBCH partially delayed injury-induced neurodegeneration. Conversely, mutations in DNAJC6 and ALDHA1 led to spontaneous degeneration of distal axons and synapses. A more detailed genetic analysis of DNAJC5/CSP mutants confirmed that loss of DNAJC5/CSP was neuroprotective, robustly delaying degeneration in axonal and synaptic compartments. Our study has identified conserved molecular responses occurring within synapse-enriched fractions of the mouse brain during the early stages of neurodegeneration, focused on functional networks modulating synaptic transmission and incorporating molecular chaperones, cytoskeletal modifiers, and calcium-binding proteins. We propose that the proteins and functional pathways identified in the current study represent attractive targets for developing therapeutics aimed at modulating synaptic and axonal stability and neurodegeneration in vivo.
In diseases affecting the nervous system, such as Alzheimer's disease and motor neuron disease, the breakdown of synaptic connections between neurons is a critical early event, contributing to disease onset and progression. However, we still know very little about the molecular machinery present in synaptic and axonal compartments of neurons that regulate their stability and cause breakdown during neurodegeneration. In this study we examined the protein composition of healthy and degenerating synapse-enriched fractions isolated from the brains of mice in order to identify early molecular changes occurring during neurodegeneration. We identified a range of proteins and cellular pathways that were modulated in synapse-enriched fractions during the early phases of degeneration, many of which were already known to regulate synaptic function. Similar molecular alterations were found in synapse-enriched fractions prepared from mouse models of Huntington's disease (HD) and spinocerebellar ataxia type 5. Data from these proteomic studies were then used to design experiments in Drosophila, in which we found that at least six of the individual proteins modified in degenerating synapses from mice were capable of independently regulating neuronal stability and degeneration in vivo. Designing novel therapeutics to target these proteins and pathways may help to delay or prevent neurodegeneration across a range of diseases.