Amyloid oligomers and their membrane toxicity - A perspective study

Amyloidosis is a condition involving a disparate group of pathologies characterized by the extracellular deposition of insoluble fibrils composed of broken-down proteins. These proteins can accumulate locally, causing peculiar symptoms, or in a widespread way, involving many organs and. causing severe systemic failure. The damage that is created is related not only to the accumulation of. amyloid fibrils but above all to the precursor oligomers of the fibrils that manage to enter the cell in a very particular way. This article analyzes the current state of research related to the entry of these oligomers into the cell membrane and the theories related to their toxicity. The paper proposed here not only aims to review the contents in the literature but also proposes a new vision of amyloid toxicity. that could occur in a multiphase process catalyzed by the cell membrane itself. In this process, the denaturation of the lipid bilayer is followed by the stabilization of a pore through energetically favorable self-assembly processes which are achieved through particular oligomeric structures.

Misfolded protein oligomers: mechanisms of formation, cytotoxic effects, and pharmacological approaches against protein misfolding diseases

The conversion of native peptides and proteins into amyloid aggregates is a hallmark of over 50 human disorders, including Alzheimer's and Parkinson's diseases. Increasing evidence implicates misfolded protein oligomers produced during the amyloid formation process as the primary cytotoxic agents in many of these devastating conditions. In this review, we analyze the processes by which oligomers are formed, their structures, physicochemical properties, population dynamics, and the mechanisms of their cytotoxicity. We then focus on drug discovery strategies that target the formation of oligomers and their ability to disrupt cell physiology and trigger degenerative processes.

Protein misfolding and related human diseases: A comprehensive review of toxicity, proteins involved, and current therapeutic strategies

Most of the cell's chemical reactions and structural components are facilitated by proteins. But proteins are highly dynamic molecules, where numerous modifications or changes in the cellular environment can affect their native conformational fold leading to protein aggregation. Various stress conditions, such as oxidative stress, mutations and metal toxicity may cause protein misfolding and aggregation by shifting the conformational equilibrium towards more aggregation-prone states. Most of the protein misfolding diseases (PMDs) involve aggregation of protein. We have discussed such proteins like Aβ peptide, α-synuclein, amylin and lysozyme involved in Alzheimer's, Parkinson's, type II diabetes and non-neuropathic systemic amyloidosis respectively. Till date, all advances in PMDs therapeutics help symptomatically but do not prevent the root cause of the disease, i.e., the aggregation of protein involved in the diseases. Current efforts focused on developing therapies for PMDs have employed diverse strategies; repositioning pre-existing drugs as it saves time and money; natural compounds that are touted as potential drug candidates have an advantage of being taken in diet normally and will induce lesser side effects. This review also covers recently developed therapeutic strategies like antisense drugs and disaggregases which has yielded therapeutic agents that have transitioned from preclinical studies into human clinical trials.

Discovery of Molecular Networks of Neuroprotection Conferred by Brahmi Extract in Aβ42-Induced Toxicity Model of Drosophila melanogaster Using a Quantitative Proteomic Approach

Accumulation of Aβ₄₂ peptides forming plaque in various regions of the brain is a hallmark of Alzheimer's disease (AD) progression. However, to date, there is no effective management strategy reported for attenuation of Aβ₄₂-induced toxicity in the early stages of the disease. Alternate medicinal systems such as Ayurveda in the past few decades show promising results in the management of neuronal complications. Medhya Rasayana such as Brahmi is known for its neuroprotective properties via resolving memory-related issues, while the underlying molecular mechanism of the same remains unclear. In the present study, we aimed to understand the neuroprotective effects of the aqueous extract of Bacopa monnieri and Centella asiatica (both commonly known as Brahmi) against the Aβ₄₂ expressing model of the Drosophila melanogaster. By applying a quantitative proteomics approach, the study identified > 90% of differentially expressed proteins from Aβ₄₂ expressing D. melanogaster were either restored to their original expression pattern or showed no change in expression pattern upon receiving either Brahmi extract treatment. The Brahmi restored proteins were part of neuronal pathways associated with cell cycle re-entry, apoptosis, and mitochondrial dynamics. The neuroprotective effect of Brahmi was also validated by negative geotaxis behavioral analysis suggesting its protective role against behavioral deficits exerted by Aβ₄₂ toxicity. We believe that these discoveries will provide a platform for developing novel therapeutics for AD management by deciphering molecular targets of neuroprotection conferred by an aqueous extract of Bacopa monnieri or Centella asiatica.

