High‐affinity sodium–vitamin C co‐transporters (SVCT) expression in embryonic mouse neurons

Abstract

The sodium±vitamin C co-transporters SVCT1 and SVCT2 transport the reduced form of vitamin C, ascorbic acid. High expression of the SVCT2 has been demonstrated in adult neurons and choroid plexus cells by in situ hybridization. Additionally, embryonic mesencephalic dopaminergic neurons express the SVCT2 transporter. However, there have not been molecular and kinetic analyses addressing the expression of SVCTs in cortical embryonic neurons. In this work, we con®rmed the expression of a SVCT2-like transporter in different regions of the fetal mouse brain and in primary cultures of neurons by RT-PCR. Kinetic analysis of the ascorbic acid uptake demonstrated the presence of two af®nity constants, 103 mM and 8 mM. A K m of 103 mM corresponds to a similar af®nity constant reported for SVCT2, while the K m of 8 mM might suggest the expression of a very high af®nity transporter for ascorbic acid. Our uptake analyses also suggest that neurons take up dehydroascorbic acid, the oxidized form of vitamin C, through the glucose transporters. We consider that the early expression of SVCTs transporters in neurons is important in the uptake of vitamin C, an essential molecule for the fetal brain physiology. Vitamin C that is found at high concentration in fetal brain may function in preventing oxidative free radical damage, because antioxidant radical enzymes mature only late in the developing brain.

Typical and atypical stem cells in the brain, vitamin C effect and neuropathology

“…Therefore, SVCTs are the major homeostatic regulators of vitamin C levels in the central nervous system (CNS; Rice, 2000), actively uptaking its reduced form, L-ascorbate (Caprile et al, 2009;Castro et al, 2001;Garcia et al, 2005). L-ascorbate is transported against its concentration gradient ( Daruwala et al, 1999;Tsukaguchi et al, 1999), and thus the energy required for the transport is supplied by sodium exchange through the Na + /K + -ATPase pump (Castro et al, 2001). The relationship between sodium and ascorbate transport was found to be 2:1, and the order of union to the substrate was sodium-ascorbate-sodium (Godoy et al, 2007).…”

“…Each transporter has a distinct distribution. For example, in situ hybridization and immunohistochemical experiments have shown that SVCT2 is expressed mostly in CNS within choroid plexus cells (Garcia et al, 2005;Tsukaguchi et al, 1999), embryonic neurons (Castro et al, 2001), tanycytes (Garcia et al, 2005) and adult cortical and hippocampal neurons (Astuya et al, 2005) (Figure 4A-B). Although most astrocytes do not express SVCT2, our results indicate that astrocytes of the external area of the entorhinal cortex express SVCT2 ( Figure 4C,E).…”

“…High levels of SVCT2 expression were observed in the cerebellum, hippocampus, and hypothalamus (Castro et al, 2001). Kinetic analysis of AA uptake in primary neuronal cultures showed two affi nity constants, 8 and 103 uM.…”

Recycling of Vitamin C by a Bystander Effect

“…Only reduced vitamin C, ascorbic acid, is present in human plasma and in cells and tissues, and the cellular content of vitamin C can exceed by several orders of magnitude the plasma levels of the vitamin (2). Human cells transport ascorbic acid in a concentrative manner through sodium-ascorbic acid co-transporters (3)(4)(5)(6)(7)(8); they also transport oxidized vitamin C, dehydroascorbic acid, down its concentration gradient through facilitative glucose transporters (9 -18). Although all cells express facilitative glucose transporters, the absolute specificity of these transporters for dehydroascorbic acid is in apparent contradiction to evidence indicating that vitamin C is present in human plasma only as ascorbic acid.…”

Brain Energy Metabolism in Health and Disease

“…The molecular basis of ascorbic acid efflux is not yet well known. Neurons can take up ascorbic acid efficiently because they express SVCT2 (Castro et al, 2001). Ascorbic acid is oxidized within neurons Fig.…”