Experimental Neurology 179, 6–8 (2003)doi:10.1006/exnr.2002.8082

COMMENTARY

Rotenone Neurotoxicity: A New Window on Environmental Causesof Parkinson’s Disease and Related Brain Amyloidoses

The classical features of Parkinson’s disease (PD)have been well established (3–6, 11). This notwith-standing, fundamental mechanisms underlying braindegeneration in PD have been far less tractable toelucidate, and this undoubtedly accounts for the factthere are no preventive interventions or therapies thatarrest the onset/progression of PD, although some dis-abling impairments of PD can be ameliorated to avariable extent by symptomatic treatments (3–6, 11).However, the past 5 years have brought spectacularresearch advances that have ushered in fresh optimismabout the prospects for discovering new and possiblymore effective therapies for PD that target mecha-nisms of neurodegeneration in PD (3–6, 11). For exam-ple, a growing number of studies are building a con-vincing array of complementary data implicating�-synuclein abnormalities in the onset/progression ofPD (3–6, 11), and this will hasten development of moreeffective PD therapies because these insights pro-vide new directions for research to elucidate mecha-nisms underlying PD (1, 11). Moreover, �-synucleinpathologies are the defining brain lesions of phenotyp-ically diverse neurodegenerative disorders known assynucleinopathies that appear to share common dis-ease mechanisms (3–6, 11) (see Table 1). Thus, ad-vances in understanding the role of �-synuclein in theonset/progression of PD, as summarized briefly below,are clarifying mechanisms underlying this and relateddisorders characterized by filamentous aggregates of�-synuclein that form intracellular amyloid deposits(11).

Since 1997, a remarkable paradigm shift in under-standing PD has resulted from converging lines of ev-idence linking �-synuclein to mechanisms underlyingPD and related neurodegenerative brain amyloidoses.As reviewed in more detail elsewhere (3–6, 11), keyobservations supporting this view are: (1) Missensemutations in the �-synuclein gene cause familial PD(FPD); (2) antibodies to �-synuclein detect the hall-mark lesions of PD, known as Lewy bodies (LBs) anddystrophic Lewy neurites (LNs), although these lesionsalso occur in other disorders such as dementia withLBs (DLB), Alzheimer’s disease (AD), and the LB vari-ant of AD (LBVAD); (3) insoluble �-synuclein filaments

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are recovered from purified LBs; (4) recombinant�-synuclein (but not the related homologues �- and�-synuclein) assembles into LB-like filaments and res-idues 71–82 in �-synuclein are essential for filamentassembly; (5) �-synuclein transgenic (TG) mice andflies develop a neurodegenerative phenotype with LBsand LNs similar to PD; (6) cortical LBs detected withantibodies to �-synuclein correlate with dementia inPD, DLB, and LBVAD; (7) antibodies to �-synucleindetect LBs in more familial AD, sporadic AD, and el-derly Down’s syndrome (DS) brains than previouslyreported; (8) �-synuclein is a building block of glialcytoplasmic inclusions (GCIs) in neurodegenerationwith brain iron accumulation type-1 and multiple systematrophy (MSA); (9) epitope specific anti-�-synuclein anti-bodies detect regions throughout �-synuclein in LBs andGCIs; (10) �-synuclein in LBs, GCIs, and related le-sions is abnormally phosphorylated, ubiquitinated,and nitrated; (11) cells transfected with �-synucleinand treated with nitric oxide generators develop LB-like �-synuclein inclusions; (12) bigenic TG mice over-expressing mutant human amyloid-� (A�) precursorproteins (APP) and �-synuclein show an augmentationin �-synuclein inclusions; (13) coexpression of chaper-ones or �-synuclein with �-synuclein in TG animalsleads to an amelioration of the disease phenotype seenin single �-synuclein TG animals, while drugs thatincrease expression of chaperones in �-synuclein TGflies ameliorate neurodegeneration. In addition, whileone might still justify retaining the name PD for thoseforms of neurodegenerative movement disorders asso-ciated with �-synuclein-rich LBs and LNs, it is clearthat neurodegenerative parkinsonian diseases are ge-netically and phenotypically heterogeneous, whichsuggests the need for substantial revisions in currentnomenclature for these disorders. Indeed, since 1997 atleast 10 gene loci have been linked to autosomal dom-inant familial parkinsonism alone (8), and one of theseloci on chromosome 17 harbors mutations in the taugene that are pathogenic for disease as result of theaccumulation of filamentous tau inclusions with amy-loid-like properties similar to those of LBs and GCIs.Finally, it should be emphasized that LBs are com-plex amyloid deposits (not unlike the senile amyloid

plaques of AD) that sequester a number of differentproteins and other components including some that arenonspecific elements and others (possibly neurofila-ments, chaperones, parkin, synphilin, etc.) that maycontribute to the toxicity of LBs (3–6, 11).

