title: Preferential Extracellular Generation of the Active Parkinsonian Toxin MPP+ by Transporter-Independent Export of the Intermediate MPDP+.
abstract: OBJECTIVE
  1-Methyl-4-phenyl-tetrahydropyridine (MPTP) is among the most widely used neurotoxins for inducing experimental parkinsonism. MPTP causes parkinsonian symptoms in mice, primates, and humans by killing a subpopulation of dopaminergic neurons. Extrapolations of data obtained using MPTP-based parkinsonism models to human disease are common; however, the precise mechanism by which MPTP is converted into its active neurotoxic metabolite, 1-methyl-4-phenyl-pyridinium (MPP(+)), has not been fully elucidated. In this study, we aimed to address two unanswered questions related to MPTP toxicology: (1) Why are MPTP-converting astrocytes largely spared from toxicity? (2) How does MPP(+) reach the extracellular space?
  
  
  RESULTS
  In MPTP-treated astrocytes, we discovered that the membrane-impermeable MPP(+), which is generally assumed to be formed inside astrocytes, is almost exclusively detected outside of these cells. Instead of a transporter-mediated export, we found that the intermediate, 1-methyl-4-phenyl-2,3-dihydropyridinium (MPDP(+)), and/or its uncharged conjugate base passively diffused across cell membranes and that MPP(+) was formed predominately by the extracellular oxidation of MPDP(+) into MPP(+). This nonenzymatic extracellular conversion of MPDP(+) was promoted by O2, a more alkaline pH, and dopamine autoxidation products.
  
  
  CONCLUSIONS
  Our data indicate that MPTP metabolism is compartmentalized between intracellular and extracellular environments, explain the absence of toxicity in MPTP-converting astrocytes, and provide a rationale for the preferential formation of MPP(+) in the extracellular space. The mechanism of transporter-independent extracellular MPP(+) formation described here indicates that extracellular genesis of MPP(+) from MPDP is a necessary prerequisite for the selective uptake of this toxin by catecholaminergic neurons.
authors.affiliation: 1 Department of Biology, University of Konstanz , Konstanz, Germany .
authors.forename: Stefan
authors.initials: S
authors.lastname: Schildknecht
authors.affiliation: 1 Department of Biology, University of Konstanz , Konstanz, Germany .
authors.forename: Regina
authors.initials: R
authors.lastname: Pape
authors.affiliation: 2 Metabolomics Junior Research Group, Luxembourg Centre for Systems Biomedicine, University of Luxembourg , Esch-Belval, Luxembourg .
authors.forename: Johannes
authors.initials: J
authors.lastname: Meiser
authors.affiliation: 1 Department of Biology, University of Konstanz , Konstanz, Germany .
authors.forename: Christiaan
authors.initials: C
authors.lastname: Karreman
authors.affiliation: 3 Department of Chemistry and Konstanz Research School Chemical Biology, University of Konstanz , Konstanz, Germany .
authors.forename: Tobias
authors.initials: T
authors.lastname: Strittmatter
authors.affiliation: 1 Department of Biology, University of Konstanz , Konstanz, Germany .
authors.forename: Meike
authors.initials: M
authors.lastname: Odermatt
authors.affiliation: 1 Department of Biology, University of Konstanz , Konstanz, Germany .
authors.forename: Erica
authors.initials: E
authors.lastname: Cirri
authors.affiliation: 3 Department of Chemistry and Konstanz Research School Chemical Biology, University of Konstanz , Konstanz, Germany .
authors.forename: Anke
authors.initials: A
authors.lastname: Friemel
authors.affiliation: 4 MCAT GmbH , Konstanz, Germany .
authors.forename: Markus
authors.initials: M
authors.lastname: Ringwald
authors.affiliation: 5 CNS Disease Research, Boehringer Ingelheim Pharma GmbH & Co. KG , Biberach an der Riss, Germany .
authors.forename: Noemi
authors.initials: N
authors.lastname: Pasquarelli
authors.affiliation: 5 CNS Disease Research, Boehringer Ingelheim Pharma GmbH & Co. KG , Biberach an der Riss, Germany .
authors.forename: Boris
authors.initials: B
authors.lastname: Ferger
authors.affiliation: 1 Department of Biology, University of Konstanz , Konstanz, Germany .
authors.forename: Thomas
authors.initials: T
authors.lastname: Brunner
authors.affiliation: 3 Department of Chemistry and Konstanz Research School Chemical Biology, University of Konstanz , Konstanz, Germany .
authors.forename: Andreas
authors.initials: A
authors.lastname: Marx
authors.affiliation: 6 Institute of Chemistry, University of Potsdam , Potsdam, Germany .
authors.forename: Heiko M
authors.initials: HM
authors.lastname: Möller
authors.affiliation: 2 Metabolomics Junior Research Group, Luxembourg Centre for Systems Biomedicine, University of Luxembourg , Esch-Belval, Luxembourg .
authors.forename: Karsten
authors.initials: K
authors.lastname: Hiller
authors.affiliation: 1 Department of Biology, University of Konstanz , Konstanz, Germany .
authors.forename: Marcel
authors.initials: M
authors.lastname: Leist
mesh_terms: D015632:1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine / Q000378:metabolism; D015655:1-Methyl-4-phenylpyridinium / Q000378:metabolism*; D001253:Astrocytes / Q000378:metabolism; D001692:Biological Transport; D002395:Catecholamines / Q000378:metabolism; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D004058:Diffusion; D059290:Dopaminergic Neurons / Q000378:metabolism*; D045604:Extracellular Fluid / Q000378:metabolism; D006801:Humans; D008995:Monoamine Oxidase / Q000378:metabolism; D010084:Oxidation-Reduction; D010300:Parkinson Disease / Q000378:metabolism* / Q000473:pathology; D011726:Pyridinium Compounds / Q000378:metabolism
chemical_list: D002395:Catecholamines; D011726:Pyridinium Compounds; C044755:1-methyl-4-phenyl-2,3-dihydropyridinium; D015632:1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine; D008995:Monoamine Oxidase; D015655:1-Methyl-4-phenylpyridinium