The Role of Brain Barriers in Maintaining Brain Vitamin Levels

Abstract

It is increasingly recognized that tissue-specific nutrient deficiencies can exist in the absence of whole-body deficiency and that these deficiencies may result from disease or disease-related physiological processes. Brain and central nervous system tissues require adequate nutrient levels to function. Many nutrients are concentrated in the cerebrospinal fluid relative to the serum in healthy individuals, and other nutrients resist depletion in the presence of whole-body nutrient depletion. The endothelial, epithelial, and arachnoid brain barriers work in concert to selectively transport, concentrate, and maintain levels of the specific nutrients required by the brain while also blocking the passage of blood-borne toxins and pathogens to brain and central nervous system tissues. These barriers preserve nutrient levels within the brain and actively concentrate nutrients within the cerebrospinal fluid and brain. The roles of physical and energetic barriers, including the blood–brain and blood–nerve barriers, in maintaining brain nutrient levels in health and disease are discussed.

INTRODUCTION

Estimates of nutrient intake levels that sustain adequate nutrition for physiological function in healthy populations have been derived for all of the essential nutrients. These recommended reference values for nutrient intakes are referred to as the Dietary Reference Intakes (DRIs), and they include the Recommended Dietary Allowance and the tolerable upper intake level. There are excellent reviews on the process and considerations for establishing DRIs (19) and their use in developing nutrition policy, programs, and guidance for the public (135). The DRIs have not previously been intended as guidance for individuals or populations suffering from chronic and other diseases, although this has recently changed with the publication of the National Academies of Sciences, Engineering, and Medicine’s consensus study report Guiding Principles for Developing Dietary Reference Intakes Based on Chronic Disease (93). Disease and its management can affect biomarkers of whole-body nutritional status and may also deplete nutrient concentrations only in specific tissues. This nutrient depletion may be due to disease-associated factors, including inflammation, infection, autoantibodies, oxidative stress, or other pathophysiological processes that accelerate nutrient catabolism or excretion, as well as to impaired nutrient utilization or transport across barriers, such as the gut, brain, or other organs. Understanding the effects of disease on nutrient requirements has been limited, in part, by a lack of suitable biomarkers of tissue-specific nutrient status.

Proper functioning of the central nervous system (CNS) requires access to adequate nutrient levels within the tissues of the CNS that create a microenvironment supporting neuronal growth and activity. Maintaining these microenvironments requires that both nutrients and other solutes are properly transported across the blood–brain and blood–nerve barriers and that nutrient levels are maintained within these tissues. This review highlights the role of brain barriers in maintaining brain vitamin levels and the effects of disease-induced barrier break down on brain vitamin deficiency.

THE STRUCTURE AND FUNCTION OF BRAIN BARRIERS

The endothelial, epithelial, and arachnoid barriers work in concert to selectively transport, and in many cases concentrate and maintain, levels of the specific nutrients required by the brain while also blocking the passage of blood-borne toxins and pathogens to brain and CNS tissue (1, 118) (Figures 1 and 2).

Figure 1.

Structure of the blood–brain and blood–cerebrospinal fluid barriers. (a) Structure of the neurovascular unit of the endothelial blood–brain barrier. (b) Structure of the epithelial blood–brain barrier at the choroid plexus. Adapted with permission from Reference 76, distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0).

Figure 2.

Cellular composition and location of the blood–brain, blood–cerebrospinal fluid, and arachnoid barriers. (a) Cellular composition and anatomical location of the blood–brain barrier of the capillary endothelia, which perfuse the brain parenchyma; the image shows a cerebral vein. (b) Cellular composition and anatomical location of the epithelial blood–cerebrospinal fluid barrier, present in each ventricular choroid plexus. (c) Cellular composition and anatomical location of the arachnoid barrier surrounding the brain parenchyma. (d) Structure of a villus within a choroid plexus, which includes endothelial cells, epithelial cells, and fenestrated capillaries. Adapted with permission from Reference 1.

