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Visanji et al. Acta Neuropathologica Communications 2013, 1:2http://www.actaneurocomms.org/1/1/2

REVIEW Open Access

The prion hypothesis in Parkinson's disease: Braakto the futureNaomi P Visanji1, Patricia L Brooks2, Lili-Naz Hazrati2 and Anthony E Lang1*

Abstract

Parkinson’s disease (PD) is a progressive neurodegenerative disorder typified by the presence of intraneuronalinclusions containing aggregated alpha synuclein (αsyn). The progression of parkinsonian pathology and clinicalphenotype has been broadly demonstrated to follow a specific pattern, most notably described by Braak andcolleagues. In more recent times it has been hypothesized that αsyn itself may be a critical factor in mediatingtransmission of disease pathology from one brain area to another. Here we investigate the growing body ofevidence demonstrating the ability of αsyn to spread transcellularly and induce pathological aggregation affectingneurons by permissive templating and provide a critical analysis of some irregularities in the hypothesis that theprogression of PD pathology may be mediated by such a prion-like process. Finally we discuss some key questionsthat remain unanswered which are vital to determining the potential contribution of a prion-like process to thepathogenesis of PD.

Keywords: Prion, Parkinson's disease, Braak staging, Alpha synuclein

Braak staging of Parkinson’s diseaseBraak and colleagues devised their widely accepted stagingsystem of PD progression by assessing the regional distri-bution of αsyn immunoreactive structures in the brains of110 αsyn positive subjects (69 incidental and 41 clinicallydiagnosed PD patients). According to the Braak model,the disease process commences in the lower brainstem inthe dorsal motor nucleus of the vagus nerve (DMV), aswell as anterior olfactory structures. The disease thenascends rostrally from the DMV through susceptible re-gions of the medulla, pontine tegmentum, midbrain, andbasal forebrain, eventually reaching the cerebral cortex(Figure 1) [1]. This is a non-random, progressive process,with specific nuclei and neuronal types giving rise to thedevelopment of Lewy pathology in a stereotypic pattern.As the pathology advances upwards from the brainstem,both the severity of the lesions and the clinical manifesta-tions of the disease increase [2].

* Correspondence: [email protected] of Patient Based Clinical Research, Toronto Western ResearchInstitute, and the Edmond J. Safra Program in Parkinson’s Disease, TorontoWestern Hospital, McLaughlin Pavilion, 7th Floor Rm 7-403, 399 BathurstStreet, Toronto, Ontario M5T 2S8, CanadaFull list of author information is available at the end of the article

© 2013 Visanji et al.; licensee BioMed Central LCommons Attribution License (http://creativecreproduction in any medium, provided the or

The Braak model of PD staging hinges on the notionthat Lewy pathology is not random: vulnerable sites in thebrain are affected in a predictable topographic sequence(Figure 1). In support of this, Braak and colleagues havedemonstrated that the cell types in the central nervoussystem (CNS) exhibiting a propensity for developing Lewypathology share common features. Specifically, projectionneurons with a long, thin, unmyelinated or poorly myelin-ated axons are particularly susceptible to developinglesions [3,4]. In agreement with this, cortical and subcor-tical projection neurons giving rise to long, sturdy, heavilymyelinated axons appear to be resistant to pathology [3].According to the Braak model, it is this apparent selectivevulnerability of specific neuronal types that accounts forthe predictable regional spread of Lewy pathology ob-served in PD.Although the Braak model outlines a pathogenic process

that initiates in the brainstem and advances rostrally, it isunclear if the brainstem is the site of the initial diseaseprocess. Indeed it is known that PD extends beyond theboundaries of the CNS, affecting the peripheral nervoussystem (PNS), and in particular the enteric nervous system(ENS). This largely stems from the clinical observationthat PD patients frequently have a wide range of non-motor symptoms related to dysfunction of the PNS, which

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Figure 1 Staging of Lewy pathology according to the Braak model. Schematic summarizing the progression of Parkinson’s disease asproposed by Braak and colleagues [1]. According to the Braak model, αsyn deposits in specific brain regions and neuronal types giving rise toLewy pathology in a stereotypic, temporal pattern that ascends caudo-rostrally from the lower brainstem (including the dorsal motor nucleus ofthe vagus nerve in the medulla then the coeruleus-subcoeruleus complex, raphe nuclei, gigantocellular reticular nucleus in the medulla aandpons) through susceptible regions of the midbrain (substantia nigra and the pedunculopontine tegmental nucleus) and forebrain (e.g., amygdala)and into the cerebral cortex (e.g., anteromedial temporal mesocortex, cingulate cortex and later neocortical structures). It is hypothesized that thedisease initiates in the periphery, gaining access to the CNS through retrograde transport along projection neurons from the gastrointestinal tract.As the disease progresses, the severity of lesions in the susceptible regions increases.

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often precede the motor symptoms of PD. In support ofthis, Lewy pathology is found throughout the PNS in bothclinically diagnosed PD patients [5] as well as cases of clin-ically asymptomatic incidental Lewy body disease (ILBD)[6-8]. The finding of Lewy Bodies in the brains of asymp-tomatic individuals without parkinsonism (i.e., ILBD) is ofparticular interest as these cases are thought to representpresymptomatic PD in the very beginning stages of thedisease process [7].Braak and colleagues have put forth the notion that

the pathogenic process of PD begins when an environ-mental insult enters the body and subsequently gains ac-cess to the CNS, where it then spreads trans-synapticallyfrom one vulnerable brain region to the next [3,9]. This“dual-hit” hypothesis postulates that the unidentifiedneurotropic pathogen enters the brain through both anasal and a gastric route. The nasal route is used to ex-plain the early involvement of olfactory structures, aswell as the olfactory dysfunction that is common in earlyPD [3,9]. In support of this, Lewy pathology is found inboth the anterior olfactory nucleus as well as olfactorybulb mitral cells, the projection neurons that receive in-put from the olfactory epithelium [1,10].Despite the initial involvement of the olfactory system,

Braak et al. [3] do not consider olfactory structures tobe the point of departure for Lewy pathology to the rest

of the brain. Instead, they propose that the unknownpathogen gains access to the brain through the gastricsystem. The gastric route would fit with the early in-volvement of the ENS in PD. Like olfactory impairment,gastrointestinal dysfunction is common in PD andseems to emerge early in the disease course [11], oftenpreceding clinical parkinsonism by many years [12].Lewy pathology has long been known to occur inthe gastrointestinal tract of PD patients and is well-documented in early, mid- and late PD [13-17]. Its pres-ence in the premotor phase of the disease has beensupported by a recent study of three patients who werefound to have αsyn staining in bowel biopsy samplesobtained 2-5 years before they presented with signs ofPD; such staining was not seen in 23 healthy controls[16]. In accordance with the Braak model, the cells inthe ENS that are vulnerable to Lewy pathology share thekey characteristics of those vulnerable in the CNS (i.e.projection neurons with a long, thin, unmyelinatedaxon) [18]. If the ENS is the site of induction for a po-tential PD-causing agent, a pathogenic insult enteringthe body via the gastrointestinal tract must then gain ac-cess to the CNS. The visceromotor projection neuronsof the DMV give rise to preganglionic fibers that innerv-ate the ENS. Braak et al. [3] have proposed it is theseunmyelinated vagal preganglionic neurons that could


provide the route for a pathogen to be retrogradelytransported from the ENS into the CNS. This notion issupported by the observation that these vagal projectionneurons are some of the first cells in the brain to displayLewy pathology [3]. It may therefore be that a patho-genic agent enters the body and gains access to thegastrointestinal tract, invades vulnerable neurons in theENS and is subsequently retrogradely transported tothe CNS through vagal preganglionic fibers. Indeed, tworecent reviews have proposed a similar mechanism bywhich misfolded proteins in a host of neurodegenerativediseases may spread from cell-to-cell within the ENSeventually reaching the CNS [19,20]. Thus, within theENS, misfolded proteins, including αsyn, undergo spon-taneous glycation to form advanced glycation end prod-ucts (AGEs), which are excreted via exosomes. Once inthe extracellular space, AGEs are recognized by cellsurface receptors for advanced glycation end products(RAGEs) which act to internalize the misfolded AGE.AGE-RAGE binding activates multiple intracellular sig-naling pathways including a positive feedback loopwhereby there is an increased cell surface expression ofRAGE along with induction of oxidative stress and in-flammation. Through this positive feedback, there iscell-to-cell spreading of disease throughout the entericwall, and perhaps beyond. In the context of Parkinson’sdisease, it is proposed that once in the DMV, the patho-gen follows an ascending course, spreading from onesusceptible cell group to the next (Figure 1).

