Vitamin C Deficiency in the Young Brain-Findings from Experimental Animal Models

Abstract

Severe and long-term vitamin C deficiency can lead to fatal scurvy, which is fortunately considered rare today. However, a moderate state of vitamin C (vitC) deficiency (hypovitaminosis C)-defined as a plasma concentration below 23 μM-is estimated to affect up to 10% of the population in the Western world, albeit clinical hallmarks in addition to scurvy have not been linked to vitC deficiency. The brain maintains a high vitC content and uniquely high levels during deficiency, supporting vitC's importance in the brain. Actions include both antioxidant and co-factor functions, rendering vitamin C deficiency likely to affect several targets in the brain, and it could be particularly significant during development where a high cellular metabolism and an immature antioxidant system might increase sensitivity. However, investigations of a non-scorbutic state of vitC deficiency and effects on the developing young brain are scarce. This narrative review provides a comprehensive overview of the complex mechanisms that regulate vitC homeostasis in vivo and in the brain in particular. Functions of vitC in the brain and the potential consequences of deficiency during brain development are highlighted, based primarily on findings from experimental animal models. Perspectives for future investigations of vitC are outlined.

Keywords: brain; deficiency; development; vitamin C.

Figure 1

Schematic outline of ascorbic acid oxidation and nomenclature. Ascorbate (ASC) readily quenches free radicals by donating an electron. thereby forming the ascorbate free radical with a half-life ranging from 10⁻³ s to minutes. The ascorbate free radical can be reduced back to ASC or subjected to further oxidation and produce dehydroascorbic acid (DHA). In turn, DHA may be hydrolyzed, irreversibly altering the molecular structure to 2,3-diketogulonic acid with no vitamin C activity, and proceed to be metabolized and cleared [34]. Alternatively, DHA is reduced by glutathione (GSH) to form the ascorbate free radical and subsequently ASC, or even directly to ASC by enzymatic reaction [34,52,53] (The figure is reproduced without modifications from [28] and in accordance with CC by 4.0).

Figure 2

The distribution of vitamin C in the body. Distribution of vitamin C (vitC) in vivo is highly differential. Some organs have concentration-dependent mechanisms for retention of vitamin C, maintaining high levels during times of inadequate supply at the expense of other organs. In addition, the concentration-dependent absorption and re-absorption mechanisms contribute to the homeostatic control of vitC in the body. The brain upholds relatively high levels compared to other organs, with neurons displaying up to 10 mM. Inserted graphs show the dose–concentration curves measured in guinea pigs subjected to different dietary vitC doses, with estimated curve fitting (Hill equation); (a) liver, (b) kidney and (c) brain with cortex, cerebellum and hippocampus levels depicted individually. In the brain, the hippocampus achieves saturation (A) at a higher dose, but with a smaller concentration maximum (Cmax) compared to cortex and cerebellum (B), illustrating a regional difference in vitC distribution within the brain. In the liver and kidney, saturation is not as clear, suggesting a more direct reflection of the increasing plasma concentration compared to the brain. This supports that vitC transport to the brain is different from that of other organs and allows for the brain to be favored in vitC distribution. Moreover, the dose–concentration relationship underlines that accurate tissue levels of vitC are difficult to extrapolate from plasma levels. (Reproduced and modified from [28,73] and in accordance with CC by 4.0.).

Figure 3

Schematic overview of mechanisms of vitamin C uptake and recycling in the brain. (1) Vitamin C (vitC) primarily enters the brain either by SVCT2-mediated ascorbate (ASC) transport through the epithelial cells of the choroid plexus to the cerebrospinal fluid (CSF), or as dehydroascorbic acid (DHA) via glucose transporter 1 (GLUT1) situated on the blood–brain barrier (BBB) endothelia (2). DHA may be recycled to ASC within the BBB-endothelial cells or released directly to the extracellular matrix. Passive diffusion of ASC and DHA may also occur; however, efflux mechanisms regulating vitC release are yet largely unaccounted for. (3) Extracellular ASC mainly enters neurons through SVCT2 transporters. Intracellularly, ASC may be oxidized leading to formation of the ascorbate free radical (AFR). AFR can then form ASC and DHA. DHA may be recycled to ASC through reduction, be transported out of the neuron e.g., by diffusion, or cleared (degraded). Neurons also possess GLUT3 transporters, allowing for facilitated diffusion of DHA. Together, these mechanisms enable the increase in concentration of high intracellular ASC in neurons, reaching as much as 10 μM. ASC may be released from neurons in response to glutamate uptake. (4) Astrocytes do not express SVCT transporters but can take up DHA through GLUT1 facilitated diffusion. DHA is recycled to ASC maintaining a concentration gradient across the astrocyte plasma membrane and promoting the continued DHA uptake and enabling the increase in concentration of intracellular ASC. ASC can then be released from the astrocytes to the extracellular matrix for subsequent uptake to neurons. This can be, e.g., in conjunction with neuronal glutamate release, where glutamate uptake and clearance by astrocytes prompts the release of ASC. AFR: Ascorbate free radical; ASC: Ascorbate; BBB: Blood–brain barrier; CSF: Cerebrospinal fluid; DHA: dehydroascorbic acid; GLUT: glucose transporter; SVCT: sodium coupled vitamin C co-transporter; VitC: vitamin C.

Figure 4

Overview of potential targets of vitamin C deficiency in the brain. Functions of vitC have not yet been completely disclosed and it is linked to several and different roles within the brain. The most well-known role is ensuring the hydroxylation and subsequent assembly of collagen in its triple helical structure. Failure to form functional collagen is seen during long-term and severe vitC deficiency and leads to the breakdown of connective tissue structures, e.g., in vascular walls, hallmarking scurvy. VitC is also linked to the formation of vasculature through hypoxia-inducible factors (HIFs). A lack of vitC may reduce hydroxylation and subsequently accumulation of HIF1α, leading to deviated angiogenesis. In addition, oxidative stress may activate HIFs, thereby increasing levels further. Acting as co-factor in the regulation of methylation of nucleic acids, vitC deficiency is linked to alterations in DNA and histone methylation patterns and subsequent alterations in the epigenetic regulation of gene expression. VitC also acts as co-factor in carnitine synthesis and, though most likely due to alterations in excretion, carnitine deficiency is associated with low vitC status and consequent reductions in mitochondrial fatty acid metabolism, compromising cellular energy metabolism. In turn, accumulating reactive oxygen species and oxidative stress in vitC deficiency may lead to peroxidation of cellular membrane lipids, compromising cellular function and viability. Directly associated with neurotransmitter synthesis, vitC is a co-factor in the hydroxylation of dopamine, leading to norepinephrine, and provides reducing equivalents for tetra-hydrobiopterin necessary for the synthesis of dopamine and serotonin. Lastly, vitC deficiency reduces the re-uptake of extracellular glutamate, which in turn may lead to excitotoxic damage in the brain. Together, these functions of vitC highlights several likely effects of states of deficiency with putative serious consequences for cellular health and brain function.