(3.27)--阿尔茨海默病中的淀粉样蛋白毒性.pdf
Rev.Neurosci.2018;aopAllison B.Reiss*,Hirra A.Arain,Mark M.Stecker,Nicolle M.Siegart and Lora J.KasselmanAmyloid toxicity in Alzheimers diseaseAbstract:A major feature of Alzheimers disease(AD)pathology is the plaque composed of aggregated amyloid-(A)peptide.Although these plaques may have harmful properties,there is much evidence to impli-cate soluble oligomeric A as the primary noxious form.A oligomers can be generated both extracellularly and intracellularly.A is toxic to neurons in a myriad of ways.It can cause pore formation resulting in the leakage of ions,disruption of cellular calcium balance,and loss of membrane potential.It can promote apoptosis,cause synaptic loss,and disrupt the cytoskeleton.Current treat-ments for AD are limited and palliative.Much research and effort is being devoted to reducing A production as an approach to slowing or preventing the development of AD.A formation results from the amyloidogenic cleavage of human amyloid precursor protein(APP).Reconfigur-ing this process to disfavor amyloid generation might be possible through the reduction of APP or inhibition of enzymes that convert the precursor protein to amyloid.Keywords:amyloid oligomers;apolipoprotein E;mito-chondria;oxidative stress;synapse.IntroductionAlzheimers disease(AD)is a chronic neurodegenerative disorder and the most common cause of gradually pro-gressive cognitive and functional decline in the elderly,accounting for 6080%of all dementias(Barnes and Yaffe,2011).A growing population of older adults is expected to bring a rapid increase in the number of patients with AD.The toll on the quality of life is augmented by the lack of effective treatments.Brains of AD patients exhibit cortical atrophy result-ing from massive neuronal loss.As the major component of senile plaques,amyloid-(A)peptide is considered as a crucial factor underlying neuronal and synaptic dysfunction in the progression of AD.Among its destruc-tive actions,A disrupts synaptic plasticity,inhibits hippocampal long-term potentiation(LTP;a form of syn-aptic plasticity that is thought to underlie learning and memory),and induces the production of reactive oxygen species(ROS;Lustbader etal.,2004;Malenka and Bear,2004;Chang etal.,2006).This review focuses on the state of our rapidly evolv-ing knowledge of the various aspects of amyloid neu-rotoxicity.Amyloid species,cellular localization,and interaction with channels,receptors,and the synapse are all considered.We consider interactions between apolipo-proteins and amyloid as well as the role of microglia in amyloid clearance.Toxic amyloid:oligomers or mature fibrilsThe aggregation and accumulation of A peptide in the brain is a pathologic hallmark of AD.Amyloid monomers are derived from the larger amyloid precursor protein(APP)that is present on the surfaces of many cells,including neurons.In the pathway to A formation,APP undergoes proteolysis by-secretase,the-site APP cleaving enzyme 1(BACE1),and-secretase to form A fragments(Rossner etal.,2006;Figure 1).There are two major species of A in the brain:A42 and A40.Although A40 is several-fold more abundant than A42 in soluble form,A42 is the major constituent of amyloid plaques(Jan etal.,2008).The process of fibrillogenesis begins with amyloid monomers that assemble to form a variety of oligomeric species that aggregate further to form short,flexible,irreg-ular protofibrils then ultimately mature and elongate into insoluble fibrils characterized by a repeating substruc-ture of-strands oriented perpendicularly to the fiber axis.Once A aggregates extracellularly to form fibrils,it becomes resistant to proteolytic cleavage.A mono-mers exist in a dynamic equilibrium of a variety of con-formations with the-sheet form capable of aggregating to oligomers and higher structures.Initially,A40,or the even more aggregation-prone A42,misfolds then aggre-gates with other monomers to form unstable soluble A oligomers(Walsh and Selkoe,2007).Oligomeric amyloid is an intermediate step in the formation of mature fibrils.*Corresponding author:Allison B.Reiss,Winthrop Research Institute,NYU Winthrop Hospital,101 Mineola Boulevard,Mineola,NY 11501,USA,e-mail:AReissNYUWinthrop.orgHirra A.Arain,Mark M.Stecker,Nicolle M.Siegart and Lora J.Kasselman:Winthrop Research Institute,NYU Winthrop Hospital,101 Mineola Boulevard,Mineola,NY 11501,USABrought to you by|Kings College LondonAuthenticatedDownload Date|5/22/18 6:47 PM2A.B.Reiss etal.:Amyloid toxicity in Alzheimers diseaseElectron microscopy showed that the minimal portion of A peptide sufficient to form fibrils is the 1423 decapep-tide,with residues 1721 involved in-sheet formation(Tjernberg etal.,1999;Bu etal.,2007).Amyloid fibrils accumulate and deposit in extracellular plaques,a hall-mark of AD.The relative toxicity of fibrillar versus oligomeric amyloid is an area of controversy.The size of the soluble amyloid that is most damaging(dimers,trimers,or larger)is an area of active investigation(Bao etal.,2012).Research suggests some toxicity from each species,although a growing consensus has formed for the greater toxicity of amyloid oligomers(Cleary etal.,2005).There remains,however,some divide in the research community regard-ing the main type of amyloid that is toxic.Here,we will summarize studies on the toxicity of both physical confor-mations of amyloid in human and animal systems.Although monomeric A at physiologic concentrations appears to be nontoxic,multiple lines of evidence point to low molecular weight oligomers rather than fibrils as the primary neurotoxic agents in AD(Glabe,2008).Oligom-ers formed by A peptides may be most harmful because they interact with cell membranes,disrupting their integ-rity(Stefani,2010).Stphan etal.