NeuroMolecular Medicine 1 Volume 4, 2003

Overview of Protein Aggregation in Single, Double,and Triple Neurodegenerative Brain Amyloidoses

John Q. Trojanowski*,1 and Mark P. Mattson2

1The Center for Neurodegenerative Disease Research, Department of Pathologyand Laboratory Medicine, Institute on Aging, University of Pennsylvania School of Medicine,

Philadelphia, PA 19104 and 2Laboratory of Neurosciences, National Instituteon Aging Intramural Research Program, Baltimore, MD 21224

Received July 17, 2003; Accepted July 18, 2003

NeuroMolecular MedicineCopyright © 2003 Humana Press Inc.All rights of any nature whatsoever reserved.ISSN 1535-1084/03/04:1–5/$25.00

*Author to whom all correspondence and reprint requests should be addressed. E-mail: trojanow@mail.med.upenn.edu

The recognition of a common mechanistic themeshared by Alzheimer’s disease (AD) and many otherneurodegenerative disorders has emerged with theincreasing appreciation that a large number of thesedisorders are characterized neuropathologically byintracellular and/or extracellular aggregates of pro-teinaceous fibrils or amyloids (Table 1) and thatthese lesions are not mere markers of the diseasestate but are directly implicated in progressive braindegeneration (Hardy and Allsop, 1991; Hardy, 1997;Hardy et al., 1998; Trojanowski and Lee, 2000; Leeet al., 2001; Bucciantini et al., 2002; Mudher andLovestone, 2002; Taylor et al., 2002; Giasson et al.,2003; Soto, 2003; Trojanowski and Lee, 2003). Thus,despite differences in the molecular composition ofthe filamentous lesions in neurodegenerative dis-orders, such as AD, Parkinson’s disease (PD), andrelated synucleinopathies; frontotemporal demen-tias and related tauopathies; prion disorders; amy-otrophic lateral sclerosis; and trinucleotide repeatdiseases, growing evidence suggests that similarpathological mechanisms may underlie all of thesedisorders. Specifically, the onset and/or progression

of brain degeneration in AD and other neurode-generative disorders may be linked mechanisticallyto abnormal interactions between brain proteinsthat lead to the assembly of the disease proteins intofilaments and the aggregation of these filamentswithin brain cells or in the extracellular space.

Sporadic and familial AD (FAD) are among themost common and well known of this group of dis-eases, and in AD these filamentous lesions are exem-plified by intracytoplasmic neurofibrillary tangles(NFTs) as well as extracellular amyloid or senileplaques (SPs; Hardy and Allsop, 1991; Hardy, 1997;Hardy et al., 1998; Trojanowski and Lee, 2000; Buc-ciantini et al., 2002; Mudher and Lovestone, 2002;Taylor et al., 2002; Soto, 2003;). Although filamen-tous lesions formed by distinct proteins are recog-nized as diagnostic hallmarks of specific disorders,sporadic AD and FAD illustrate some of the com-plex and poorly understood overlap among theseneurodegenerative diseases. For example, the het-erogeneous dementing disorders classified as ADoverlap with a large group of distinct neurodegen-erative disorders that are collectively known as

2 Trojanowski and Mattson

NeuroMolecular Medicine Volume 4, 2003

tauopathies, and tauopathies are characterized byprominent tau-rich tangle pathology throughoutthe brain (Lee et al., 2001). However, AD also over-laps with another diverse group of disorders knownas synucleinopathies, which are characterized byfilamentous α-synuclein brain pathology (Tro-janowski and Lee, 2003). Thus, although the diag-nostic hallmarks of AD are numerous SPs composedof Aβ fibrils and intraneuronal NFTs formed byaggregated tau filaments, NFTs are similar to the

filamentous tau inclusions characteristic of neu-rodegenerative tauopathies, many of which do notshow other diagnostic disease-specific lesions.Notably, tau gene mutations have been shown tocause familial frontotemporal dementias andparkinsonism linked to chromosome 17 in manykindreds (Lee et al., 2001). Moreover, Lewy bodies(LBs), the hallmark intracytoplasmic neuronal inclu-sions of PD, also occur in the most common sub-type of AD known as the LB variant of AD, and

Table 1Abnormal Protein–Protein Interactions: Mechanisms of Disease in Diverse Neurodegenerative Disorders

Disease Lesion/components Location

AD a,b,c SPs/Aβ ExtracellularNFTs/PHFtau IntracytoplasmicLBs/α-synuclein Intracytoplasmic

ALSa Spheroids/NF subunits, SOD1 IntracytoplasmicDLBc LBs/α-synuclein IntracytoplasmicDSa,b,c SPs/Aβ Extracellular

