Figure 7: Suggested model of human cerebral folate transport.

Schematic drawing of the brain including the cerebrospinal fluid circulation. The rectangle indicates the magnified area containing the choroid plexus and the ependyma. At physiological plasma concentrations (6–40 nM) 5MTHF (F) is primarily taken up by FRα at the basolateral membrane of the human choroid plexus. The 5MTHF-FRα complex is internalized by receptor-mediated endocytosis, translocated into GPI-anchored protein-enriched early endosomal compartments (GEECs) and further transferred to MVBs. MVBs are late endosomal compartments localized in the endocytic route. A minor proportion of internalized 5MTHF is translocated to PCFT-positive vesicles and transported into the cytoplasma. ILVs of MVBs containing FRα are generated by inward budding of the limiting membrane. These ILVs are released as exosomes into the CSF after fusion of the MVB with the apical cell membrane. Fusion of the limiting membrane of MVBs with the apical membrane explains the presence of FRα molecules on the apical surface of the choroid plexus cells too. FRα-containing exosomes circulate in the CSF, cross the ependymal cell layer and are distributed in the brain parenchyma. FRα-positive exosomes might initially be taken up by astrocytes and from these further delivered to neurons. FRα-negative exosomes hardly cross the ependymal cell layer and are targeted to other brain cells, such as microglia. At higher than normal plasma concentrations of 5MTHF (>100 nM) alternative mechanisms of uptake and transport may provide for transcellular 5MTHF delivery across the choroid plexus.