Parkinson’s disease as a prion disorderTransplanted dopamine neurons in humansThe Braak model puts forth evidence for the progressive,stereotypic spread of an unknown pathogenic insultfrom one affected brain region to the next. Building onthis, there has been considerable interest in recent yearsin the possibility that this disease progression is medi-ated in a prion-like fashion, with the spread and seedingof sequentially involved brain areas by misfolded pro-teins. The suggestion that PD may be a prion disorderstems from the observation of Lewy body pathology inembryonic dopamine neurons transplanted into the pu-tamen of human PD patients [21-24]. In four separatecase reports, autopsies performed 10-22 years post-transplantation found evidence of αsyn-positive, Lewy-body-like inclusions in grafted human dopamineneurons. Such inclusions are not normally seen in neuronsof such a young age. These inclusions stained positively forthioflavin-S (indicating the presence of β-sheets) and ubi-quitin and the affected transplants showed reduced dopa-mine transporter and tyrosine hydroxylase levels. Inconcert with the discussed evidence of a sequential stagingof PD pathology, these observations lead to the proposalthat the progression of PD may be mediated by a prion-

like process [25,26]. Furthermore, in contrast to Braak etal.’s thinking, the proposal was made that αsyn itself maybe the pathogenic factor underlying the spread of the dis-ease. Adding weight to this proposal, similar prion-likemechanisms have been proposed for other proteins in-volved in several other neurodegenerative diseases, includ-ing Alzheimer’s, Huntington’s and amyotrophic lateralsclerosis [27-29].

In vitro evidencePrions are composed of the PrPsc protein, a misfoldedform of endogenous PrPc, and underlie disorders suchas Creutzfeldt-Jakob disease, bovine spongiform enceph-alopathy and scrapie [30]. The conversion of alpha hel-ical PrPc to the β-sheet rich PrPsc confers an infectivitythat is the defining feature of a prion [31]. Indeed, as allother known infectious agents contain either DNA orRNA, the term prion, derived from the words protein-aceous and infectious, was given to this unique PrPscprotein [32]. Though the precise molecular mechanismsunderlying the propagation of the prion protein are un-known, it is universally accepted that PrPsc acts as atemplate upon which native PrPc is refolded into PrPsc.Extensive in vitro studies have shown that, when incu-bated at 37°C, monomeric αsyn forms fibrils reminis-cent of those contained in Lewy bodies in PD [33-35].Furthermore, existing fibrils can act as seeds, promot-ing fibrillization of surrounding monomeric αsyn [36].Closer inspection of the nature of the seed-induced fi-brils has revealed that addition of seeds produced fromαsyn bearing the A30P mutation to wildtype monomericαsyn leads to the generation of fibrils with the samecharacter as A30P fibrils [37]. Thus it would appear thatthe assembly of wildtype fibrils induced by A30P seedsinvolves a conformational change in wildtype αsyn tothat of A30P. This process is remarkably similar to thatdescribed for the templated conversion of PrPc to PrPsc.In the same study it was noted that the addition ofwildtype seeds to monomeric A30P αsyn did not involvea conversion to the wildtype fibril. This feature of tem-plated conversion seems to apply to the A30P αsyn mu-tation responsible for a small number of familial cases ofPD but not to the wildtype form of αsyn responsible forthe majority of cases of PD. Thus, in the context of thisreview regarding PD we would define a prion-like mech-anism of disease propagation as a mechanism by whichnative protein with rich α-helical structure is refoldedinto a toxic form with high β-sheet structure trigger-ing the misfolding of further native protein in a self-perpetuating process. A form of the protein capable oftriggering this process can be conferred from one cell toanother, thus inducing progressive neurodegeneration.Beyond the test tube, a series of in vitro tissue culture

experiments have demonstrated the following features


that make αsyn an attractive candidate as the pathogenicfactor implicated in the prion-like spread of PD path-ology: 1) Fibrillar αsyn has been shown to be transportedvia axonal transport [38]. 2) Monomeric and aggregatedforms of αsyn, contained in cytosolic vesicles, are re-leased from neurons by an unconventional exocytoticmechanism [39]. 3) Aggregated forms of αsyn (fibrillarand non-fibrillar oligomers) are taken up into neuronsvia conventional endocytosis, whereby they can movethrough the endosomal pathway and are eventuallydegraded by the lysosome [40]. 4) Transduction ofpreformed fibrillar αsyn into cells overexpressing αsynresults in the formation of insoluble intracellular inclu-sions recapitulating several key features of Lewy bodiesincluding size, subcellular localisation, β-pleated sheetconformation, and hyperphosphorylation and polyu-biquitination of constituent αsyn [41]. Furthermore,using C-Myc tagged preformed αsyn fibrils, it has beendemonstrated that soluble endogenous αsyn becomesrecruited to these insoluble intracellular inclusions even-tually comprising the major component [41]. Severalother studies have employed co-cultures in a wide var-iety of cell lines to demonstrate the ability of exogenousαsyn, be it monomeric, oligomeric or fibrillar, to infiltrate

Exosomerelease

Cell injuryand leaking

Transynaptic

5

6

1

Nanotube

Figure 2 Potential mechanisms of neuron-to-neuron transmission ofvia (1) leakage from injured cells with compromised membrane integrity. Eand gain access to neighboring neurons (2). Synuclein can also be transmiSynuclein can be packaged into exosomes which are released and taken uconnection between two cells potentially allowing synuclein to transfer freby direct synaptic contact (6).

surrounding cells and induce a pathological aggregativeresponse [42,43]. These cellular mechanisms, by whichαsyn can be both released from and taken up by neurons,are illustrated in Figure 2. Thus, at the in vitro level thereis compelling evidence that αsyn is capable of spreadingan aggregation-inducing disease-like pathology from oneneuron to another.

In vivo studiesMore recent studies have investigated the translation ofthese findings in vivo. Desplats et al. grafted green fluor-escent protein (GFP) labelled mouse cortical neuronalstem cells (MCNSCs) into the hippocampus of miceoverexpressing human αsyn [43]. Using double immuno-fluorescence, these authors demonstrated that sevendays post grafting ~2.5% of MCNSCs had taken up αsynfrom the host tissue; 28 days post grafting this numberhad risen to ~15%. Although these data demonstratehost to graft transmission of αsyn in vivo, ultrastructuralanalysis of inclusion bodies within grafted cells found noevidence of the fibril or aggregate formation followinginfiltration by αsyn that was evident in their in vitrostudies. A more recent study mimicked more closely theclinical situation grafting murine mesencephalic neurons

