Choroid plexus transcytosis and exosome shuttling deliver folate into brain parenchyma

Abstract

Loss of folate receptor-alpha function is associated with cerebral folate transport deficiency and childhood-onset neurodegeneration. To clarify the mechanism of cerebral folate transport at the blood-cerebrospinal fluid barrier, we investigate the transport of 5-methyltetrahydrofolate in polarized cells. Here we identify folate receptor-alpha-positive intralumenal vesicles within multivesicular bodies and demonstrate the directional cotransport of human folate receptor-alpha, and labelled folate from the basolateral to the apical membrane in rat choroid plexus cells. Both the apical medium of folate receptor-alpha-transfected rat choroid plexus cells and human cerebrospinal fluid contain folate receptor-alpha-positive exosomes. Loss of folate receptor-alpha-expressing cerebrospinal fluid exosomes correlates with severely reduced 5-methyltetrahydrofolate concentration, corroborating the importance of the folate receptor-alpha-mediated folate transport in the cerebrospinal fluid. Intraventricular injections of folate receptor-alpha-positive and -negative exosomes into mouse brains demonstrate folate receptor-alpha-dependent delivery of exosomes into the brain parenchyma. Our results unravel a new pathway of folate receptor-alpha-dependent exosome-mediated folate delivery into the brain parenchyma and opens new avenues for cerebral drug targeting.