Chronic Glucocorticoids Consumption Triggers and Worsens Experimental Alzheimer's Disease-Like Pathology by Detrimental Immune Modulations

INTRODUCTION

Among the risk factors identified in the sporadic forms of Alzheimer's disease (AD), environmental and lifestyle elements are of growing interest. Clinical observations suggest that stressful events can anticipate AD onset, while stress-related disorders can promote AD. Here, we tested the hypothesis that a chronic treatment with glucocorticoids is sufficient to trigger or exacerbate AD molecular hallmarks.

METHODS

We first validated a rat model of experimental chronic glucocorticoids (GC) consumption (corticosterone [CORT] in drinking water for 4 weeks). Then, to evaluate the consequences of chronic GC consumption on the onset of amyloid-β (Aβ) toxicity, animals chronically treated with GC were intracerebroventricularly injected with an oligomeric solution of Aβ25-35 (oAβ) (acute model of AD). We evaluated AD-related cognitive deficits and pathogenic mechanisms, with a special emphasis on neuroinflammatory markers.

RESULTS

Chronic CORT consumption caused the inhibition of the nonamyloidogenic pathways, the impairment of Aβ clearance processes and the induction of amyloidogenic pathways in the hippocampus. The principal enzymes involved in glucocorticoid receptor activation and Tau phosphorylation were upregulated. Importantly, the AD-like phenotype triggered by chronic CORT was analogous to the one caused by oAβ. These molecular commonalities across models were independent from inflammation, as chronic CORT was immunosuppressive while oAβ was pro-inflammatory. When chronic CORT consumption anticipated the induction of the oAβ pathology, we found a potentiation of neuroinflammatory processes associated with an exacerbation of synaptic and memory deficits but also an aggravation of AD-related hallmarks.

DISCUSSION/CONCLUSION

This study unravels new functional outcomes identifying chronic CORT consumption as a main risk factor for AD and suggests that glucocorticoid-based therapies should be prescribed with caution in populations with AD risk.

Maysin plays a protective role against α-Synuclein oligomers cytotoxicity by triggering autophagy activation

Parkinson's disease (PD) is a widespread neurodegenerative disorder characterized by the progressive loss of neurons. The accumulation of aggregated forms of the α-Synuclein (Syn) protein is the main cause of neurotoxicity in PD by disrupting cellular homeostasis until neuronal death. Scientific research is constantly looking for natural products as preventive agents against the progression of several neurodisorders due their safety and non-toxic nature. Neuroprotective phytochemicals include Maysin (Mys), the most abundant C-glycosilflavone in corn silk. In this work, the Mys protective role against damage by Syn amyloid aggregates - oligomers and fibrils - was investigated in SH-SY5Y human neuroblastoma cells obtaining novel and interesting information concerning the Mys molecular mechanism of action. Mys showed effectiveness in preventing the typical toxic events induced by Syn amyloid aggregates, i.e. oxidative stress and imbalance of intracellular calcium homeostasis. Mys exhibited a cytoprotective role, especially against Syn oligomers injury, activating an autophagic degradative process, thus playing a key role on several features of amyloid neurotoxicity. Therefore, Mys could be proposed for the first time to the scientific community as an interesting novel natural compound that might allow to develop alternative strategies to prevent the damage of Syn oligomers involved in Parkinson's disease.