Transmembrane2

4

Endocytosis &Exocytosis 3

synuclein. Alpha synuclein can be released into the extracellular spacextracellular synuclein could then directly translocate the cell membranetted from cell-to-cell via conventional exocytosis and endocytosis (3).p by surrounding cells (4).Tunneling nanotubes can form a directely from one cell to another (5). Finally, synuclein could be transmitted


into the striatum of mice overexpressing human αsyn[42]. Six months post grafting a small number of graftedcells were found to contain small human (i.e., host) αsynpositive punctae. These authors also demostrated thatthe structural form of αsyn had no effect on the abilityfor cellular uptake in vivo. Thus monomeric, oligomericand fibrillar recombinant αsyn were all found in neur-onal cells following a stereotaxic injection in rats. Inter-estingly, this phenomenon of αsyn uptake is not limitedto neurons, but has also been demonstrated in astrocytesboth in cell culture and in vivo, whereupon it triggers aninflammatory response in the recipient cells [44]. As in-flammation is increasingly implicated in a host of neuro-degenerative diseases, this process may represent analternative means by which αsyn can mediate the pa-thological progression of PD. A recent study using atransgenic mouse model of synucleinopathy (TgM83),demonstrated that inoculation of young asymptomaticmice with brain homogenates prepared from oldersymptomatic mice accelerated the presence of both αsynhyperphosphorylated at serine 129 and aggregated αsynand significantly decreased survival time [45]. Further-more, this hastened pathology was absent in αsyn knock-out animals inoculated with the same brain homogenatesprepared from older symtomatic transgenic mice. Thissuggests a strong role for endogenous host αsyn in thetransmission of pathology from affected to unaffected sitesand is consistent with a prion-like mechanism of diseasepropagation.In the aforementioned studies there was no evidence

of a spread of pathologic αsyn to more widespread areasof the brain distal to the sites of application. However,two recent studies have expanded upon this. Angotet al., (2012) used a viral vector (AAV2/6-hu αsyn) toengineer rat nigral neurons to express human αsynwhich, over time, was transported from nigral cell bodiesto the terminals in the striatum [46]. These authors thengrafted rat ventral mesencephalic neurons into thestriatum and observed transmission of human αsynexpressed by the virally transduced nigrostriatal cellsinto the grafted tissue. Indeed, the extent of transfer ofthe virally expressed human αsyn was shown to be of amuch greater magnitude than that observed in previousstudies examining the transfer of αsyn into grafted tis-sue. Furthermore, human αsyn immunoreactivity withinthe grafted cells was shown to be surrounded by astrong immunoreactivity for rodent αsyn, suggesting thatthe human αsyn supplied by the host tissue had induceda seeding effect in the rodent αsyn present in the graftedcells. This study demonstrated for the first time thatαsyn can induce a seeding effect in vivo following axonaltransport along the nigrostriatal pathway. However, amore recent study by Luk et al [47] has demonstratedthat this phenomenon can be even more widespread. In

this study, brain homogenates from aged transgenicmice, containing significant levels of aggregated A53Thuman αsyn (the first mutation of αsyn discovered inhumans), were injected into the neocortex and striatumof younger animals long before pathology normally de-velops. 30 days post injection some level of αsyn path-ology was evident in the vicinity of the injection sites.However, 90 days post injection αsyn pathology, reminis-cent of Lewy bodies and surrounded by astrogliosis andmicrogliosis, was widespread throughout the brain.These inclusions were not apparent in age-matched con-trol animals innoculated with saline or with homoge-nates prepared from younger animals. The critical roleof host αsyn to this process was also demonstrated bythe observed lack of pathology following innoculation inαsyn null mice. In addition, detailed examination dem-onstrated a recruitment of endogenous murine αsyn tothe pathological inclusions further supporting a permis-sive templating prion-like process. Mapping studies illus-trated that it was those brain regions with the mostneuronal connections to the sites of innoculation thatdisplayed the most severe pathology (frontal cortex andthalamus) and that there was a relative sparing of re-gions close by but lacking direct neuronal innervation.Indeed, based on evidence from their studies the authorssuggest that the distribution of αsyn pathology followinginnoculation is suggestive of a trans-synaptic spread.This contrasts with the recent in vitro findings ofFreundt et al (2012), who found that axon to somatransfer of αsyn did not require synaptic contacts [38].Although there is now considerable in vivo evidence of

cell-to-cell transmission of αsyn and recruitment of en-dogenous αsyn to inclusions reminiscent of Lewy bodies,a crucial outstanding issue is establishing whether thereis a link between this and the process of neurode-generation. A recent elegant study has suggested acausative link between progressive Lewy inclusion for-mation and neurodegeneration of nigral dopaminergiccells with correspoding deficit in striatal dopamine andmotor function [48]. The authors demonstrated a cleartime and connectivity dependent spread of Lewy body-like pathology following intrastriatal injection of fibrillarαsyn in mice, with expression seen earliest in those brainregions highly innervated by the striatal injection site(cortical layers IV and V, and the olfactory bulb) and lat-ter spread to more distally connected areas (ventral stri-atum, thalamus, occipital cortex, and commisural andbrainstem fibers). Pathology was particularly abundantin the dopaminergic cells of the substantia nigra parscompacta. Intriguingly, at 30 days post injection, ap-proximately 30% of nigral cells contained αsyn-positiveinclusions, which was accompanied by negligible cellloss. At 100 days post injection, both the number of cellsbearing αsyn-positive inclusions, the total number of


nigral cells and striatal levels of dopamine, declinedconcommittantly. These data strongly suggest that ag-gregation of αsyn may directly precipitate the loss ofnigral dopaminergic neurons that is a fundamental char-acteristic of PD.

Challenges to reconciling Braak staging with a prion-likespread of pathology in PDClearly there is increasing evidence, both experimentaland clinical, demonstrating that exogenously appliedαsyn can infiltrate surrounding cells and initiate a PD-like pathological response. However, it remains to beshown what initiates this process and whether it can ac-count for disease progression in the human brain. Herewe evaluate the prion hypothesis of PD progression tak-ing into account the current understanding of theneuroanatomy and pathology of PD and highlight somecritical remaining questions that need to be addressedbefore accepting PD as a prion disorder.The conserved propagation patterns of αsyn pathology

described by the Braak model could indicate a prion-likespread of αsyn. According to Braak, vulnerable brain re-gions are affected in a predictable sequence, progressingin a stereotypical caudal-rostral pattern starting in thelower brainstem. One could hypothesize that pathogenicαsyn is transported to anatomically connected brain re-gions along the prescribed route of progression, seedingαsyn aggregation in each vulnerable region encountered.Whether the pattern and progression of pathology canbe generalized to the predictable, sequential involvementof vulnerable sites as Braak describes, however, remainscontroversial. The Braak model postulates that Lewypathology in the lower brainstem is necessary for thelater appearance of Lewy pathology in more rostralstructures [1]. However, this may be an artifact of theirpre-selection of cases on the basis of involvement ofDMV. Multiple studies have challenged the Braak sta-ging scheme, reporting cases with inclusions throughoutthe brain but with preservation of medullary nuclei[8,49-51]. In fact, recent reports suggest that the Braaksystem fails to classify upwards of 50% of αsyn immuno-reactive cases [50,52,53]. The pathological regional het-erogeneity between PD cases suggests Braak’s proposedpathway is not the only possible route of spread andpathology may even emerge simultaneously in multiplesubcortical and cortical regions. Such cell autonomousprotein misfolding would argue against a Braak model ofPD staging. One might expect that a cell autonomousprocess would better explain genetic causes of PD asso-ciated with Lewy bodies. However, that the site of initialemergence of disease may well be variable and perhapsmultiple (possibly within certain anatomical constraints)leading to a heterogeneous phenotype, does not precludethat a prion-like mechanism still might govern spread of

disease through the brain from these multiple and varieddeparture points.Challenges also exist for the proposed “gut to brain”

spread of Lewy pathology. Rather than suggesting initialinvolvement of the ENS with subsequent spread to theDMV, Beach et al (2010) have advocated a "brain to gut"direction of transmission given their findings of a rostro-caudal gradient of phosphorylated αsyn in the GI tractand the known richest vagal innervation of the uppergut [5]. The recent report of Del Tredici and Braak(2012) showing evidence for involvement of the spinalcord only after involvement of the brain also raises ques-tion about the earliest involvement of at least some ENSregions [54].Another prerequisite of the Braak model is that the

