Functional characterization and identification of the gene of the proton-organic cation antiporter

• Applicant: Professor Dr. Jürgen Brockmöller, since 8/2021
• Subject Area: Pharmacology
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 461080000

Numerous medically important substances are hydrophilic and positively charged at typical pH values in organisms. Such hydrophilic organic cations cannot efficiently pass through biological membranes by passive diffusion for intestinal absorption or to cross the blood-brain barrier. These organic cations include drugs like clonidine, diphenhydramine, varenicline, and oxycodone, but also addictive substances like nicotine or cocaine. So far, it is postulated that many of such substances are transported by a so-called proton-organic cation (H+/OC) antiporter. However, the gene encoding this transporter has not yet been identified. This gene likely belongs to the solute carrier (SLC) superfamily of transporters which encompasses more than 450 membrane encoding genes.Whether a substance is a substrate of the H+/OC antiporter is concluded from concentration-dependent, pH-dependent, and sodium-independent uptake as well as from an antiporter activity and susceptibility to inhibition by imipramine. And, typically, H+/OC antiporter substrates are not significantly transported by other organic cation transporters like OCT1-3, OCTN1-2, MATE1-2, or PMAT. Here we want to characterize comprehensively the substrate spectrum of the H+/OC antiporter using cell-based transport assays and bioinformatics methods, as currently only less than 50 substrates are known. Furthermore, we aim to identify the gene encoding this H+/OC antiporter and to characterize its tissue expression and transcriptional regulation. We will apply diverse methods for the identification of this gene including functional screening of the most likely candidate genes (including possibly relevant orphan genes) in combination with transcriptomics, and we will apply affinity labeling with typical substrates and proteomics. Knowing the gene coding for the H+/OC antiporter would be medically and scientifically most important. It might result in a better understanding of absorption, distribution, and elimination of drugs and other substances in the human body. The high functional activity of the H+/OC antiporter in endothelial cells from the blood-brain barrier highlights its particular role for drug transport there. Thus, this study should significantly improve our understanding of the passage of many drugs across the blood-brain barrier, which is most relevant for both, their beneficial and toxic effects in the brain. Also, the identification of the H+/OC antiporter gene would contribute to a better understanding of drug-drug interactions at the blood-brain-barrier and other barriers in the human body. Finally, once we know the relevant gene, we could systematically analyze genomic variation in this gene to achieve a better understanding of the large interindividual variation in the pharmacokinetics of some H+/OC antiporter substrates. Finally, the characterization of the H+/OC antiporter and identification of its gene may contribute to targeted drug development and individualized drug therapy.

• DFG Programme: Research Grants
• Ehemaliger Antragsteller: Dr. Salim Ansari, until 8/2021