Thus, sporadic neurodegenerative synucleinopathies(Table 1) may result in part from oxidative/nitrativedamage to �-synuclein as well as phosphorylation andubiquitination, while the A53T and A30P missensemutations in the alpha-synuclein gene of rare FPDpatients may be pathogenic by promoting the fibrillo-genesis of �-synuclein and the formation of �-synucleinamyloid aggregates or inclusions (5, 6, 11). However, itunknown precisely how genetic/epigenetic factorscause wild-type �-synuclein to form insoluble filamentsthat aggregate into amyloid deposits.

The deleterious effects of �-synuclein aggregates oncentral nervous system (CNS) neurons (e.g., LBs) inPD or glial cells (e.g., GCIs) in MSA remain to beelucidated. However, LBs are likely to impair the func-tion/viability of affected neurons based on studies ofTG models of neurodegenerative diseases that havebeen engineered to overexpress �-synuclein in theCNS. Notably, the availability of TG animal models ofLB-like inclusions generated by expressing mutantand wild-type human �-synuclein in the CNS will en-able experimental tests of this hypothesis as well asthe screening of new drugs that might reverse thedisease phenotype (1, 5, 11).

However, the recent development of nontransgenicrat models of PD-like brain degeneration based on theadministration of rotenone, a common pesticide, hasopened up promising new avenues for the investigationof environmental causes of PD (2), and in the currentissue of Experimental Neurology, Sherer et al. refinethis model further (7). Significantly, this rotenonemodel of a PD-like disorder will stimulate further re-search into clues linking environmental toxins to neu-

rodegenerative disease that have emerged from awealth of epidemiological data on PD (reviewed in 2and 7), as well as other disorders such as Guam Par-kinsonism/Dementia Complex (PDC) (10), and from aremarkable line of research into a PD-like syndromethat developed in drug addicts who a abused a mis-manufactured “designer drug” known as N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Indeed, assummarized more extensively in the current and priorreports from Greenamyre’s group (2, 7), the discoveryof MPTP represented a major advance in efforts tomodel environmentally induced PD in animals. Nota-bly, MPTP toxicity was shown to result from inhibitionof complex I in the mitochondrial electron transportchain by a metabolite of MPTP known as 1-methyl-4-pyridinium (MPP�) that preferentially accumulated indopaminergic neurons. Although there appear to beparallels between neurotoxicity of MPTP and rotenone,and other studies suggest the existence of systemicdefects in complex I activity in PD patients, MPTPmodels do not lead to robust LBs and LNs, which limitstheir utility for investigating links between environ-mental causes of sporadic PD and the formation of thefilamentous �-synuclein amyloid deposits.

Thus, the development of a rotenone-induced PD-like phenotype in rats characterized by highly selectivenigrostriatal dopaminergic degeneration and �-synucleininclusions represents a significant advance in the de-velopment of animal models of PD to investigate envi-ronmental causes of this and other neurodegenerativedisease. Moreover, because rotenone also inhibits com-plex I, the rotenone model links mitochondrial dysfunc-tion and pesticide exposure to mechanisms of sporadicPD (2, 7). However, the difficulties associated withjugular vein administration of rotenone in the originalmodel limited its widespread use for elucidating therole of complex I inhibitors in the onset/progression ofPD (2, 7). Thus, Sherer et al. now describe in this issueof Experimental Neurology a more exploitable rote-none-induced model of PD based on chronic, subcuta-neous administration of rotenone to rats using osmoticminipumps (7). Significantly, these rats also develop ahighly selective nigrostriatal dopaminergic degenera-tion associated with �-synuclein inclusions, and it islikely that the availability of this model system willaccelerate the pace of research into pesticide andother environmental causes of PD and related synu-cleinopathies.

Equally or perhaps more significant, the rotenonemodel provides an approximate roadmap for investi-gating the role of other neurotoxins in PD as well ashow such toxins might induce or increase the risk fordeveloping other late-life neurodegenerative diseases.For example, while fibrillar A� deposits known as se-nile plaques (SPs) and intraneuronal aggregates of taufibrils known as neurofibrillary tangles (NFTs) are di-agnostic amyloid lesions of AD, �50% of patients with

TABLE 1

Neurodegenerative Diseases with Prominent Filamentous�-Synuclein Amyloid Lesions

Sporadic and Familial Parkinson’s diseaseSporadic and Familial Alzheimer’s diseaseDementia with LBsGuam Parkinsonism/Dementia ComplexMultiple system atrophyDown syndromeNeuronal degeneration with brain iron accumulation type 1Prion diseasesProgressive supranuclear palsyPure autonomic failureREM sleep behavior disorder

Note. Sporadic and hereditary neurodegenerative diseases charac-terized by filamentous �-synuclein brain lesions are listed here, andthose in bold are disorders in which �-synuclein pathology is thepredominant form of brain amyloid.