severity of the lesions in the affected brain regions willincrease as the disease progresses [1]. It would thereforebe expected that the DMV, Braak’s point of departurefor Lewy pathology in the brain, would have more se-vere pathology than the rostral regions said to be af-fected later in the disease. This, however, is not true inall cases. In one study, upwards of 65% of cases hadαsyn loads in the substantia nigra and locus coeruleusthat were roughly equivalent to that found in the DMV[55]. Further still, there are reports of severe involve-ment of the substantia nigra without any prominentDMV pathology [56,57]. This lack of significant αsynpathology in the DMV compared to more rostral struc-tures cannot be explained by neuronal loss, as thesecases had well preserved neurons. For the Braak-prionhypothesis to account for these cases, pathogenic αsynwould have to spread and seed very rapidly to all vulner-able brain regions once the disease process is initiated,which does not fit with the protracted disease course ofidiopathic PD. Alternatively, mechanisms underlyingseeding and subsequent aggregation and cell death maybe dissociated.One possible explanation for the inconsistencies be-

tween the Braak model and the conflicting reports maybe that spreading of pathology can occur in both an an-terograde and retrograde direction. Braak and colleaguesinitially hypothesized that while the olfactory system isaffected very early in the disease process, olfactory struc-tures are not the point of departure for Lewy pathologyto the rest of the brain, despite connections with corticaland subcortical regions [1]. Subsequent observationsfrom a growing number of groups, including Braak andcolleagues, however, have suggested that Lewy pathologymay indeed progress along olfactory pathways [9,58-60].Lerner and Bagic [26] subsequently expanded upon thisby proposing a complementary hypothesis to the Braakmodel, in which they emphasize the possibility of an-terograde spread of the pathogen through the olfactorytubercle and bulb to the brainstem, in addition to the


retrograde spread from the dorsal motor nucleus of thevagus, as described by Braak et al. [1]. The addition ofthis olfactory route of progression may reconcile somediscrepancies with the original Braak model. Although,the bi-directional spread of a pathogen may be at oddswith the Braak staging scheme, it is not inconsistentwith the concept of neuronal spread of a pathogen in aprion-like fashion. Indeed, Lerner and Bagic suggestedsuch a prion-like mechanism may underlie the neurode-generative process in PD [26].A further explanation for the reported inconsistencies

with the Braak model may be the clinical diversity ofPD. Different subtypes of PD may have different under-lying pathological patterns. In support of this, Hallidayet al. [61] found that PD patients with a younger age ofonset and a long clinical course had pathology that fitwith the Braak model, whereas patients with older onsetand shorter disease duration did not. This suggests thatclinical phenotype may predict the pattern of pathologythat underlies PD. The fact that Braak and colleaguesonly selected cases in which the DMV was positive forαsyn immunoreactivity may account for some of theseapparent discrepancies, as they may have only examineda select subtype of PD. More recent large-scale neuro-pathological studies tend to include all cases that arepositive for αsyn immunoreactivity, regardless of whichbrain regions were affected, and as a result may includea more diverse profile of PD and its subtypes. It maytherefore be that a prion-like spread of αsyn in a patternpredicted by the Braak model does indeed occur in PDbut only in certain subtypes of the disease.A key outstanding issue when considering using Lewy

body pathology to define the progression of PD is regard-ing whether the Lewy body itself is harmful or protective.Traditionally, the presence of Lewy bodies in sites proneto neurodegeneration has supported the dogma that theLewy body is detrimental to neuronal survival. However,in recent years many studies have suggested that Lewybody formation is a protective cellular mechanism to se-quester toxic αsyn aggregates and even aid their degrad-ation by an aggresome-like mechanism [62-65]. In somecases cells containing Lewy bodies have been described ashaving a healthier morphology than neighboring Lewybody negative cells [66]. Furthermore, in vitro studies havedemonstrated that cell death can precede Lewy body for-mation [67] and in PD it has been shown that the majorityof cells undergoing apoptosis do not contain Lewy bodies[68]. As the Lewy body may in fact represent a cell that isresisting neurodegeneration, caution must be used inemploying it to define disease progression. Indeed, a recentstudy examining the relationship between nigral cell loss,distribution of Lewy bodies and disease duration in PDfound no evidence to support a correlation between Lewybody distribution or density and neurodegeneration [69].

It is vital to keep this issue in mind when also consid-ering the selective susceptibility of certain cell types andspecific brain regions to Lewy pathology. While theBraak model describes projection neurons with a long,thin, unmyelinated axons as the cell type that is proneto Lewy pathology [3,4], it is not known why these neu-rons are susceptible. Although many explanations couldbe provided to account for differences between the hu-man disease and animal models, it is interesting to notethat the recent important study by Luk et al. [47], whichprovides some of the strongest evidence to date fortranscellular propagation of αsyn pathology, demon-strated most marked involvement of heavily myelinatedprojections, including the internal capsule. Furthermore,it is not clear why some regions with Braak’s “vulner-able” cell types are affected whereas many other anatom-ically connected/neighboring regions with similar celltypes are not. Such cryptic susceptibility is furtherhighlighted in Braak’s detailed description of a non-geographical pattern of Lewy pathology in specific sub-cortical structures in earlier disease versus a morediffuse spreading throughout the cortex in later stages[1]. It therefore seems that αsyn propagates differentlyaccording to the type of brain tissue it encounters; how-ever, it is not clear how or why this would be the case.To further complicate matters, according to the Braak

model, the neuronal type prone to developing Lewypathology in PD is specific to PD and distinguishes PDfrom other synucleinopathies [3]. Therefore, the prion-like spread of αsyn must discriminate between cell typesacross different synucleinopathies. Furthermore, whileabnormal aggregation of αsyn is the dominant patho-logical hallmark of synucleinopathies, the nature of αsynaggregation is distinct between different disorders. Forexample, PD, Parkinson’s disease dementia (PDD) anddementia with Lewy bodies (DLB) are characterized byαsyn deposits in neuronal Lewy bodies and Lewyneurites, whereas multiple system atrophy (MSA) is de-fined by abnormal filamentous deposition of αsyn in thenuclei and cytoplasm of both oligodendrocytes andneurons [70-72]. If prion-like αsyn seeding plays an im-portant role in the pathogenesis of neurodegenerativedisease then how and why αsyn behaves differently indifferent diseases must be addressed (see below). In thiscontext, using the Lewy body as a structural marker forthe progression of the disease may not be appropriate.For example Kramer et al., have demonstrated that, incontrast to the relatively small number of cortical Lewybodies, there are a number of small presynaptic αsyn ag-gregates in DLB, which may account for the significantcognitive impairment in this disease [63]. Similarly,Milber et al found evidence of neuronal dysfunction pre-ceding Lewy pathology in the nigra in PD [73]. Thesedata suggest that synaptic dysfunction can occur without


the presence of Lewy bodies per se and that perhapsusing the Lewy body alone as a marker of pathologicalprogression is not sufficient to predict the spread ofdisease.Finally, if the progression of PD can be explained by a

Braak-prion-like spread, a critical question that must beaddressed is what initiates the process in the first place?Braak’s dual-hit hypothesis postulates that an environ-mental neurotropic insult is responsible [3,9]. While theBraak model proposes a viral pathogen, a recent study inmice demonstrated that a peripherally administered en-vironmental toxin (i.e. rotenone) may induce a Braak-like spread of αsyn pathology in the CNS [74]. Althoughthis has not been shown in the human condition, suchresults do support the hypothesis of an environmentalinsult triggering the Braak’s cascade of pathology seen inPD. What is not clear, however, is how an environmentalinsult could induce the involvement of systems outsideof Braak’s continuous chain of projection neurons thatare presumably isolated from the environment. Perhapsone of the most challenging facts to reconcile with thistheory is the very early involvement of the cardiac sym-pathetic nerves seen in PD and in the presumed pre-clinical state of incidental Lewy body disease [75].