Sigma-1 Receptor Agonists Induce Oxidative Stress in Mitochondria and Enhance Complex I Activity in Physiological Condition but Protect Against Pathological Oxidative Stress

The sigma1 receptor (σ1R) is a chaperone protein residing at mitochondria-associated endoplasmic reticulum (ER) membranes (MAMs), where it modulates Ca2+ exchange between the ER and mitochondria by interacting with inositol-1,4,5 trisphosphate receptors (IP3Rs). The σ1R is highly expressed in the central nervous system and its activation stimulates neuromodulation and neuroprotection, for instance in Alzheimer's disease (AD) models in vitro and in vivo. σ1R effects on mitochondria pathophysiology and the downstream signaling are still not fully understood. We here evaluated the impacts of σ1R ligands in mouse mitochondria preparations on reactive oxygen species (ROS) production, mitochondrial respiration, and complex activities, in physiological condition and after direct application of amyloid Aβ1-42 peptide. σ1R agonists (2-(4-morpholinethyl)-1-phenylcyclohexanecarboxylate hydrochloride (PRE-084), tetrahydro-N,N-dimethyl-5,5-diphenyl-3-furanmethanamine (ANAVEX1-41, AN1-41), (S)-1-(2,8-dimethyl-1-thia-3,8-diazaspiro[4.5]dec-3-yl)-3-(1H-indol-3-yl)propan-1-one (ANAVEX3-71, AN3-71), dehydroepiandrosterone-3 sulfate (DHEA), donepezil) increased mitochondrial ROS in a σ1R antagonist-sensitive manner but decreased Aβ1-42-induced increase in ROS. σ1R ligands (agonists or antagonists) did not impact respiration but attenuated Aβ1-42-induced alteration. σ1R agonists (PRE-084, AN1-41, tetrahydro-N,N-dimethyl-2,2-diphenyl-3-furanmethanamine hydrochloride (ANAVEX2-73, AN2-73), AN3-71) increased complex I activity, in a Ca2+-dependent and σ1R antagonist-sensitive manner. σ1R ligands failed to affect complex II, III, and IV activities. The increase in complex I activity explain the σ1R-induced increase in ROS since ligands failed to affect other sources of ROS accumulation in mitochondria and homogenates, namely NADPH oxidase (NOX) and superoxide dismutase (SOD) activities. Furthermore, Aβ1-42 significantly decreased the activity of complexes I and IV and σ1R agonists attenuated the Aβ1-42-induced complex I and IV dysfunctions. σ1R activity in mitochondria therefore results in a Ying-Yang effect, by triggering moderate ROS increase acting as a physiological signal and promoting a marked anti-oxidant effect in pathological (Aβ) conditions.

Amyloid toxicity is enhanced after pharmacological or genetic invalidation of the σ1 receptor

The sigma-1 receptor (S1R) is a molecular chaperone which activity modulates several intracellular signals including calcium mobilization at mitochondria-associated endoplasmic reticulum membranes. S1R agonists are potent neuroprotectants against neurodegenerative insults and particularly in rodent models of Alzheimer's disease (AD). We here analyzed whether S1R inactivation modifies vulnerability to amyloid toxicity in AD models. Two strategies were used: (1) amyloid β[25-35] (Aβ25-35) peptide (1, 3, 9nmol) was injected intracerebroventricularly in mice treated repeatedly with the S1R antagonist NE-100 or in S1RKO mice, and (2) WT, APPSweInd, S1RKO, and APPSweInd/S1RKO mice were created and female littermates analyzed at 8 months of age. Learning deficits, oxidative stress, Bax level and BDNF content in the hippocampus were analyzed. Aβ25-35 induced learning impairment, oxidative stress, Bax induction and BDNF alteration at lower dose in NE-100-treated mice or S1RKO mice as compared to WT animals. The extent of learning deficits and biochemical alterations were also higher in APPSweInd/S1RKO mice as compared to WT, APPSweInd, and S1RKO animals. S1R inactivation or altered S1R expression augmented the pathological status in pharmacologic and genetic AD mouse models. These observations, in relation with the well-known protective effects of S1R agonists, are coherent with a role of signal amplifier in neurodegeneration and neuroprotection proposed for S1R in AD and related neurodegenerative disorders.