NFTs/PHFtau IntracytoplasmicLBs/α-synuclein Intracytoplasmic

NBIA 1c LBs/α-synuclein IntracytoplasmicGCIs/α-synuclein Intracytoplasmic

LBVAD (AD+DLB)c SPs/Aβ ExtracellularNFTs/PHFtau IntracytoplasmicLBs/α-synuclein Intracytoplasmic

MSAc GCIs/α-synuclein IntracytoplasmicNIID Inclusions/expanded poly-glutamine tracts IntranuclearPrion diseasesa Amyloid plaques/prions ExtracellularTauopathiesa,b Tangles/abnormal tau IntracytoplasmicTri-nucleotide repeat diseases Inclusions/Expanded poly-glutamine tracts Intranuclear and

Intradendritic

*This table lists hereditary and sporadic neurodegenerative disorders characterized neuropathologically byprominent filamentous lesions. Most lesions are in nuclei, cell bodies and processes of neurons and/or glia, butsome are extracellular (i.e., SPs).

Abbr: Aβ, amyloid-beta peptide; AD, Alzheimer’s disease; ALS, amytrophic lateral sclerosis; DLB, dementiawith Lewy bodies; DS, Down syndrome; GCIs, glial cytoplasmic inclusions; LBs, Lewy bodies; LBVAD, Lewybody variant of Alzheimer’s disease; MSA, multiple system atrophy; NBIA 1, neurodegeneration with brainiron accumulation type 1; NF, neurofilaments; NFTs, neurofibrillary tangles; NIID, neuronal intranuclear inclu-sion disease; PD, Parkinson’s disease; PHFtau, paired helical filament tau; SOD1, superoxide dismutase 1; SPs,senile plaques.

aBoth hereditary and sporadic forms of these disorders occur. bNeurodegenerative diseases with prominent tau pathology are tauopathies. cNeurodegenerative diseases with prominent synuclein pathology are synucleinopathies.

Protein Aggregation in Neurodegenerative Brain Amyloidoses 3

NeuroMolecular Medicine Volume 4, 2003

numerous cortical LBs are the defining brain lesionsof dementia with LBs, which is similar to AD clin-ically but distinct from AD pathologically (Tro-janowski and Lee, 2000; Trojanowski and Lee,2003). Further, α-synuclein gene mutations causefamilial PD in rare kindreds, and these mutationsmay be pathogenic by altering the properties ofα-synuclein, thereby promoting the formation ofα-synuclein filaments that aggregate into LBs (Tro-janowski and Lee, 2003). However, it is now knownthat FAD mutations and trisomy 21 lead to abun-dant accumulations of LBs and Lewy neurites ordystrophic processes containing protein aggregatescomposed of α-synuclein filaments in the brains ofmost FAD and elderly Down syndrome (DS)patients, respectively. However, it is unclear howthese genetic abnormalities promote the formationof LBs from wild-type α-synuclein proteins encodedby a normal gene. Nonetheless, the accumulationof α-synuclein into filamentous inclusions appearsto play a mechanistic role in the pathogenesis of anumber of progressive neurological disorders,including PD, dementia with LBs, DS, FAD, LB vari-ant of AD, sporadic AD, multiple system atrophy,and other synucleinopathies, whereas amyloiddeposits formed by other subunit proteins are char-acteristic of prion disorders, amyotrophic lateralsclerosis, and trinucleotide repeat diseases (Tro-janowski and Lee, 2000; Trojanowski and Lee, 2003).

Thus, many neurodegenerative diseases (but notall of the disorders listed in Table I or reviewed inthis issue) share an enigmatic symmetry, that is, mis-sense mutations in the gene encoding the diseaseprotein cause a familial variant of the disorder aswell as its hallmark brain lesions but the same brainlesions also form from the corresponding wild-typebrain protein in sporadic variants of the disease.Moreover, AD is one of the more striking examplesof a “triple brain amyloidosis,” that is, a neurode-generative disorder wherein at least three differentbuilding block proteins (τ, α-synuclein) or amyloidβ-peptide fragments (Aβ) of a larger amyloid pre-cursor protein fibrillize and aggregate into patho-logical deposits of amyloid within (NFTs, LBs) andoutside (SPs) neurons. However, there are exam-ples of other triple brain amyloidoses, such as DSand Mariana Island dementia or Guam Parkinson–dementia complex, that also show evidence ofaccumulations of amyloid deposits formed by τ,α-synuclein, and Aβ, and there is increasing recog-

