Aldehyde dehydrogenase inhibition as a pathogenic mechanism in Parkinson disease - PubMed (original) (raw)

. 2013 Jan 8;110(2):636-41.

doi: 10.1073/pnas.1220399110. Epub 2012 Dec 24.

Shannon L Rhodes, Aaron Lulla, Niall P Murphy, Hoa A Lam, Kelley C O'Donnell, Lisa Barnhill, John E Casida, Myles Cockburn, Alvaro Sagasti, Mark C Stahl, Nigel T Maidment, Beate Ritz, Jeff M Bronstein

Affiliations

Aldehyde dehydrogenase inhibition as a pathogenic mechanism in Parkinson disease

Arthur G Fitzmaurice et al. Proc Natl Acad Sci U S A. 2013.

Abstract

Parkinson disease (PD) is a neurodegenerative disorder particularly characterized by the loss of dopaminergic neurons in the substantia nigra. Pesticide exposure has been associated with PD occurrence, and we previously reported that the fungicide benomyl interferes with several cellular processes potentially relevant to PD pathogenesis. Here we propose that benomyl, via its bioactivated thiocarbamate sulfoxide metabolite, inhibits aldehyde dehydrogenase (ALDH), leading to accumulation of the reactive dopamine metabolite 3,4-dihydroxyphenylacetaldehyde (DOPAL), preferential degeneration of dopaminergic neurons, and development of PD. This hypothesis is supported by multiple lines of evidence. (i) We previously showed in mice the metabolism of benomyl to S-methyl N-butylthiocarbamate sulfoxide, which inhibits ALDH at nanomolar levels. We report here that benomyl exposure in primary mesencephalic neurons (ii) inhibits ALDH and (iii) alters dopamine homeostasis. It induces selective dopaminergic neuronal damage (iv) in vitro in primary mesencephalic cultures and (v) in vivo in a zebrafish system. (vi) In vitro cell loss was attenuated by reducing DOPAL formation. (vii) In our epidemiology study, higher exposure to benomyl was associated with increased PD risk. This ALDH model for PD etiology may help explain the selective vulnerability of dopaminergic neurons in PD and provide a potential mechanism through which environmental toxicants contribute to PD pathogenesis.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Fig. 1.

Fig. 1.

ALDH inhibition as the proposed mechanism of benomyl-induced Parkinson disease. Benomyl is efficiently metabolized to potent ALDH inhibitors—BIC, MBT, and particularly MBT-SO—so exposure leads to the accumulation of the toxic dopamine metabolite DOPAL. This offers a possible explanation for the selective toxicity to dopaminergic neurons observed in PD pathogenesis. GSH, glutathione.

Fig. 2.

Fig. 2.

Inhibitory actions of benomyl and its metabolites. (A) Exposure to benomyl or MBT, but not carbendazim, inhibited ex vivo ALDH activity in mesencephalic neurons dissociated from 2-d-old rat pups (n = 3–11). (B) Benomyl, BIC, and MBT inhibited in vitro mitochondrial ALDH activity (n = 2–8 at each concentration tested). Carbendazim did not significantly inhibit mALDH activity at up to 20 μM (n = 4). (C) Benomyl and carbendazim inhibited 26S UPS activity in SK-_N_-MC neuroblastoma cells (n = 4–14 at each concentration tested), whereas MBT exposure up to 10 μM had no effect (n = 5). (D) Benomyl/BIC/MBT inhibit ALDH activity at lower concentrations than benomyl/carbendazim inhibit the 26S UPS. Data are expressed as percent relative to vehicle controls (0.01% DMSO), except for 26S UPS inhibition which is relative to treatment with the 26S UPS inhibitor lactacystin (5 μM). *P < 0.01, **P < 0.0001; benomyl (○), BIC (◇), MBT (☐), carbendazim (Δ). mALDH, mitochondrial aldehyde dehydrogenase; 26S UPS, ubiquitin-proteasome system.

Fig. 3.

Fig. 3.

Dopaminergic neuronal damage in primary mesencephalic cultures exposed to benomyl or its metabolites. Representative field of view (20×) shows untreated immunoreactive (A) dopaminergic neurons (TH+) and (B) neuronal nuclei (NeuN+). (C) Benomyl neurotoxicity was recapitulated by MBT exposure, whereas carbendazim was not toxic to TH+ neurons at 1 μM. NeuN+ counts were unaffected by any treatment. Because MBT is either the proximal or penultimate benomyl metabolite that inhibits ALDH activity, there appears to be an association between neuronal damage and ALDH inhibition; proteasomal inhibition by the carbendazim moiety is not sufficient to kill cells under the same conditions. Data are expressed as percent relative to vehicle controls (0.01% DMSO). *P < 0.05, **P < 0.01, ***P < 0.0001.

Fig. 4.

Fig. 4.

Monoamine oxidase (MAO) inhibitor protects against neurotoxicity because of DOPAL accumulation. Neuronal loss resulting from 1 μM benomyl or MBT exposure was mitigated by cotreatment with the MAO inhibitor pargyline (200 μM, n = 13–28). Because MAO inhibition reduces the metabolism of dopamine to DOPAL, this suggests that DOPAL is toxic to dopaminergic neurons and that benomyl is toxic via DOPAL accumulation as a result of ALDH inhibition. Data are expressed as percent relative to vehicle controls (0.01% DMSO). *P = 0.0027, **P = 2.4 × 10−4, ***P = 6.1 × 10−5.

Fig. 5.

Fig. 5.

Aminergic neuronal damage in Danio rerio embryos exposed to benomyl. Representative confocal images of zebrafish embryos (A, C, and E) unexposed or (B, D, and F) bathed in 1 μM benomyl from 5 h until 120 h postfertilization are shown. (G) Neuronal counts (A and B) decreased in VMAT2+ anterior and diencephalic clusters of ETvmat2:GFP zebrafish exposed to benomyl (solid bars) but were unaffected in (C and D) Rohon-Beard and (E and F) trigeminal neurons in Tg(isl1[ss]:Gal4-VP16,UAS:EGFP) zf154 zebrafish. (H) Measurement of total fluorescence yielded similar results. Data are expressed as percent relative to vehicle controls (0.01% DMSO). *P < 0.1, **P < 0.05. De, diencephalon; LC, locus coeruleus; OB, olfactory bulb; Te, telencephalon.

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