(2001)injected amyloid peptides into the rat dorsal dentate gyrus and found significant impairment in synaptic transmission,cogni-tive decline,and cell death only when an aggregation of amyloid occurred.The toxicity of A peptides correlated well with their propensity to aggregate.Monomeric A was not harmful to cells(Giuffrida etal.,2009).The microinjection of fibrillar A into the brains of aged rhesus monkeys has been found to cause a signifi-cant loss of neuronal cells and the activation of microglia at the site of injection(Geula etal.,1998).In a study of primate brain pathology,neuronal and axonal loss were observed in the vicinity of fibrillar plaques in the cere-bral cortex in aged rhesus monkeys and in humans with and without AD(Shah etal.,2010).Diffuse,nonfibrillar plaques were not associated with neuronal loss.Although the literature suggests that fibrillar amyloid has cytotoxic properties,numerous studies have demon-strated that fibril burden does not directly correlate with cognitive decline and neuronal death(Benilova et al.,2012).The disease severity of AD has been correlated IntracellularExtracellularA oligomerA monomer-Secretase cleavageAPPLipid raftCCCCCCCCCCCCCCBACE-1-Secretase complexCleavageFigure 1:Schematic of APP processing and amyloid formation in lipid rafts.APP generates a nonamyloidogenic product when cleaved by-secretase.The sequential cleavage by BACE1 and-secretase generates predominantly an A40 protein fragment,but-secretase is somewhat imprecise and can cleave at more than one site,creating a slightly longer form,A42,510%of the time.Both species tend to misfold and aggregate,generating oligomers,but A42 is the more fibrillo-genic.Oligomers aggregate to form protofibrils and fibrils,forming plaques.An equilibrium may exist between oligomers and fibrils in which fibrils can serve as a source for oligomers,leading to deleterious effects.The process of amyloid formation occurs on lipid rafts,regions of membrane bilayers containing distinct markers,increased cholesterol,sphingolipids,glycolipids,and others.APP is distributed between lipid and nonlipid raft membrane regions,whereas both BACE1 and-secretase subunits undergo posttranslational S-palmitoyla-tion,which targets them to lipid raft microdomains.High intracellular cholesterol(C)increases the partitioning of APP,-and-secretase into lipid rafts.Brought to you by|Kings College LondonAuthenticatedDownload Date|5/22/18 6:47 PMA.B.Reiss etal.:Amyloid toxicity in Alzheimers disease3with soluble but not aggregated A(McLean etal.,1999;Nslund et al.,2000).This finding shifted the focus of research to nonfibrillar forms of amyloid.Evidence sup-ports the primacy of oligomeric amyloid,a soluble inter-mediate stage in the formation of mature fibrillar amyloid,as the main source of toxicity(Kayed etal.,2003).High molecular weight soluble A extracted from human AD brain and injected into mouse brain is relatively inactive,but when dissociated into low molecular weight species(about 870kDa),the A becomes cytotoxic(Yang etal.,2017).The destabilization of larger soluble oligomers may promote AD.Large,insoluble deposits are also posited to function either as reservoirs of the bioactive oligomers or as a sequestration mechanism that renders them inert(Caughey and Lansbury,2003;Shahnawaz and Soto,2012).One potential explanation for the toxicity of oligom-ers focuses on the exposed hydrophobic amino acids that become concealed in the fibrillar amyloid.Exposed amino acids,particularly lysine and arginine,may be responsi-ble for electrostatic and hydrophobic interactions with cells that can be reduced by covering or modifying these amino acids(Yoshiike etal.,2007).It has been proposed that aggregated amyloid may be neuroprotective in that it sequesters the oligomers,shielding the cells from these amino acids(Sakono and Zako,2010).An alternative pos-sibility is that the toxic effects caused by fibrillar amyloid are due to the shedding of oligomers from fibrils injected into model organisms.Therefore,it may have been the oli-gomer,not the fibril,causing the toxicity(Tipping etal.,2015).A study comparing effects of fibrillar versus oligomeric amyloid infused in vivo into the left lateral ventricle of rats found that both caused a