nition that τ or α-synuclein intraneuronal inclusionsmay converge with extracellular deposits of Aβ in“double brain amyloidoses,” as exemplified by theabundant τ inclusions in a member of the Contursikindred with familial PD, the presence of LBs or NFTsin patients with prion disease, the co-occurrenceof PD with abundant Aβ deposits, and dementia orLBs with progressive supranuclear palsy in somepatients. Accordingly, clarification of this enigmaticsymmetry in any one of these disorders is likely tohave a profound impact on understanding the mech-anisms that underlie other of these diseases as wellas on efforts to develop novel therapies to treat them.For this reason, this special issue of NeuroMolecularMedicine focuses on shared underlying mechanismscommon to these and other disorders mentionedhere, including the specific pathobiology of the dif-ferent types of amyloid that are characteristic of eachof these diseases, as well as cellular mechanismsthat may promote or inhibit accumulations ofprotein aggregates (oxidative stress, proteosome/ubiquitin systems, chaperone/heat shock proteinresponses, and genetic mutations) with the expec-tation that insights into these mechanism will accel-erate the pace of the successful discovery of drugsto treat these neurodegenerative brain amyloidoses(Welch and Gambetti, 1998; Auluck et al., 2002;Auluck and Bonini, 2002; Bonini, 2002; Giasson etal., 2002a; Giasson et al., 2002b; Loo and Clarke,2003; Soto, 2003).

Taken together, a growing number of recentadvances into understanding brain amyloidosis inthese disorders prompt consideration of pathogenicscenarios wherein synergistic interactions betweenτ, Aβ, and α-synuclein amyloid may occur or theearly intermediates and protofibrillar species of τ,Aβ and α-synuclein mediate brain degeneration inAD (Taylor et al., 2002; Giasson et al., 2003). Theseuncertainties notwithstanding, the insights intobrain amyloidosis in AD and related neurodegen-erative diseases mentioned above argue that aninformed and accurate view of the sequence ofevents leading to brain degeneration in triple anddouble brain amyloidoses will come with additionalnew data from experimental studies (e.g., in cell cul-ture and animal model systems) that rigorously testcompeting hypotheses about the cascade of eventsleading to brain degeneration in AD and other neu-rodegenerative brain amyloidoses. For example,hypothetical events that might underlie protein

4 Trojanowski and Mattson

NeuroMolecular Medicine Volume 4, 2003

fibrillization and aggregate formation or the toxic-ity associated with misfolded proteins includeincreased intracellular oxidative stress coupled witha failure of normal cellular antioxidant mechanismsor impairments in molecular chaperones and pro-tein refolding mechanisms or excitotoxicity(Markesbery, 1997; Auluck et al., 2002; Auluck andBonini, 2002; Bonini, 2002; Bucciantini et al., 2002;Giasson et al., 2002a; Giasson et al., 2002b; Taylor etal., 2002; Giasson et al., 2003; Mattson, 2003; Soto,2003). Indeed, there may be a plethora of pathwaysleading to protein misfolding with the subsequentformation of toxic amyloid deposits (be they formedby τ, Aβ, α-synuclein, prions, or polyglutaminetracts), and a much larger array of proteins than pre-viously anticipated may be vulnerable to misfoldor fibrillize and cause disease as a result of a vari-ety of noxious or stressful cellular perturbations.

Nonetheless, even in the absence of a completeunderstanding of these processes, sufficient infor-mation is available now to embark on drug dis-covery efforts to develop more effective therapiesfor protein misfolding diseases, including neu-rodegenerative brain amyloidoses, such as AD,synucleinopathies, and tauopathies (Mayeux andSano, 1999; Irizarry and Hyman, 2001; Trojanowski,2002; Loo and Clarke, 2003; Soto, 2003). For exam-ple, compounds have been identified that preventthe conversion of normal proteins into abnormalconformers or variants with structural propertiesthat predispose the pathological proteins to formpotentially toxic filamentous aggregates, and it isalso plausible to speculate that some of these agentsmay have therapeutic efficacy in more than one neu-rodegenerative disorder. Moreover, several promi-nent events that occur downstream of the proteinmisfolding and aggregation that lead to synapticdysfunction and cell death appear to be sharedamong the different brain amyloidoses, includingoxidative stress and perturbed cellular calciumhomeostasis (Mattson et al., 2002). Thus, althoughmore profound insights into abnormal protein-protein interactions and protein misfolding fromcontinuing research advances will coalesce in thefuture to clarify the earliest upstream events andthe downstream events in neurodegenerative brainamyloidoses, it is nonetheless timely now to embarkon efforts to discover new and better therapies forAD, synucleinopathies, tauopathies, prion diseases,

trinucleotide repeat disorders, and other devastat-ing neurodegenerative disorders caused by abnor-mal filamentous aggregates.

Acknowledgments

The co-editors of this special issue of NeuroMol-ecular Medicine thank all investigators who con-tributed to make it a success. The researchsummarized here was supported by the NationalInstitute on Aging of the National Institutes ofHealth, the Alzheimer ’s Association, and theMichael J. Fox Foundation. Additional informationon neurodegenerative diseases can be obtained athttp://www.med.upenn.edu/cndr. Finally, theresearch summarized here has been made possibleby the families of our patients whose generous sup-port of our efforts made this research possible.