Molecular simulation aspects of amyloid peptides at membrane interface

The interactions of amyloid peptides with cell membranes play an important role in maintaining the integrity and functionality of cell membrane. A thorough molecular-level understanding of the structure, dynamics, and interactions between amyloid peptides and cell membranes is critical to amyloid aggregation and toxicity mechanisms for the bench-to-bedside applications. Here we review the most recent computational studies of amyloid peptides at model cell membranes. Different mechanisms of action of amyloid peptides on/in cell membranes, targeted by different computational techniques at different lengthscales and timescales, are rationally discussed. Finally, we have proposed some new insights into the remaining challenges and perspectives for future studies to improve our understanding of the activity of amyloid peptides associated with protein-misfolding diseases. This article is part of a Special Issue entitled: Protein Aggregation and Misfolding at the Cell Membrane Interface edited by Ayyalusamy Ramamoorthy.

Nanobodies raised against monomeric ɑ-synuclein inhibit fibril formation and destabilize toxic oligomeric species

Background

The aggregation of the protein ɑ-synuclein (ɑS) underlies a range of increasingly common neurodegenerative disorders including Parkinson’s disease. One widely explored therapeutic strategy for these conditions is the use of antibodies to target aggregated ɑS, although a detailed molecular-level mechanism of the action of such species remains elusive. Here, we characterize ɑS aggregation in vitro in the presence of two ɑS-specific single-domain antibodies (nanobodies), NbSyn2 and NbSyn87, which bind to the highly accessible C-terminal region of ɑS.

Results

We show that both nanobodies inhibit the formation of ɑS fibrils. Furthermore, using single-molecule fluorescence techniques, we demonstrate that nanobody binding promotes a rapid conformational conversion from more stable oligomers to less stable oligomers of ɑS, leading to a dramatic reduction in oligomer-induced cellular toxicity.

Conclusions

The results indicate a novel mechanism by which diseases associated with protein aggregation can be inhibited, and suggest that NbSyn2 and NbSyn87 could have significant therapeutic potential.

Electronic supplementary material

The online version of this article (doi:10.1186/s12915-017-0390-6) contains supplementary material, which is available to authorized users.

Leucettine L41, a DYRK1A-preferential DYRKs/CLKs inhibitor, prevents memory impairments and neurotoxicity induced by oligomeric Aβ25-35 peptide administration in mice

Dual-specificity tyrosine phosphorylation-regulated kinases (DYRKs) and cdc2-like kinases (CLKs) are implicated in the onset and progression of Down syndrome (DS) and Alzheimer's disease (AD). DYRK1A has emerged as a possible link between amyloid-β (Aβ) and Tau, the major pathological proteins in AD. We here assessed the neuroprotective potential of a novel inhibitor of DYRKs/CLKs. The Leucettine L41, acting preferentially on DYRK1A, was tested in Aβ25-35-treated mice, a nontransgenic model of AD-like toxicity. We co-injected intracerebroventricularly oligomeric Aβ25-35 peptide and L41 in Swiss male mice. After 7 days, they were submitted to behavioral tests addressing spatial and non-spatial, short- and long-term memories. The oxidative stress, apoptotic markers, kinases involved in Tau phosphorylation, and synaptic integrity were analyzed by Western blot and ELISA in the hippocampus. L41, tested at 0.4, 1.2, 4 µg, prevented the Aβ25-35-induced memory deficits in the Y-maze, passive avoidance and water-maze tests, with the most active dose being 4 µg. The inhibitor prevented the Aβ25-35-induced oxidative stress, as revealed by measures of lipid peroxidation levels and reactive oxygen species accumulation, and abolished Aβ25-35-induced expression of pro-apoptotic markers. L41 prevented the Aβ25-35-induced decrease of AKT activation and increase of glycogen synthase kinase-3β (GSK-3β) activation, resulting in a decrease of Tau phosphorylation. Finally, L41 restored Aβ25-35-reduced levels of synaptic markers. The novel DYRK1A-preferential inhibitor L41 therefore prevented Aβ25-35-induced memory impairments and neurotoxicity in the mouse hippocampus. These in vivo data highlighted particularly DYRK1A as a major kinase involved in Aβ pathology and suggested therapeutic developments for DYRK1A inhibitors in AD.