7COMMENTARY

familial or sporadic AD as well as elderly DS patientswith AD develop LBs similar to those in PD (5, 6, 11).Thus, AD is a “triple brain amyloidosis,” wherein atleast three different building-block proteins (tau,�-synuclein) or peptide fragments (i.e., A�) of APPfibrillize and aggregate into pathological deposits ofamyloid within (NFTs, LBs) and outside (SPs) neurons.However, there are examples of other triple brain amy-loidoses such as DS and Guam PDC that also accumu-late amyloid deposits formed by tau, �-synuclein, andA�, and there is evidence to suggest that Guam PDCmight be caused by neurotoxins derived from cycadnuts of the sago palm or by other environmental factorssince unequivocal evidence of a genetic etiology of thisdisorder has not yet emerged (11). Furthermore, thereis increasing recognition that tau or �-synuclein intra-neuronal inclusions may converge with extracellularA� deposits in “double brain amyloidoses” as exempli-fied by the co-occurrence of PD with abundant A� de-posits and dementia, or PD with progressive supranu-clear palsy (11). Thus, the rotenone model of PDprompts the intriguing and as yet completely unan-swered question of what other forms of pathologicalprotein misfolding, aggregation, and amyloidosismight be induced by rotenone or other environmentaltoxins as a result of compromised mitochondrial func-tion, impaired bioenergetics, oxidative damage, orother mechanisms (2, 5, 7) that eventually culminate ina neurodegenerative brain amyloidosis?

Since many neurodegenerative diseases character-ized by brain amyloidosis share an enigmatic symme-try such that missense mutations in the gene encodingthe disease protein cause a familial variant of the dis-order as well as its hallmark brain lesions, while thesame brain lesions also form from the correspondingwild-type brain protein in sporadic variants of the dis-ease, it is likely that clarification of this enigmaticsymmetry in any one of these disorders will have aprofound impact on understanding the mechanismsthat underlie other of these brain amyloidoses as wellas on efforts to develop novel therapies to treat them(11). Although genetic research has contributed enor-mously to our understanding of the role of genetic pointmutations in familial PD, AD, and other hereditaryneurodegenerative brain amyloidoses, far less progresshas been made in elucidating how environmental fac-tors including neurotoxins contribute to the onset/pro-gression of the sporadic counterparts of these diseases.Therein lies one of the most promising contributions ofthe rotenone model of PD because it adds considerablenew traction to research efforts designed to unravel thecomplex cascade of events that might begin with asingle exposure to, or multiple and chronic encounterswith, naturally occurring or manufactured compoundsin the environment, and then culminate over the spaceof a few years or many decades in the progressiveabnormal accumulation of proteins that form brain

amyloid deposits and result in the onset of a late-lifeneurodegenerative disease (2, 5, 7, 9, 11, 12).

ACKNOWLEDGMENTS

The studies summarized here from our Center for Neurodegenera-tive Disease Research (CNDR) were supported by grants from theNational Institute on Aging of the National Institutes of Health andthe Alzheimer’s Association. Due to space limitations in this Com-mentary, the citations listed here emphasize reviews wherein manyof the key primary references on neurodegenerative brain amyloid-oses are found. For more information on these diseases, see theCNDR website (http://www.uphs.upenn.edu/cndr/).

REFERENCES

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2. Betarbet, R., T. B. Sherer, G. Mackenzie, M. Garcia-Osuma,A. V. Panov, and T. Greenamyre. 2000. Chronic systemic pes-ticide exposure reproduces features of Parkinson’s disease. Nat.Neurosci. 3: 1301–1306.

3. Dickson, D. W. 2001. Alpha-synuclein and the Lewy body dis-orders. Curr. Opin. Neurol. 14: 423–432.

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8. Skipper, L., and M. Farrer. 2002. Parkinson’s genetics: Molec-ular insights for the new millennium. NeuroToxicology 23: 503–514.

9. Thiruchelvam, M., E. K. Richfield, B. M. Goodman, R. B. Baggs,and D. A. Cory-Slechta. 2002. Developmental exposure to thepesticides paraquat and maneb and the Parkinson’s diseasephenotype. NeuroToxicology 23: 621–633.

10. Trojanowski, J. Q., T. Ishihara, M. Higuchi, Y. Yoshiyama, M.Hong, B. Zhang, M. F. Forman, V. Zhukareva, and V. M.-Y. Lee.2002. Amyotrophic lateral sclerosis/parkinsonism dementiacomplex: Transgenic mice provide insights into mechanismsunderlying a common tauopathy in an ethnic minority onGuam. Exp. Neurol. 176: 1–11.

11. Trojanowski, J. Q., and V. M.-Y. Lee. 2002. Parkinson’s diseaseand related synucleinopathies are a new class of nervous sys-tem amyloidoses. NeuroToxicology 23: 457–460.

12. Uversky, V. N., J. Li, K. Bower, and A. L. Fink. 2002. Syner-gistic effects of pesticides and metals on the fibrillation of al-pha-synuclein: Implications for Parkinson’s disease. Neuro-Toxicology 23: 527–536.

John Q. TrojanowskiCenter for Neurodegenerative Disease ResearchDepartment of Pathology and Laboratory Medicine, and Institute

on AgingUniversity of Pennsylvania School of MedicinePhiladelphia, Pennsylvania 19104

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