Other challenges to the prion hypothesisIn addition to the issues outlined above, there are criticalquestions in the defining of PD as a prion disorder thatmust be addressed. One substantial issue is indeed se-mantic. As the word prion itself derives from the words“protein” and “infectious”, to define PD as a prion dis-order mediated by αsyn, we must demonstrate that αsynis capable of “infectivity”. It has been suggested that, dueto an apparent lack of microbiological transmissibilityand propagation within communities, all amyloidogenicproteins capable of permissive templating be termed“prionoids” [27]. It remains possible that difficulties inverifying an infectious nature of αsyn may be due to thelength of time required for the process to occur in vivo.Indeed in human post mortem studies, Lewy-like path-ology was not observed in grafted tissue 18 months postgraft [76]. Accumulation was apparent four years postgraft, but not until 10-14 years post graft was full Lewypathology evident [21]. There are likely several featuresof both the host and the "infectious" agent that influencethe cell to cell transfer, effectiveness of permissive tem-plating, and the distribution and nature of the resultingpathology. For example, in classical prion diseases, thebiochemical nature of the prion protein PrPsc and thegenetic makeup of the host (M/V at codon 129) havemajor influences on the nature and distribution of theunderlying pathology (and as a consequence the clinicalmanifestations). The same types of processes may influ-ence the distribution of Lewy body pathology in PD and

even determine whether the final disease presentation isPD or that of another synucleinopathy. Furthermore, ifindeed αsyn is responsible for a prion-like propagationof the disease, understanding the relatively delayedpropagation of αsyn pathology post symptom onset, ascompared to the rapid process of conventional prion dis-orders once symptomatic, could inform the developmentof therapies to slow the progression of both PD andprion diseases.We have reviewed considerable experimental evidence

supporting the premise that αsyn can be released fromand enter neighbouring neurons, and induce aggregationof native αsyn in vitro. However, there are some caveats tothese experimental settings that necessitate caution in ex-trapolating these data to the human brain in PD. Firstly,not all tissue culture studies find that supplying exogenousαsyn in the surrounding media is sufficient for infiltrationinto cells. Indeed in order to get meaningful internalisa-tion Luk and colleagues had to make use of cationic lipo-somes to facilitate entry of αsyn into the cell [41]. Afurther caution is required in interpreting the significanceof data demonstrating the transmission of αsyn in animalstudies employing mice genetically engineered to over-express αsyn, thus presumably enhancing any host-to-graft transmission, although more recent workdemonstrates similar changes in non-transgenic animals[48]. Furthermore, the applicability of animal studiesemploying direct stereotaxic application of αsyn to thebrain to the initiation and propagation of idiopathic PDremains to be shown. Although the recent results of Lukand colleagues in non-transgenic animals [48] are impres-sive and very promising, it must be admitted that the rele-vance of using very high concentrations of preformedartificial αsyn fibrils to human PD is unknown. Thus, fur-ther studies are required to demonstrate unequivocallythat enough αsyn can be released and infiltrate surround-ing tissues, by physiological means, to induce aggregationin vivo. Secondly, there is controversy in the literature re-garding the structural assembly of αsyn required to induceinclusion formation. In the many studies employingco-culture and those stereotaxically implanting cellsoverexpressing αsyn, the structural nature of the infiltrat-ing αsyn is unclear [42,77]. Tissue culture studies haveshown that monomeric, oligomeric and fibrillar αsyn canbe taken up by cells [42]. Thus, in co-culture experiments,and those stereotaxically implanting cells overexpressingαsyn, it could be any of these structural forms leading tothe aggregate formation within the host. It should also beemphasized that much of this discussion hinges on theconcept that the insoluble fibrillar protein aggregates areresponsible for cell death, whereas there is increasing evi-dence implicating soluble oligomers as potential culpritsin the pathogenesis of neurodegenerative diseases such asPD [78]. Thirdly, the degradation of different assemblies


of internalised αsyn requires further evaluation since somecell culture studies suggest that exogenously appliedmonomeric αsyn is quickly degraded by the lysosomewhereas fibrillar αsyn endures [41]. Conversely, other cellculture studies have found that oligomeric αsyn is de-graded by the lysosome, and that monomeric αsyn rapidlyand directly translocates to cellular membranes thusresisting lysosomal degradation and is therefore the morelikely disease spreading form [40]. Finally, although the re-cent studies by Angot et al and Luk et al. [46,47] havedemonstrated a putative trans-synaptic spread of αsynpathology in vivo, crucially, the transfer of αsyn throughthe neuroanatomical pathways implied in Braak’s stagingscheme has not yet been shown. Indeed in the study ofLuk et al. [47], stereotaxically applied αsyn was demon-strated to travel in both an anterograde and retrogrademanner, being evident in both the thalamus and thesubstantia nigra pars compacta following a striatal applica-tion in mice. As mentioned above, such a bidirectionalspread seems at odds with Braak staging in PD, althoughdoes not argue against a prion-like disease mechanism.Future in vitro studies should be performed to investigatethe ability of αsyn applied in the ENS or olfactory bullb tospread along the pathways described by Braak staging.The occurrence of Lewy bodies in transplanted fetal

nigral neurons in patients with PD has been a strongincentive to consider a prion-like spread of pathology.However, not all case reports find pathology in graftedtissue [79]. In those that do, Lewy bodies are present“within a minority” of grafted cells [22] although itshould be noted that the proportion of transplantedneurons reported to contain Lewy bodies is almostidentical to the proportion of Lewy body positive neu-rons in the substantia nigra in Parkinson’s disease. Des-pite this, it must be acknowledged that Lewy bodiescontain much more than aggregated αsyn and occur inmany disorders in which αsyn is not believed to be aprimary contributor to the overall pathogenesis. Amore plausible explanation may be that multiple othertoxic factors that are known to exist in the PD brain,such as proinflammatory cytokines, reactive oxygenspecies (supported by the extent of neuromelanizationuncharacteristic of such young neurons [21]) and pro-apoptotic factors, may damage the cell leading to a dis-ruption of cellular processes and explain the presenceof Lewy bodies in previously healthy grafted dopamin-ergic cells. This is not to say that αsyn cannot be impli-cated in such alternate cell death mechanisms. Forexample, as previously discussed, αsyn can be taken upinto microglia and induce the production and excretionof inflammatory mediators.A final challenge to the prion-like propagation of αsyn

pathology accounting for PD is the observation thatLewy pathology is not necessary for nigral degeneration

and the clinical presence of parkinsonism. While patho-logical studies on genetic forms of PD are limited, it isclear that at least some of these patients do not showclassic Lewy pathology. Indeed, in the case of LRRK2mutations even within a single family, a spectrum ofpathology has been observed among family memberswith manifesting PD, ranging from those completelylacking any synuclein pathology, to those with signifi-cant αsyn aggregation [80-84]. Furthermore a recentstudy comparing neuronal dysfunction in relation toBraak stage in patients with PD, individuals with ILBDand controls has demonstrated that both cellular dys-function (loss of tyrosine hydroxylase) and neuro-degeneration can precede Lewy body pathology in ILBD[73]. These findings are in direct contrast to those ofLuk and colleagues [47] who suggest quite the reverse,that synuclein pathology precedes cell loss. Clearly, fur-ther studies will need to be performed to address theseconcerns.

ConclusionsRecent years have seen a tremendous surge of interest inthe possibility that neurodegenerative diseases, includingPD, could develop and progress via non-cell autono-mous means, spreading by transcellular mechanismswith seeding and subsequent permissive templating, in aprion-like fashion. If these mechanisms are indeed inte-gral to the pathogenesis of PD and other diseases, thiscould have important therapeutic implications, as re-cently discussed in more detail elsewhere [85,86]. Thesepossibilities include treatments designed to block the re-lease or uptake of the pathogenic protein, increase itsextracellular clearance by microglia, inhibit protein as-sembly by, for example, nanomaterials [87] and increasedegradation of aggregates, for example, by lysosomes.Currently available drugs may have additional inhibitoryeffects on the endocytosis mechanism that mediates theinternalization of αsyn oligomers [88]. Another approachthat could be available for clinical study in the near fu-ture might be the use of monoclonal antibodies orimmunization therapy, which might be expected to havegreater potential for benefit if there is indeed an extra-cellular phase of αsyn transmission. In light of the factthat there is currently no available therapy to slow orhalt the pathological progression of PD, the potentialtherapeutic implications of a therapy targeted at themechanism responsible for the transmission of path-ology from one region to another are formidable. Thus,it is vital that further in vitro and in vivo studies areperformed to validate the potential involvement of αsynas a prion-like factor and tease out the mechanisms bywhich αsyn may be responsible for disease progressionin the human condition. However, in view of the unre-solved challenges highlighted in this review, caution


should be taken in uncritically accepting a role for aprion-like process, particularly in all cases of PD. Indeed,the mechanisms contributing to the progression of thedisease may be as variable as the disease itself.

