Diversity of human choline transporters: substrate specificity, kinetic properties, and inhibitor sensitivity

Carrier-mediated uptake of choline is rate-limiting for acetylcholine biosynthesis and various other biological processes. To date, 16 solute carrier (SLC) proteins have been identified that may facilitate choline permeation across the outer cell membrane. However, their biochemical functions have not yet been experimentally compared. We overexpressed 16 SLC proteins with known choline-transporting capacity and compared their choline transport kinetics. Additionally, we evaluated their capacity to transport choline analogues as well as metabolites involved in its biosynthesis and degradation or modulators of cholinergic neurotransmission to gain insight into the biological functions of the SLCs. Furthermore, we investigated whether the transporters could be distinguished by their sensitivity to inhibition by hemicholinium-3 and other substances. Of the 16 SLCs tested, seven (SLC5A7, SLC35F2, SLC35F3, SLC35F4, SLC25F5, SLC35G4, and SLC44A5) exhibited K_M values for choline transport in the range of 12 to 50 µM, closely aligning with physiological plasma choline concentrations. Among them, SLC5A7 displayed over tenfold higher intrinsic clearance than any of the others. SLC22A1-3 were confirmed as choline transporters, albeit with low affinity. Hemicholinium-3 most strongly inhibited SLC5A7 and also significantly inhibited SLC35F2-5 and SLC35G4. Choline transport by these six transporters was inhibited by about 50 % at 100 µM decynium-22 and verapamil.. In humans, multiple SLCs may contribute to cellular choline uptake, depending on physiologic conditions and their yet incompletely characterized expression patterns. The present data may also enhance our understanding of inherited and environmental modulation of these transporters with possible consequence, for instance, on motor and cognitive functions.