Choroid plexus transcytosis and exosome shuttling deliver folate into brain parenchyma

Marcel Grapp,Nicolas Snaidero,Robert Steinfeld,Mikael Simons,Arne Wrede,Michaela Schweizer,Hans-Joachim Galla,Johanna Bückers,Philip S Low,Jutta Gärtner,Henning Urlaub,Sabine Hüwel

doi:10.1038/ncomms3123

Abstract

Loss of folate receptor-α 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-α-positive intralumenal vesicles within multivesicular bodies and demonstrate the directional cotransport of human folate receptor-α, and labelled folate from the basolateral to the apical membrane in rat choroid plexus cells. Both the apical medium of folate receptor-α-transfected rat choroid plexus cells and human cerebrospinal fluid contain folate receptor-α-positive exosomes. Loss of folate receptor-α-expressing cerebrospinal fluid exosomes correlates with severely reduced 5-methyltetrahydrofolate concentration, corroborating the importance of the folate receptor-α-mediated folate transport in the cerebrospinal fluid. Intraventricular injections of folate receptor-α-positive and -negative exosomes into mouse brains demonstrate folate receptor-α-dependent delivery of exosomes into the brain parenchyma. Our results unravel a new pathway of folate receptor-α-dependent exosome-mediated folate delivery into the brain parenchyma and opens new avenues for cerebral drug targeting.

Highlights

Loss of folate receptor-a function is associated with cerebral folate transport deficiency and childhood-onset neurodegeneration
As the functional loss of FRa is associated with very low 5MTHF concentration in the cerebrospinal fluid (CSF) but normal plasma concentration of 5MTHF, and as FRa is abundantly expressed in the choroid plexus, we have hypothesized that FRa provides the major route for the blood–CSF transport of 5MTHF
We demonstrate that FRa mediates the transcellular 5MTHF transport into the CSF and that FRa-containing exosomes are targeted into the brain parenchyma

Summary

Introduction

Loss of folate receptor-a function is associated with cerebral folate transport deficiency and childhood-onset neurodegeneration. We identify folate receptor-a-positive intralumenal vesicles within multivesicular bodies and demonstrate the directional cotransport of human folate receptora, and labelled folate from the basolateral to the apical membrane in rat choroid plexus cells. The uptake of folates into enterocytes in the small intestine is mediated by the proton-coupled folate transporter (PCFT; SLC46A1). This transporter has a pH optimum of 5.5 and functions as proton cotransporter with a Km of about 1 mM3. We have recently described cerebral folate transport deficiency, a childhood neurodegenerative disease that is caused by mutations in the FOLR1 gene leading to a functional loss of FRa8,9. We demonstrate that FRa mediates the transcellular 5MTHF transport into the CSF and that FRa-containing exosomes are targeted into the brain parenchyma

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