AbbreviationsPD: Parkinson’s disease; αsyn: Alpha synuclein; DMV: Dorsal motor nucleus ofthe vagus nerve; CNS: Central nervous system; PNS: Peripheral nervoussystem; ENS: Enteric nervous system; AGEs: Advanced glycation endproducts; RAGEs: Receptors for advanced glycation end products; GFP: Greenfluorescent protein; MCNSCs: Mouse cortical neuronal stem cells;ILBD: Incidental Lewy body disease; PDD: Parkinson’s disease dementia;DLB: Dementia with Lewy bodies.

Competing interestsNPV, PLB & L-NH declare no conflicts of interest. AEL has served as anadvisor for Abbott, Allon Therapeutics, Astra Zenica, Avanir Pharmaceuticals,Biovail, Boerhinger-Ingelheim, BMS Cephalon, Ceregene, Eisai, GSK, LundbeckA/S, Medtronic, Merck Serono, MSD, Novartis, Santhera, Solvay, and Teva;received grants from Canadian Institutes of Health Research, DystoniaMedical Research Foundation, Michael J. Fox Foundation, National ParkinsonFoundation, Parkinson Society of Canada, and Ontario Problem GamblingResearch Centre; received publishing royalties from Saunders, Wiley-Blackwell,Johns Hopkins Press, and Cambridge University Press; and has served as anexpert witness in cases related to the welding industry.

Authors’ contributionsNPV & PLB carried out literature searches, assisted in generation of Figuresand writing of the manuscript. AEL & L-NH assisted in writing of themanuscript. All authors read and approved the final manuscript.

Author details1Division of Patient Based Clinical Research, Toronto Western ResearchInstitute, and the Edmond J. Safra Program in Parkinson’s Disease, TorontoWestern Hospital, McLaughlin Pavilion, 7th Floor Rm 7-403, 399 BathurstStreet, Toronto, Ontario M5T 2S8, Canada. 2Tanz Centre for Research inNeurodegenerative Diseases, University of Toronto, 6 Queens Park CrescentWest, Toronto, Ontario M5S 3H2, Canada.

Received: 21 February 2013 Accepted: 22 February 2013Published: 8 May 2013

References1. Braak H, Del Tredici K, Rub U, de Vos RAI, Steur E, Braak E: Staging of brain

pathology related to sporadic Parkinson's disease. Neurobiol Aging 2003,24:197–211.

2. Braak H, Rub U, Jansen Steur EN, Del Tredici K, de Vos RA: Cognitive statuscorrelates with neuropathologic stage in Parkinson disease. Neurology2005, 64:1404–1410.

3. Braak H, Rub U, Gai WP, Del Tredici K: Idiopathic Parkinson's disease:possible routes by which vulnerable neuronal types may be subject toneuroinvasion by an unknown pathogen. Journal of Neural Transmission2003, 110:517–536.

4. Braak H, Braak E: Pathoanatomy of Parkinson's disease. Journal ofNeurology 2000, 247:3–10.

5. Beach TG, Adler CH, Sue LI, Vedders L, Lue L, White Iii CL, Akiyama H,Caviness JN, Shill HA, Sabbagh MN, Walker DG: Multi-organ distribution ofphosphorylated alpha-synuclein histopathology in subjects with Lewybody disorders. Acta Neuropathol 2010, 119:689–702.

6. Bloch A, Probst A, Bissig H, Adams H, Tolnay M: Alpha-synuclein pathologyof the spinal and peripheral autonomic nervous system in neurologicallyunimpaired elderly subjects. Neuropathol Appl Neurobiol 2006, 32:284–295.

7. Dickson DW, Fujishiro H, DelleDonne A, Menke J, Ahmed Z, Klos KJ, JosephsKA, Frigerio R, Burnett M, Parisi JE, Ahlskog JE: Evidence that incidentalLewy body disease is pre-symptomatic Parkinson's disease.Acta Neuropathol 2008, 115:437–444.

8. Parkkinen L, Kauppinen T, Pirttila T, Autere JM, Alafuzoff I: alpha-Synucleinpathology does not predict extrapyramidal symptoms or dementia.Ann Neurol 2005, 57:82–91.

9. Hawkes CH, Del Tredici K, Braak H: Parkinson's disease: a dual-hithypothesis. Neuropathol Appl Neurobiol 2007, 33:599–614.

10. Daniel SE, Hawkes CH: Preliminary diagnosis of Parkinson's disease byolfactory bulb pathology. Lancet 1992, 340:186–186.

11. Pfeiffer RF: Gastrointestinal dysfunction in Parkinson's disease. LancetNeurol 2003, 2:107–116.

12. Abbott RD, Petrovitch H, White LR, Masaki KH, Tanner CM, Curb JD, Grandinetti A,Blanchette PL, Popper JS, Ross GW: Frequency of bowel movements and thefuture risk of Parkinson's disease. Neurology 2001, 57:456–462.

13. Braak H, de Vos RA, Bohl J, Del Tredici K: Gastric alpha-synucleinimmunoreactive inclusions in Meissner's and Auerbach's plexuses incases staged for Parkinson's disease-related brain pathology.Neurosci Lett 2006, 396:67–72.

14. Lebouvier T, Chaumette T, Damier P, Coron E, Touchefeu Y, Vrignaud S,Naveilhan P, Galmiche JP: Bruley des Varannes S, Derkinderen P,Neunlist M: Pathological lesions in colonic biopsies during Parkinson'sdisease. Gut 2008, 57:1741–1743.

15. Lebouvier T, Neunlist M, Bruley des Varannes S, Coron E, Drouard A,N'Guyen JM, Chaumette T, Tasselli M, Paillusson S, Flamand M, et al: Colonicbiopsies to assess the neuropathology of Parkinson's disease and itsrelationship with symptoms. PLoS One 2010, 5:e12728.

16. Shannon KM, Keshavarzian A, Mutlu E, Dodiya HB, Daian D, Jaglin JA,Kordower JH: Alpha-synuclein in colonic submucosa in early untreatedParkinson's disease. Mov Disord 2012, 27:709–715.

17. Wakabayashi K, Takahashi H, Takeda S, Ohama E, Ikuta F: Parkinson'sdisease: the presence of Lewy bodies in Auerbach's and Meissner'splexuses. Acta Neuropathol 1988, 76:217–221.

18. Braak H, Del Tredici K: Poor and protracted myelination as a contributoryfactor to neurodegenerative disorders. Neurobiol Aging 2004, 25:19–23.

19. Natale G, Ferrucci M, Lazzeri G, Paparelli A, Fornai F: Transmission of prionswithin the gut and towards the central nervous system. Prion 2011,5:142–149.

20. Natale G, Pasquali L, Paparelli A, Fornai F: Parallel manifestations ofneuropathologies in the enteric and central nervous systems.Neurogastroenterol Motil 2011, 23:1056–1065.

21. Kordower JH, Chu Y, Hauser RA, Freeman TB, Olanow CW: Lewy body-likepathology in long-term embryonic nigral transplants in Parkinson'sdisease. Nature medicine 2008, 14:504–506.

22. Kordower JH, Chu Y, Hauser RA, Olanow CW, Freeman TB: Transplanteddopaminergic neurons develop PD pathologic changes: a second casereport. Mov Disord 2008, 23:2303–2306.

23. Kurowska Z, Englund E, Widner H, Lindvall O, Li J-Y, Brundin P: Signs ofDegeneration in 12–22-Year Old Grafts of Mesencephalic DopamineNeurons in Patients with Parkinson's Disease. Journal of Parkinson's disease2011, 1:83–92.