References

Auluck P. K. and Bonini N. M. (2002) Pharmacologi-cal prevention of Parkinson disease in Drosophila.Nat. Med. 8, 1185–1186.

Auluck P. K., Chan H. Y. E., Trojanowski J. Q., LeeV. M.-Y., and Bonini N. M. (2002) Chaperone sup-pression of alpha-synuclein toxicity in a Drosophilamodel of Parkinson’s disease. Science 295, 865–868.

Bonini N. M. (2002) Chaperoning brain degeneration.Proc. Natl. Acad. Sci. USA 99, 16407–16411.

Bucciantini M., Giannoni E., Chiti F., et al. (2002)Inherent toxicity of aggregates implies a commonmechanism of protein misfolding disease. Nature416, 507–511.

Giasson B. I., Forman M. S., Higuchi M., et al. (2003)Initiation and synergistic fibrillization of tau andalpha-synuclein. Science. 300, 636–639.

Giasson B. I., Lee V. M.-Y., Ischiropoulos H., and Tro-janowski J. Q. (2002a) The relationship betweenoxidative/nitrative stress and pathological inclu-sions in Alzheimer’s and Parkinson’s disease. FreeRadic. Biol. Med. 32, 1264–1275.

Giasson B. I., Duda J. E., Murray I. V., et al. (2002b)Oxidative damage linked to neurodegenerationby selective alpha-synuclein nitration in synucle-inopathy lesions. Science. 290, 985–989.

Hardy J., Duff K., Hardy K. G., Perez-Tur J., and HuttonM. (1998) Genetic dissection of Alzheimer’s dis-

Protein Aggregation in Neurodegenerative Brain Amyloidoses 5

NeuroMolecular Medicine Volume 4, 2003

ease and related dementias: amyloid and its rela-tionship to tau. Nat. Neurosci. 1, 355–358.

Hardy J. (1997) Amyloid the presenilins andAlzheimer’s disease. Trends Neurosci. 20, 154–159.

Hardy J. and Allsop D. (1991) Amyloid deposition asthe central event in the aetiology of Alzheimer’sdisease. Trends Pharmcol. Sci. 12, 383–388.

Irizarry M. C. and Hyman B. T. Alzheimer disease ther-apeutics. (2001) J. Neuropath. Exp. Neurol.60,923–928.

Lee V. M.-Y., Goedert M., and Trojanowski J. Q. (2001)generative tauopathies. Annu. Rev. Neurosci. 24,21–1159.

Loo T. W. and Clarke D. M. (2003) Application of chem-ical chaperones to the rescue of folding defects.Methods Mol. Biol. 232, 231–244.

Markesbery W. R. (1997) Oxidative stress hypothesis inAlzheimer’s disease. Free Radic. Biol. Med.20,154–159.

Mattson M. P. (2003) Excitotoxic and excitoprotectivemechanisms: abundant targets for the preventionand treatment of neurodegenerative disorders.Neuromol. Med., 3, 65–94.

Mattson M. P., Chan S. L., and Duan W. (2002) Modi-fication of brain aging and neurodegenerativedisorders by genes, diet, and behavior. Physiol. Rev.82, 637–672.

Mayeux R. and Sano M. (1999) Treatment of Alzheimer’sdisease. N. Engl. J. Med. 341, 1670–1679.

Mudher A. and Lovestone S. (2002) Alzheimer ’sdisease—do tauists and Baptists finally shakehands? Trends Neurosci. 25, 22–26.

Soto C. (2003) Unfolding the role of protein misfold-ing in neurodegenerative diseases. Nat. Rev. Neu-rosci. 4, 49–60.

Taylor J. P., Hardy J., and Fischbeck K. H. (2002) Toxicproteins in neurodegenerative disease. Science296, 1991–1995.

Trojanowski J. Q. and Lee V. M-Y. (2003) Parkin-son’s disease and related alpha-synucleinopathiesare brain amyloidoses. Ann. NY Acad. Sci. 991,107–111.

Welch W. J. and Gambetti P. (1998) Chaperoning braindiseases. Nature 392, 23–24.

Trojanowski J. Q. (2002) Emerging Alzheimer’s dis-ease therapies: focusing on the future. NeurobiolAging 23, 985–990.

Trojanowski J. Q. and Lee V. M.-Y. (2000) “Fatal attrac-tions” of proteins: a comprehensive hypotheticalmechanism underlying Alzheimer’s disease andother neurodegenerative disorders. Ann. N Y Acad.Sci. 924, 62–67.