decline in spatial learning and memory,but the severity of impairment was greater in rats that received oligomers(He etal.,2012).Concomitantly,the rats given oligomers exhibited more neurodegenera-tion and a greater inflammatory response,manifested as the activation of Toll-like receptor 4 and tumor necrosis factor-(TNF-)in the hippocampus.The injection of oligomeric A into the lateral ventri-cle of adult cynomolgus macaque brains led to the accu-mulation in the entorhinal cortex,hippocampus(dentate gyrus),striatum,and amygdala with consequent induc-tion of phosphorylation,microglial activation,and synaptic loss.No aggregates of A were detected(Walsh etal.,2005;Forny-Germano etal.,2014).This supports the importance of oligomers over fibrils in AD in a primate model with a large brain that is able to perform complex behaviors.At this time,although oligomers seem to be the most directly toxic,the relative damage potential from each species warrants further study,particularly in light of the uncertain fluidity of conversion from one form to another.Amyloid cytotoxicity to the neuronal cell membraneAmyloid has been found to be located mainly on lipid rafts,microdomains of distinct lipid composition(mostly composted of sphingolipids and cholesterol)on cell mem-branes that are resistant to solubilization by nonionic detergents(Brown and London,2000).The amyloidogenic processing of APP takes place in lipid rafts because the key enzymes BACE1 and-secretase are located in these microdomains along with a reservoir of APP(Ehehalt et al.,2003;Figure 1).Specific lipid conditions within these rafts can accelerate APP processing to produce more amyloid and enhance aggregation and cell membrane interaction(Hattori etal.,2006).Amyloid fibrils aggregate mostly in the extracellular space around neuronal and glial cells.Lipid membranes can serve as nucleation sites for the accelerated assem-bly of fibrillar A aggregates(Matsuzaki,2007).It has been proposed that amyloid causes toxic effects through membrane interactions(Butterfield and Lashuel,2010).The mechanisms of toxicity may include the induction of membrane channels or pores,activation of calcium chan-nels such as N-methyl-D-aspartate(NMDA),membrane thinning,and/or membrane destabilization.APP and amyloid monomers and oligomers may exhibit detergent-like properties,destabilizing the membrane by removing lipid molecules(Sheikh et al.,2012;Gunn et al.,2016;Figure 2).One commonly recognized mechanism of amyloid toxicity is that amyloid peptides disrupt ionic homeosta-sis leading to uncontrolled Ca2+influx and thus elevated cytoplasmic Ca2+.As the healthy neuron at rest maintains a large Ca2+gradient between the extracellular space and the cytosol,the loss of this gradient is highly damaging and can lead to the depletion of vesicular transmitter stores(Parodi etal.,2010;Figure 2).The channel hypoth-esis explains Ca2+influx as resulting from the formation of a hollow structure in the membrane composed of small,annular A oligomers with a hydrophobic exterior and hydrophilic interior.These A peptides wedge and aggre-gate together within the membrane,span the bilayer,and create a pore that functions as a Ca2+-sensitive ion channel(Jang etal.,2007;Lal etal.,2007).It has been shown that amyloid peptides,mostly oligomers,do incor-porate into the lipid bilayer.This was visually depicted Brought to you by|Kings College LondonAuthenticatedDownload Date|5/22/18 6:47 PM4A.B.Reiss etal.:Amyloid toxicity in Alzheimers diseasethrough a pore-like morphology image in a high-resolu-tion atomic force imager(Jang etal.,2010).Pore forma-tion has also been evident in postmortem brain tissues of AD patients(Inoue,2008).This mechanism has further been supported by observations of pore-like activity,pre-dominantly an increase in the neuronal influx of Ca2+,in neurons(Seplveda,2014;Figure 2).Calcium dyshomeo-stasis resulting from pore formation could plausibly cause mitochondrial dysfunction,in turn inducing a cascade of ROS,leading to neuronal apoptosis and death(Dykens,1994;Fernndez-Morales etal.,2012).The relative impor-tance of amyloid pore formation is controversial,with researchers obtaining contradictory results regarding amyloid peptides correlating to an increased influx of calcium(Arispe etal.,2010;Lazzari etal.,2015).Despite the emphasis on prefibrillar oligomers as stated above,adverse effects of fibrillar amyloid are still of concern(Meyer-Luehmann etal.,2008).Soluble and fibrillar forms may work in concert or fibrillar amyloid may act as a reservoir of oligomers.In murine models,areas around plaques exhibit degenerative changes such as increased ROS(Garcia-Alloza etal.,2006).Shorter fibrils may be more likely to inflict damage on membranes than longer fibrils because they are more disruptive to lipid bilayers(Xue etal.,2009).When fibrils fragment,there is an increase in fibril load and more ends are available for interaction with membranes.Attempts to synthesize fragments of defined,uniform size ar