24. Li JY, Englund E, Holton JL, Soulet D, Hagell P, Lees AJ, Lashley T, Quinn NP,Rehncrona S, Bjorklund A, et al: Lewy bodies in grafted neurons insubjects with Parkinson's disease suggest host-to-graft diseasepropagation. Nature medicine 2008, 14:501–503.

25. Olanow CW, Prusiner SB: Is Parkinson's disease a prion disorder? Proc NatlAcad Sci U S A 2009, 106:12571–12572.

26. Lerner A, Bagic A: Olfactory pathogenesis of idiopathic Parkinson diseaserevisited. Mov Disord 2008, 23:1076–1084.

27. Aguzzi A, Rajendran L: The transcellular spread of cytosolic amyloids,prions, and prionoids. Neuron 2009, 64:783–790.

28. Brundin P, Melki R, Kopito R: Prion-like transmission of protein aggregates inneurodegenerative diseases. Nat Rev Mol Cell Biol 2010, 11:301–307.

29. Polymenidou M, Cleveland DW: The seeds of neurodegeneration: prion-like spreading in ALS. Cell 2011, 147:498–508.

30. Prusiner SB: Prions. Proceedings of the National Academy of Sciences of theUnited States of America 1998, 95:13363–13383.

31. Pan KM, Baldwin M, Nguyen J, Gasset M, Serban A, Groth D, Mehlhorn I,Huang Z, Fletterick RJ, Cohen FE, et al: Conversion of alpha-helices intobeta-sheets features in the formation of the scrapie prion proteins.Proceedings of the National Academy of Sciences of the United States ofAmerica 1993, 90:10962–10966.

32. Prusiner SB: Novel proteinaceous infectious particles cause scrapie.Science 1982, 216:136–144.

33. Serpell LC, Berriman J, Jakes R, Goedert M, Crowther RA: Fiber diffraction ofsynthetic alpha-synuclein filaments shows amyloid-like cross-betaconformation. Proc Natl Acad Sci U S A 2000, 97:4897–4902.


34. Crowther RA, Daniel SE, Goedert M: Characterisation of isolated alpha-synuclein filaments from substantia nigra of Parkinson's disease brain.Neurosci Lett 2000, 292:128–130.

35. Biere AL, Wood SJ, Wypych J, Steavenson S, Jiang Y, Anafi D, Jacobsen FW,Jarosinski MA, Wu GM, Louis JC, et al: Parkinson's disease-associatedalpha-synuclein is more fibrillogenic than beta- and gamma-synucleinand cannot cross-seed its homologs. J Biol Chem 2000, 275:34574–34579.

36. Wood SJ, Wypych J, Steavenson S, Louis JC, Citron M, Biere AL: alpha-synuclein fibrillogenesis is nucleation-dependent. Implications for thepathogenesis of Parkinson's disease. J Biol Chem 1999, 274:19509–19512.

37. Yonetani M, Nonaka T, Masuda M, Inukai Y, Oikawa T, Hisanaga S, HasegawaM: Conversion of wild-type alpha-synuclein into mutant-type fibrils andits propagation in the presence of A30P mutant. J Biol Chem 2009,284:7940–7950.

38. Freundt EC, Maynard N, Clancy EK, Roy S, Bousset L, Sourigues Y, Covert M,Melki R, Kirkegaard K, Brahic M: Neuron-to-neuron transmission of alpha-synuclein fibrils through axonal transport. Ann Neurol 2012, 72:517–524.

39. Lee HJ, Patel S, Lee SJ: Intravesicular localization and exocytosis of alpha-synuclein and its aggregates. J Neurosci 2005, 25:6016–6024.

40. Lee HJ, Suk JE, Bae EJ, Lee JH, Paik SR, Lee SJ: Assembly-dependentendocytosis and clearance of extracellular alpha-synuclein. Int J BiochemCell Biol 2008, 40:1835–1849.

41. Luk KC, Song C, O'Brien P, Stieber A, Branch JR, Brunden KR, Trojanowski JQ,Lee VM: Exogenous alpha-synuclein fibrils seed the formation of Lewybody-like intracellular inclusions in cultured cells. Proceedings of theNational Academy of Sciences of the United States of America 2009,106:20051–20056.

42. Hansen C, Angot E, Bergstrom AL, Steiner JA, Pieri L, Paul G, Outeiro TF,Melki R, Kallunki P, Fog K, et al: alpha-Synuclein propagates from mousebrain to grafted dopaminergic neurons and seeds aggregation incultured human cells. J Clin Invest 2011, 121:715–725.

43. Desplats P, Lee HJ, Bae EJ, Patrick C, Rockenstein E, Crews L, Spencer B,Masliah E, Lee SJ: Inclusion formation and neuronal cell death throughneuron-to-neuron transmission of alpha-synuclein. Proceedings of theNational Academy of Sciences of the United States of America 2009,106:13010–13015.

44. Lee HJ, Kim C, Lee SJ: Alpha-synuclein stimulation of astrocytes: Potentialrole for neuroinflammation and neuroprotection. Oxid Med Cell Longev2010, 3:283–287.

45. Mougenot AL, Nicot S, Bencsik A, Morignat E, Verchere J, Lakhdar L,Legastelois S, Baron T: Prion-like acceleration of a synucleinopathy in atransgenic mouse model. Neurobiol Aging 2012, 33:2225–2228.

46. Angot E, Steiner JA, Lema Tome CM, Ekstrom P, Mattsson B, Bjorklund A,Brundin P: Alpha-synuclein cell-to-cell transfer and seeding in grafteddopaminergic neurons in vivo. PLoS One 2012, 7:e39465.

47. Luk KC, Kehm VM, Zhang B, O'Brien P, Trojanowski JQ, Lee VM: Intracerebralinoculation of pathological alpha-synuclein initiates a rapidly progressiveneurodegenerative alpha-synucleinopathy in mice. J Exp Med 2012,209:975–986.

48. Luk KC, Kehm V, Carroll J, Zhang B, O’Brien P, Trojanowski JQ, Lee VM-Y:Pathological a-Synuclein Transmission Initiates Parkinson-likeNeurodegeneration in Nontransgenic Mice. Science 2012, 338:949–953.

49. Hughes AJ, Daniel SE, Blankson S, Lees AJ: A clinicopathological study of100 cases of Parkinson's Disease. Arch Neurol 1993, 50:140–148.

50. Jellinger KA: A critical reappraisal of current staging of Lewy-relatedpathology in human brain. Acta Neuropathol 2008, 116:1–16.

51. Parkkinen L, Pirttila T, Alafuzoff I: Applicability of current staging/categorization of alpha-synuclein pathology and their clinical relevance.Acta Neuropathol 2008, 115:399–407.

52. Zaccai J, Brayne C, McKeith I, Matthews F, Ince PG: Patterns and stages ofalpha-synucleinopathy: Relevance in a population-based cohort.Neurology 2008, 70:1042–1048.

53. Beach TG, Adler CH, Lue LF, Sue LI, Bachalakuri J, Henry-Watson J, Sasse J,Boyer S, Shirohi S, Brooks R, et al: Unified staging system for Lewy bodydisorders: correlation with nigrostriatal degeneration, cognitiveimpairment and motor dysfunction. Acta Neuropathol 2009, 117:613–634.

54. Del Tredici K, Braak H: Spinal cord lesions in sporadic Parkinson's disease.Acta Neuropathol 2012, 124:643–664.

55. Attems J, Jellinger KA: The dorsal motor nucleus of the vagus is not anobligatory trigger site of Parkinson's disease. Neuropathol Appl Neurobiol2008, 34:466–467.

56. Kalaitzakis ME, Graeber MB, Gentleman SM, Pearce RKB: The dorsal motornucleus of the vagus is not an obligatory trigger site of Parkinson'sdisease: a critical analysis of alpha-synuclein staging. Neuropathol ApplNeurobiol 2008, 34:284–295.

