peer review of synergistic effects of pesticides and metals in parkinson's

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Synergistic Effects of Pesticides and Metals on Fibrillation of α-Synuclein: Implications for Parkinson’s DiseaseVladimir N. Uversky, Jie Li, Kiowa Bower, Anthony L. Fink*

Dept of Chem and BioChem, Univ. of California, Santa Cruz, CA 95064

A Peer Review by

Arloe P. Fontenot

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Synergistic Effects of Pesticides and Metals on Fibrillation of α-Synuclein: Implications for Parkinson’s Disease

Vladimir N. Uversky, Jie Li, Kiowa Bower, Anthony L. Fink*Dept of Chem and BioChem, Univ. of California, Santa Cruz, CA 95064

Introduction

-alpha-Synuclein, is a synuclein protein of unknown function found primarily in neural tissues; it is seen mainly in the presynaptic terminals.

-a-Synuclein is expressed predominantly in the neocortex, hippocampus, substantia nigra, thalamus, and cerebellum. Although primarily a neuronal protein; this protein can also be found in glial cells, those which bind and provide structure for the neuronal cells.

-Lewy bodies are abnormal, intraneural cytoplasmic inclusions composed of filaments of a-synuclein that radiate out from a central core. Morphologically, they appear as either classical (brain stem) or cortical.

-The main disease associated with the presence of Lewy bodies is Parkinson’s Disease, of which its cause is still unknown.

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Introduction

-alpha-Synuclein is an abundant brain protein consisting of 140 amino acids and of relatively unknown function, and belongs to a class of proteins known to be natively unfolded; i.e its purified structure is substantially disordered at neutral pH.

-Fibrils, or clusters, of a-synuclein have been found and reported from individuals with Lewy body diseases, as well as Lewy bodies sequestered in vitro.

-The authors recently reported preliminary studies which indicated certain pesticides and metals could accelerate the formation of a-synuclein fibrils, acting individually or synergistically. Both of these agents induce conformational change in a-synuclein; which is likely a critical precursor to association and fibrillation.

-These observations may serve to suggest an underlying molecular basis for PD and other Lewy body diseases.

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Introduction

Lewy Bodies

Small, dense deposits, termed Lewy bodies or Lewy neurites, stud the brains of patients with Parkinson's disease. The three sections of brain tissue shown above depict how the protein, alpha-synuclein (stained brown), packs these deposits. This finding, and others, leads many scientists to believe that alpha-synuclein plays a key role in Parkinson's disease. Researchers are now searching for ways to stop their negative actions and hopefully aid patients in the future.

Reprinted by permission from Nature, M.G. Spillantini et al., 388:839 (1997) © 2001 MACMILLAN MAGAZINES LTD.

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Introduction

Parkinson’s Disease…and a possible mechanismDescription: The motor defects of Parkinson's disease are related to the loss of dopaminergic neurons in specific brain regions. Examination of these neurons in diseased tissue has revealed the presence of Lewy bodies, dense aggregates that include the protein alpha-synuclein. A genetic basis for most cases of Parkinson's disease has not yet been identified, but mutations in alpha-synuclein have been associated with at least one rare form of the disease, and mutations in another protein, the parkin gene, are associated with another inherited form of Parkinson's disease. Parkin is found in Lewy bodies along with alpha-synuclein and the parkin protein is an enzyme, an E3 ubiquitin ligase. This ligase tags specific proteins in normal cells with ubiquitin, targeting them for destruction in the proteosome (see Proteosome pathway). One of the proteins that parkin normally targets for destruction is a specific O-glycosylated form of alpha-synuclein. Failure of parkin-mediated degradation of alpha-synuclein may be a key factor leading to the death of dopaminergic neurons. Another substrate of parkin is a GPCR-like protein called Pael-R that accumulates in the ER of affected cells and may cause neuronal cell death. The involvement of Parkin and alpha-synuclein mutations in genetic forms of Parkinson's suggest that failure of ubiquitination and protein degradation may be causative in other forms of Parkinson's. Questions remaining include the cause of the lack of effective ubiquitination in individuals lacking obvious genetic defects in this pathway and how to use this knowledge of ubiquitination and protein degradation in Parkinson's disease to identify therapeutic strategies.

Contributors: Glenn Croston, PhD.

References: Shimura, H. et al. (2001) Ubiquitination of a New Form of alpha-Synuclein by Parkin from Human Brain: Implications for Parkinson's disease. Science 293 (5528) 263-269

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Materials, Methods, & Experimental Design*a-Synuclein was expressed and prepared based on an earlier experiment by the authors

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Results – a-Synuclein fibrillation in presence of pesticides

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Results – “Dose-Dependence” a-Synuclein fibrillation in the presence of dieldrin

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Results – The [AlCl3] required to accelerate a-synuclein decreases at higher [a-synuclein]

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Results – Metal ion and pesticide-induced conformational changes in a-synuclein

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Results – Synergistic effects between pesticide (DDC) and metal (AlCl3)

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Discussion

-Of the pesticides the authors examined, those that showed the most significant accelerating effects were rotenone, DDT, 2.4-D, dieldrin, diethyldithiocarbamate, paraquat, maneb, trifluralin, parathion, and imidazoldinethione.

-MPP+, the active metabolite of MPTP that causes PD-like symptoms in humans and primates by acts to inhibit Complex I of the mitochondria had no significant effect on the kinetics (fibrillation) of a-synuclein. However, rotenone, which also inhibits Complex I of the mitochondria, accelerated a-synuclein fibrillation.

-Pesticides in general showed increasing rates of fibrillation with increasing concentration of pesticide.

-In terms of metals, trivalent metals coupled with low pH were most effective at stimulating fibrillation. The most effective ions were Al3+, Fe3+, Co3+, Cd2+, Mn2+, Cu2+, Co2+, respectively. There was a good correlation between charge density and effectiveness in stimulating a-synuclein fibrillation.

-The results were consistent with a model in which both the cations (above) and pesticides interact with a-synuclein to create conformational change of the protein to a partially-folded state with a high propensity to aggregate.

-Also noted that increasing hydrophobicity of pesticides is an important factor in stimulating aggregation, by binding to the partially-folded intermediate conformation, thereby increasing he population of the intermediate.

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Discussion

Thus, environmental factors that

stimulate the aggregation of

a-synuclein do so by binding to

and increasing the concentration of

the partially-folded intermediate

conformation that lead to

aggregation.

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Comments and Criticism

-results section was only partial for all pesticides tested

-only results for a single metal’s effects was shown

-preparation of the paper’s main target (a-synuclein) was left out of the Materials and Methods prep session, though mentioned in a previous paper. Would have been good to include anyway.

-overall, a good paper in my opinion, illustrating the effects of pesticides and certain cationic metals on the formation of Lewy bodies

Thanks!

peer review of synergistic effects of pesticides and metals in parkinson's

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