The Endothelial Barrier

The endothelial barrier, also known as the blood–brain barrier (BBB), filters harmful substances from the blood and concentrates required nutrients from the blood into interstitial fluid (ISF) in an energy-dependent manner. The ISF immediately surrounds neurons and functions to supply nutrients to them, to remove metabolic wastes from them, and in intercellular communication (65). The BBB is composed of brain microvascular endothelial cells (BMECs), which perfuse the brain and spinal cord (Figures 1a and 2a). These endothelial cells are connected by tight junctions (TJs), which function to exclude macromolecules while still allowing the paracellular diffusion of small hydrophilic compounds (70) (Figure 1a). TJs are composed of the integral membrane proteins occludin, claudin, junctional adhesion molecules, and cytoplasmic accessory proteins (70). Lipid-soluble molecules may diffuse passively through cell membranes; however, most water-soluble vitamins must be actively transported into the ISF across the BBB (118). In addition, BMECs lack the fenestrations characteristic of capillaries elsewhere in the body and show low pinocytotic activity to restrict the transcellular transport of molecules. Surrounding the BMECs are pericytes and astrocytic end-feet, which also function to restrict permeability from the blood to the ISF (Figure 1a) (115, 152). Together with interacting neurons and microglia, these elements form a complex known as the neurovascular unit (Figure 1a). The selective admission of required substances into the CNS is facilitated by specialized transporters or receptors present on the BMECs that comprise the BBB. Several nutrients required by the brain, including folate (vitamin B₉), ascorbic acid (AA; vitamin C), and glucose, are known to be concentrated into the CNS across the BBB (85, 90, 118); certain required vitamins also resist depletion in the brain, even when faced with whole-body or serum vitamin deficiency, due to the energy-dependent transporters of the BBB, which allow for concentration against chemical gradients. BMECs contain a relatively large number of mitochondria, which are necessary to provide energy to maintain vitamin concentrations within the ISF against this often-significant concentration gradient from serum to ISF (76, 92). Any defect in the BBB (e.g., loss of barrier integrity caused by degradation of the TJs of the barrier, loss of function of BBB transporters and receptors, or decreased mitochondrial capacity in the BMECs of the barrier) can lead to an imbalance in brain vitamin levels and improper functioning of the CNS.

The Epithelial Barrier

The epithelial barrier or blood–cerebrospinal fluid barrier (BCSFB) is composed of CSF-facing cuboidal epithelial cells of the choroid plexuses (CPs), which surround CP capillaries (53) (Figures 1b and 2b,d). The primary function of CP epithelial cells is to synthesize and maintain the composition of the CSF in the lateral, third, and fourth ventricles (Figure 2). To that end, CP epithelial cells contain abundant mitochondria, Golgi apparatus, smooth endoplasmic reticulum, and lysosome-like vesicles to aid in their secretory capacity. Unlike the BBB, fenestrated capillaries are present in each CP villus of the BCSFB to aid in the exchange of fluid from the serum to the ISF and the uptake of solutes across the basolateral membrane of the BCSFB epithelial cells (122). TJs join the apical membrane of epithelial cells to restrict paracellular diffusion, creating the BCSFB. The apical membrane also contains a brush border of microvilli with specific transporters for absorption and secretion, as well as cilia to aid in the ventricular circulation of the CSF (Figure 1b). The BCSFB also consists of ependymal cells, which line the brain ventricles, and the glia limitans of the brain surface, both of which are joined by gap junctions (20) (Figure 2b) and allow for the free diffusion of solutes between the CSF and ISF of the brain (150). The permeation of solutes into the brain parenchyma across the ependyma is a slow process (i.e., days) that depends on the presence of efflux transporters, which at the same time actively transport substrates from the CNS (122). As a result, the BBB, not the BCSFB, is often the main target of therapeutics due to its perfusion of the entire brain parenchyma (Figure 2).

The arachnoid barrier primarily functions to seal the brain while allowing for CSF-specific drainage (Figure 2d). It is not a site of nutrient transport due to the presence of TJs on the exterior arachnoid epithelium. The inner membrane contains punctate junctions that allow for the drainage of the CSF into the subarachnoid space and through to the sagittal sinus via arachnoid granulations (114). Together, the brain barriers allow the ISF and CSF to resist physiological fluctuations of serum and maintain a constant and specific protected neural microenvironment that is essential for the proper functioning of the CNS.

