Abstract
Vitamin C is incorporated into the cerebrospinal fluid (CSF) through choroid plexus cells. While the transfer of vitamin C from the blood to the brain has been studied functionally, the vitamin C transporter, SVCT2, has not been detected in the basolateral membrane of choroid plexus cells. Furthermore, it is unknown how its expression is induced in the developing brain and modulated in scurvy conditions. We concluded that SVCT2 is intensely expressed in the second half of embryonic brain development and postnatal stages. In postnatal and adult brain, SVCT2 is highly expressed in all choroidal plexus epithelial cells, shown by colocalization with GLUT1 in the basolateral membranes and without MCT1 colocalization, which is expressed in the apical membrane. We confirmed that choroid plexus explant cells (in vitro) form a sealed epithelial structure, which polarized basolaterally, endogenous or overexpressed SVCT2. These results are reproduced in vivo by injecting hSVCT2wt-EYFP lentivirus into the CSF. Overexpressed SVCT2 incorporates AA (intraperitoneally injected) from the blood to the CSF. Finally, we observed in Guinea pig brain under scorbutic condition, that normal distribution of SVCT2 in choroid plexus may be regulated by peripheral concentrations of vitamin C. Additionally, we observed that SVCT2 polarization also depends on the metabolic stage of the choroid plexus cells.
Highlights
Choroid plexus epithelial cells are connected by tight junctions that restrict the paracellular movement of molecules between the blood and cerebrospinal fluid (CSF)[1,2,3]
We first demonstrated that GLUT1 and SVCT2 are widely expressed in choroid plexus cells from the lateral ventricle of the mouse brain using RT-PCR and Western blot analysis (Fig. 1A,B,D,E)
Using sections of brain tissue incubated in 4% paraformaldehyde for 2 days, GLUT1 was detected in the basolateral membrane of choroid plexus cells (Fig. 1C, arrows); SVCT2 was mainly internalized (Fig. 1F)
Summary
Choroid plexus epithelial cells are connected by tight junctions that restrict the paracellular movement of molecules between the blood and cerebrospinal fluid (CSF)[1,2,3]. Different studies have been carried out to examine choroid plexus function in primary cell cultures and immortalized cell lines[6,7,8,9,10] using bicameral systems that show different characteristics of polarized epithelial cells[11,12,13,14,15] and represent a blood-CSF barrier model for transcellular transport studies[13,14,16,17]. The expression of the mRNA added to the AA incorporation studies was suggestive of the presence of an active, saturable transporter, SVCT2, which would serve to transfer AA from the blood plasma to the CSF and subsequently the brain[54,55]. A similar location has been observed in human choroid plexus papilloma cells, even though kinetic studies to analyze AA uptake indicate that SVCT2 is expressed in these cells[57]
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