Demonstration of a coupled metabolism-efflux process at the choroid plexusas a mechanism of brain protection toward xenobiotics

Brain homeostasis depends on the composition of both brain interstitial flu id and CSF. Whereas the former is largely controlled by the blood-brain bar rier, the latter is regulated by a highly specialized blood-CSF interface, the choroid plexus epithelium, which acts either by controlling the influx of bloodborne compounds, or by clearing deleterious molecules and metabolit es from CSF. To investigate mechanisms of brain protection at the choroid p lexus, the blood-CSF barrier was reconstituted in vitro by culturing epithe lial cells isolated from newborn rat choroid plexuses of either the fourth or the lateral ventricle. The cells grown in primary culture on semipermeab le membranes established a pure polarized monolayer displaying structural a nd functional barrier features, (tight junctions, high electric resistance, low permeability to paracellular markers) and maintaining tissue-specific markers (transthyretin) and specific transporters for micronutrients (amino acids, nucleosides). In particular, the high enzymatic drug metabolism cap acity of choroid plexus was preserved in the in vitro blood-CSF interface. Using this model, we demonstrated that choroid plexuses can act as an absol ute blood-CSF barrier toward 1-naphthol, a cytotoxic, lipophilic model comp ound, by a coupled metabolism-efflux mechanism. This compound was metaboliz ed in situ via uridine diphosphate glururonosyrtransferase-catalyzed conjug ation, and the cellular efflux of the glucurono-conjugate was mediated by a transporter predominantly located at the basolateral, i.e., blood-facing m embrane. The transport process was temperature-dependent, probenecid-sensit ive, and recognized other glucuronides. Efflux of 1-naphthol metabolite was inhibited by intracellular glutathione S-conjugates. This metabolism-polar ized efflux process adds a new facet to the understanding of the protective functions of choroid plexuses.