CEREBROSPINAL FLUID

Biological Functions and Synthesis

CSF is synthesized in the CP and secreted into the ventricles of the brain across the BCSFB (Figure 2). In adult humans, the ventricles contain about 150 mL of CSF, which is turned over four times per day and is in constant flux to maintain intracranial pressure (122). The CSF provides essential nutrients to the CNS, facilitates the removal of metabolic waste products, and functions as a protective cushion for the brain. Almost 500 metabolites have been quantitatively characterized in human CSF, including nutrients, vitamins, amino acids, minerals, carbohydrates, and hormones; however, the entire CSF metabolome remains under investigation (74, 151).

Stem Cell Nutriture

In addition to protecting and maintaining the required balance of ions and metabolites in the brain, the CSF relays diffusible signals that control neural stem cell proliferation in both healthy and diseased states (64). In a healthy individual, these signals control normal cortical development and contribute to the neuronal plasticity required for learning and memory (64, 116). Historically, disease or acute injury to the brain was thought to cause permanent damage to neurons. However, it has been shown that populations of stem cells are maintained in the adult brain in the subventricular zone (SVZ), and progenitor cells are maintained in the dentate gyrus of the hippocampus (25, 132). The SVZ contains stem cells that differentiate into neurons, astrocytes, and oligodendrocytes in the olfactory bulb (35). The CSF has also recently been shown to relay diverse and age-dependent signals from the lateral ventricular CP to SVZ stem cells, and these signals serve to maintain quiescence and differentially affect proliferation of unique SVZ stem cell populations (116). The subgranular zone of the hippocampal dentate gyrus is the site of adult neurogenesis of dentate granule cells. Dysregulation of neurogenesis in this niche may disrupt hippocampal processes such as learning and memory (134). There has been no systematic investigation of the role of the CSF in vitamin delivery to the cells of this niche. Similarly, the role of CSF-mediated vitamin delivery in maintaining the proliferative capacity of these cells, especially as related to disease progression, is also understudied. Such studies are limited, in part, by the lack of suitable, responsive biomarkers of the nutrient status and proliferative capacity of the stem cells.

TRANSPORT OF NUTRIENTS ACROSS BRAIN BARRIERS

Nutrient transport systems of the BBB and BCSFB are required to maintain CNS homeostasis as well as to allow for the selective concentration of specific nutrients and metabolites. Transporters may be unique to the BBB or BCSFB, or both, or they may also be present on other absorptive membranes throughout the body. In general, brain transporters are specific for their nutrient of interest; however, in some cases, a single transporter may bind multiple ligands, such as the mineral transporter divalent metal transporter 1 (DMT1) and the sodium-dependent multivitamin transporter (SMVT) (21, 121).

Carrier-Mediated Transport

Carrier-mediated transport is the primary method of nutrient transport into the CNS across the BBB (Figure 3; Table 1). The solute carrier (SLC) gene superfamily encodes transmembrane transporters, including facilitated transporters and secondary active ion-coupled transporters or exchangers (108). The major metabolic substrates of these membrane-bound transporters are small, hydrophilic molecules, including glucose, amino acids, vitamins, and other trace elements. One well-known BBB transporter is glucose transporter 1 (GLUT1, or SLC2A1), which is the primary glucose transporter located on BMEC membranes (118). GLUT1 is essential for maintaining CNS concentrations of the primary energy source utilized by the brain. Pantothenic acid (vitamin B₅; PA) and biotin (vitamin B₇) are transported by SMVT at the BBB (138). This SLC family transporter was shown to be concentrated at the luminal membrane and to be responsible for 88% and 98% of, respectively, biotin and PA transport in cultured human BMECs (138). Thiamine (vitamin B₁) is transported by thiamin transporter 1 (THTR1), and thiamine is essential for proper CNS function (2, 21). Folate is transported, in part, by the reduced folate carrier (RFC), which has been shown to be expressed on human brain microvessels (139). The sodium-dependent vitamin C transporter (SVCT2) is found primarily in the CP and is responsible for transporting the reduced form of vitamin C, AA, while GLUT1 and GLUT3 control uptake of the oxidized form, dehydroascorbic acid (DHA), into the CNS (40).

Figure 3.

Transport processes at the endothelial blood–brain barrier. Schematic representation of transport processes at the endothelial barrier, including active efflux, facilitated solute carriers, lipid-mediated passive diffusion, receptor-mediated transcytosis, and restricted paracellular transport. Adapted with permission from Reference 21.

Table 1.