Learning performances and vulnerability to amyloid toxicity in the butyrylcholinesterase knockout mouse

Butyrylcholinesterase (BChE) is an important enzyme for detoxication and metabolism of ester compounds. It also hydrolyzes the neurotransmitter acetylcholine (ACh) in pathological conditions and may play a role in Alzheimer's disease (AD). We here compared the learning ability and vulnerability to Aβ toxicity in male and female BChE knockout (KO) mice and their 129Sv wild-type (Wt) controls. Animals tested for place learning in the water-maze showed increased acquisition slopes and presence in the training quadrant during the probe test. An increased passive avoidance response was also observed for males. BChE KO mice therefore showed enhanced learning ability in spatial and non-spatial memory tests. Intracerebroventricular (ICV) injection of increasing doses of amyloid-β[25-35] (Aβ25-35) peptide oligomers resulted, in Wt mice, in learning and memory deficits, oxidative stress and decrease in ACh hippocampal content. In BChE KO mice, the Aβ25-35-induced deficit in place learning was attenuated in males and blocked in females. No change in lipid peroxidation or ACh levels was observed after Aβ25-35 treatment in male or female BChE KO mice. These data showed that the genetic invalidation of BChE in mice augmented learning capacities and lowered the vulnerability to Aβ toxicity.

Probing the interplay between amyloidogenic proteins and membranes using lipid monolayers and bilayers

Many degenerative diseases such as Alzheimer's and Parkinson's involve proteins that have a tendency to misfold and aggregate eventually forming amyloid fibers. This review describes the use of monolayers, bilayers, supported membranes, and vesicles as model systems that have helped elucidate the mechanisms and consequences of the interactions between amyloidogenic proteins and membranes. These are twofold: membranes favor the formation of amyloid structures and these induce damage in those membranes. We describe studies that show how interfaces, especially charged ones, favor amyloidogenic protein aggregation by several means. First, surfaces increase the effective protein concentration reducing a three-dimensional system to a two-dimensional one. Second, charged surfaces allow electrostatic interactions with the protein. Anionic lipids as well as rafts, rich in cholesterol and gangliosides, prove to play an especially important role. Finally, these amphipathic systems also offer a hydrophobic environment favoring conformational changes, oligomerization, and eventual formation of mature fibers. In addition, we examine several models for membrane permeabilization: protein pores, leakage induced by extraction of lipids, chaotic pores, and membrane tension, presenting illustrative examples of experimental evidence in support of these models. The picture that emerges from recent work is one where more than one mechanism is in play. Which mechanism prevails depends on the protein, its aggregation state, and the lipid environment in which the interactions occur.

The amyloid-cell membrane system. The interplay between the biophysical features of oligomers/fibrils and cell membrane defines amyloid toxicity

Amyloid cytotoxicity, structure and polymorphisms are themes of increasing importance. Present knowledge considers any peptide/protein able to undergo misfolding and aggregation generating intrinsically cytotoxic amyloids. It also describes growth and structure of amyloid fibrils and their possible disassembly, whereas reduced information is available on oligomer structure. Recent research has highlighted the importance of the environmental conditions as determinants of the amyloid polymorphisms and cytotoxicity. Another body of evidence describes chemical or biological surfaces as key sites of protein misfolding and aggregation or of interaction with amyloids and the resulting biochemical modifications inducing cell functional/viability impairment. In particular, the membrane lipid composition appears to modulate cell response to toxic amyloids, thus contributing to explain the variable vulnerability to the same amyloids of different cell types. Finally, a recent view describes amyloid toxicity as an emerging property dependent on a complex interplay between the biophysical features of early aggregates and the interacting cell membranes taken as a whole system.