MFSD7c functions as a transporter of choline at the blood-brain barrier

Abstract

AbstractMutations of MFSD7c (also known asFlvcr2), which is an orphan transporter, are linked to Fowler syndrome1, 2. Here, we useMfsd7cknockout mice and cell-based assays to reveal that MFSD7c is a choline transporter at the blood-brain barrier (BBB). We performed comprehensive metabolomics and detected differential changes of metabolites in the brains and livers ofMfsd7cknockout (Mfsd7c−/−) embryos. Particularly, we found that choline-related metabolites were altered in the brains but not in the livers ofMfsd7c−/−embryos. Thus, we hypothesized that MFSD7c regulates the levels of choline in the brain. Indeed, expression of humanMFSD7cin cells significantly increased choline uptake. Interestingly, we showed that choline uptake byMFSD7cis greatly increased by choline-metabolizing enzymes, leading us to demonstrate that MFSD7c is a facilitative transporter of choline. Furthermore, single-cell patch-clamp showed that the import of choline by MFSD7c is electrogenic. Choline transport function of MFSD7c is conserved in vertebrates, but not in yeasts. We show that human MFSD7c is a functional ortholog of HNM1, the yeast choline importer. Employing our transport assays, we showed that several missense mutations of humanMFSD7cfrom Fowler patients had abolished or reduced choline transport activity. Mice lackingMfsd7cin the CNS endothelial cells suppressed the import of exogenous choline from blood but unexpectedly had increased choline levels in the brain. Stable-isotope tracing study revealed that MFSD7c is required for exporting choline derived from lysophosphatidylcholine (LPC) in the brain. Collectively, our work identifies MFSD7c as a choline transporter at the BBB. This study suggests that defective export of choline in the brain may be a cause of Fowler syndrome.