57. Kingsbury AE, Bandopadhyay R, Silveira-Moriyama L, Ayling H, Kallis C,Sterlacci W, Maeir H, Poewe W, Lees AJ: Brain stem pathology inParkinson's disease: an evaluation of the Braak staging model. MovDisord 2010, 25:2508–2515.

58. Sengoku R, Saito Y, Ikemura M, Hatsuta H, Sakiyama Y, Kanemaru K, Arai T,Sawabe M, Tanaka N, Mochizuki H, et al: Incidence and extent of Lewybody-related alpha-synucleinopathy in aging human olfactory bulb.J Neuropathol Exp Neurol 2008, 67:1072–1083.

59. Silveira-Moriyama L, Holton JL, Kingsbury A, Ayling H, Petrie A, Sterlacci W,Poewe W, Maier H, Lees AJ, Revesz T: Regional differences in the severityof Lewy body pathology across the olfactory cortex. Neurosci Lett 2009,453:77–80.

60. Hubbard PS, Esiri MM, Reading M, McShane R, Nagy Z: Alpha-synucleinpathology in the olfactory pathways of dementia patients. J Anat 2007,211:117–124.

61. Halliday G, Hely M, Reid W, Morris J: The progression of pathology inlongitudinally followed patients with Parkinson's disease. ActaNeuropathol 2008, 115:409–415.

62. Olanow CW, Perl DP, DeMartino GN, McNaught KS: Lewy-body formation isan aggresome-related process: a hypothesis. Lancet Neurol 2004, 3:496–503.

63. Kramer ML, Schulz-Schaeffer WJ: Presynaptic alpha-synuclein aggregates,not Lewy bodies, cause neurodegeneration in dementia with Lewybodies. J Neurosci 2007, 27:1405–1410.

64. Tanaka M, Kim YM, Lee G, Junn E, Iwatsubo T, Mouradian MM: Aggresomesformed by alpha-synuclein and synphilin-1 are cytoprotective. J BiolChem 2004, 279:4625–4631.

65. Terry RD: Do neuronal inclusions kill the cell? J Neural Transm Suppl 2000,59:91–93.

66. Gertz HJ, Siegers A, Kuchinke J: Stability of cell size and nucleolar size inLewy body containing neurons of substantia nigra in Parkinson'sdisease. Brain Res 1994, 637:339–341.

67. Saha AR, Ninkina NN, Hanger DP, Anderton BH, Davies AM, Buchman VL:Induction of neuronal death by alpha-synuclein. Eur J Neurosci 2000,12:3073–3077.

68. Tompkins MM, Hill WD: Contribution of somal Lewy bodies to neuronaldeath. Brain Res 1997, 775:24–29.

69. Parkkinen L, O’Sullivan S, Collinsa C, Petriec A, Holtona J, Revesza T, Lees AJ:Disentangling the Relationship between Lewy Bodies and Nigral NeuronalLoss in Parkinson’s Disease. Journal of Parkinson's Disease 2011, 1:277–286.

70. Nakazato Y, Yamazaki H, Hirato J, Ishida Y, Yamaguchi H: Oligodendroglialmicrotubular tangles in olivopontocerebellar atrophy. J Neuropathol ExpNeurol 1990, 49:521–530.

71. Papp MI, Kahn JE, Lantos PL: Glial cytoplasmic inclusions in the CNS ofpatients with multiple system atrophy (striatonigral degeneration,olivopontocerebellar atrophy and Shy-Drager syndrome). J Neurol Sci1989, 94:79–100.

72. Wakabayashi K, Yoshimoto M, Tsuji S, Takahashi H: Alpha-synucleinimmunoreactivity in glial cytoplasmic inclusions in multiple systematrophy. Neurosci Lett 1998, 249:180–182.

73. Milber JM, Noorigian JV, Morley JF, Petrovitch H, White L, Ross GW, Duda JE:Lewy pathology is not the first sign of degeneration in vulnerableneurons in Parkinson disease. Neurology 2012, 79:2307–2314.

74. Pan-Montojo F, Anichtchik O, Dening Y, Knels L, Pursche S, Jung R, Jackson S,Gille G, Spillantini MG, Reichmann H, Funk RH: Progression of Parkinson'sdisease pathology is reproduced by intragastric administration of rotenonein mice. Plos One 2010, 5:e8762.

75. Orimo S, Takahashi A, Uchihara T, Mori F, Kakita A, Wakabayashi K, Takahashi H:Degeneration of cardiac sympathetic nerve begins in the early diseaseprocess of Parkinson's disease. Brain Pathol 2007, 17:24–30.

76. Kordower JH, Freeman TB, Snow BJ, Vingerhoets FJ, Mufson EJ, Sanberg PR,Hauser RA, Smith DA, Nauert GM, Perl DP, et al: Neuropathologicalevidence of graft survival and striatal reinnervation after thetransplantation of fetal mesencephalic tissue in a patient withParkinson's disease. N Engl J Med 1995, 332:1118–1124.

77. Danzer KM, Krebs SK, Wolff M, Birk G, Hengerer B: Seeding induced byalpha-synuclein oligomers provides evidence for spreading of alpha-synuclein pathology. J Neurochem 2009, 111:192–203.


78. Kalia LV, Kalia SK, McLean PJ, Lozano AM, Lang AE: alpha-Synucleinoligomers and clinical implications for Parkinson disease. Ann Neurol,In press.

79. Mendez I, Vinuela A, Astradsson A, Mukhida K, Hallett P, Robertson H,Tierney T, Holness R, Dagher A, Trojanowski JQ, Isacson O: Dopamineneurons implanted into people with Parkinson's disease survive withoutpathology for 14 years. Nature medicine 2008, 14:507–509.

80. Farrer M, Chan P, Chen R, Tan L, Lincoln S, Hernandez D, Forno L, Gwinn-Hardy K,Petrucelli L, Hussey J, et al: Lewy bodies and parkinsonism in families withparkin mutations. Ann Neurol 2001, 50:293–300.

81. Gasser T, Hardy J, Mizuno Y: Milestones in PD Genetics. Mov Disord 2011,26:1042–1048.

82. Mori H, Kondo T, Yokochi M, Matsumine H, Nakagawa-Hattori Y, Miyake T,Suda K, Mizuno Y: Pathologic and biochemical studies of juvenileparkinsonism linked to chromosome 6q. Neurology 1998, 51:890–892.

83. Takahashi H, Ohama E, Suzuki S, Horikawa Y, Ishikawa A, Morita T, Tsuji S,Ikuta F: Familial juvenile parkinsonism: Clinical and pathological study in afamily Neurology 1994, 44:437–441.

84. van de Warrenburg BPC, Lammens M, Lucking CB, Denefle P, Wesseling P,Booij J, Praamstra P, Quinn N, Brice A, Horstink M: Clinical and pathologicabnormalities in a family with parkinsonism and parkin gene mutations.Neurology 2001, 56:555–557.

85. Frost B, Diamond MI: Prion-like mechanisms in neurodegenerativediseases. Nature Reviews Neuroscience 2010, 11:155–159.

86. Guest WC, Silverman JM, Pokrishevsky E, O'Neill MA, Grad LI, Cashman NR:Generalization of the prion hypothesis to other neurodegenerativediseases: an imperfect fit. J Toxicol Environ Health A, 74:1433–1459.

87. Kabanov AV, Gendelman HE: Nanomedicine in the diagnosis and therapyof neurodegenerative disorders. Prog Polym Sci 2007, 32:1054–1082.

88. Konno M, Hasegawa T, Baba T, Miura E, Sugeno N, Kikuchi A, Fiesel FC,Sasaki T, Aoki M, Itoyama Y, Takeda A: Suppression of dynamin GTPasedecreases alpha-synuclein uptake by neuronal and oligodendroglialcells: a potent therapeutic target for synucleinopathy. Mol Neurodegener2012, 7:38.

doi:10.1186/2051-5960-1-2Cite this article as: Visanji et al.: The prion hypothesis in Parkinson'sdisease: Braak to the future. Acta Neuropathologica Communications 20131:2.

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