Identification of the endogenous key substrates of the human organic cation transporter OCT2 and their implication in function of dopaminergic neurons.
BACKGROUND
The etiology of neurodegenerative disorders, such as the accelerated loss of dopaminergic neurons in Parkinson's disease, is unclear. Current hypotheses suggest an abnormal function of the neuronal sodium-dependent dopamine transporter DAT to contribute to cell death in the dopaminergic system, but it has not been investigated whether sodium-independent amine transporters are implicated in the pathogenesis of Parkinson's disease.


RESULTS
By the use of a novel tandem-mass spectrometry-based substrate search technique, we have shown that the dopaminergic neuromodulators histidyl-proline diketopiperazine (cyclo(his-pro)) and salsolinol were the endogenous key substrates of the sodium-independent organic cation transporter OCT2. Quantitative real-time mRNA expression analysis revealed that OCT2 in contrast to its related transporters was preferentially expressed in the dopaminergic regions of the substantia nigra where it colocalized with DAT and tyrosine hydroxylase. By assessing cell viability with the MTT reduction assay, we found that salsolinol exhibited a selective toxicity toward OCT2-expressing cells that was prevented by cyclo(his-pro). A frequent genetic variant of OCT2 with the amino acid substitution R400C reduced the transport efficiency for the cytoprotective cyclo(his-pro) and thereby increased the susceptibility to salsolinol-induced cell death.


CONCLUSIONS
Our findings indicate that the OCT2-regulated interplay between cyclo(his-pro) and salsolinol is crucial for nigral cell integrity and that a shift in transport efficiency may impact the risk of Parkinson's disease.
Taubert
Dirk
D
Department of Pharmacology, Medical Hospital of the University of Cologne, Cologne, Germany. dirk.taubert@medizin.uni-koeln.de
Grimberg
Gundula
G
Stenzel
Werner
W
Schömig
Edgar
E
D000818:Animals; D045744:Cell Line, Tumor; D004298:Dopamine / Q000378:metabolism*; D006801:Humans; D008875:Middle Aged; D009474:Neurons / Q000187:drug effects / Q000378:metabolism*; D018696:Neuroprotective Agents / Q000494:pharmacology; D027701:Organic Cation Transport Proteins / Q000378:metabolism*; D000075102:Organic Cation Transporter 2; D010456:Peptides, Cyclic / Q000494:pharmacology; D010879:Piperazines / Q000494:pharmacology; D011485:Protein Binding; D051381:Rats; D013378:Substantia Nigra / Q000166:cytology / Q000378:metabolism; D053719:Tandem Mass Spectrometry; D014446:Tyrosine 3-Monooxygenase / Q000378:metabolism
D018696:Neuroprotective Agents; D027701:Organic Cation Transport Proteins; D000075102:Organic Cation Transporter 2; D010456:Peptides, Cyclic; D010879:Piperazines; C491013:SLC22A2 protein, human; D014446:Tyrosine 3-Monooxygenase; C015924:histidyl-proline diketopiperazine; D004298:Dopamine