Transporters at the blood–brain and blood–cerebrospinal fluid barriers responsible for delivering vitamins to the brain

Transporter and function	Vitamin (alternative name)	Brain location	Reference	Notes
Unknown: bidirectional transport via facilitated diffusion	B3 [niacin (nicotinamide)]	BBB	121
Unknown: active transport followed by conversion to NAD	B3 [niacin (nicotinamide)]	BCSFB	121
Unknown: bidirectional transport; facilitated diffusion	B6 (pyridoxine)	BBB	121
Unknown: bidirectional transport; facilitated diffusion with subsequent phosphorylation	B6 (pyridoxine)	BCSFB	121
RFC (SLC19A1): bidirectional carrier	B9 [folate (also transports thiamine monophosphate with low affinity)]	BBB and apical choroid plexus (also known as the BCSFB)	99, 121, 138, 139, 159
FRα (FOLR1): high-affinity transporter that is taken into cells via endocytosis	B9 (folate)	BCSFB	12, 44, 106
PCFT (SLC46A1)	B9 (folate)	BCSFB	12
THTR1(SLC19A2)	B1 (thiamine)	Ubiquitous	2, 21, 91, 121, 127	Ubiquitously expressed; necessary for thiamine transport in bone marrow, pancreas, inner ear; mutations cause thiamin-responsive megaloblastic anemia
THTR2 (SLC19A3)	B1 (thiamine)	Brain; localization unknown	67, 121	Genetic defect causes CNS symptoms and thiamine metabolism dysfunction syndrome 2
SLC25A19	B1 [thiamine (specifically transports thiamine pyrophosphate)]	Mitochondria	68
SLC52A2	B2 (riboflavin)	Brain	155
SMVT	B5 (pantothenic acid) B7 (biotin)	BBB and BCSFB	97,121,138	Sodium dependent
SR-B1	α-tocopherol	BBB	13, 43, 121	For ataxia related to vitamin E deficiency, high-dose supplementation is helpful
PLTP	α-tocopherol	BCSFB	33,39, 121
Afamin	α-tocopherol	BBB in vitro	61
SVCT2	C (ascorbic acid)	Basolateral BCSFB (apical unknown)	40, 118
GLUT1(SLC2A1)	C (dehydroascorbic acid)	BBB	40, 139
GLUT4	C (dehydroascorbic acid)	Astrocytes	66, 118

Abbreviations: BBB, blood–brain barrier; BCSFB, blood–cerebrospinal fluid barrier; CNS, central nervous system; NAD, nicotinamide adenine dinucleotide.

Receptor-Mediated Transport

Receptor-mediated transcytosis is an energy-dependent process used to transport proteins, peptide hormones, and carrier proteins across the BBB (Figure 3; Table 1). Receptor-mediated transport controls the entrance of large biomolecules, and as a result, it is studied extensively for therapeutic drug delivery across the BBB. This transport pathway requires large quantities of receptors on the luminal membrane of BMECs, which bind target ligands and allow for the endocytosis of the receptor–ligand complexes. Vesicular sorting occurs during transcytosis before ligands are released across the basolateral membrane (54). Iron bound to transferrin, cobalamin (vitamin B₁₂) bound to transcobalamin, low-density lipoproteins, and hormones, including insulin and leptin, are transported in this manner (21). The transferrin receptor is expressed both on BMECs and CP epithelial cells, which allows iron to enter the CNS bound to holotransferrin. α-Tocopherol, a form of lipophilic vitamin E, is transported across the BBB through selective uptake by scavenger receptor type B class 1 (SR-B1) receptors while associated with high-density lipoprotein (13), and it was shown in vitro to be transported to a lesser extent by association with afamin, a member of the albumin superfamily (61). Folate is also transported by folate receptor-α (FRα, encoded by the FOLR1 gene) from the blood to the CSF through the BCSFB.

Efflux Transporters

In addition to transporting required nutrients into the CNS, the brain barriers must also be able to export metabolic waste and other toxins from the brain parenchyma (Figure 3). Efflux transporters function to remove brain metabolites, toxic substances, and other xenobiotic compounds that can interfere with proper neurological function (108). Adenosine triphosphate (ATP)-binding cassette (ABC) transporters are the primary active efflux transporters present at the luminal membrane of BMECs. P-glycoprotein and members of the multidrug-resistance protein family, or organic anion transporter family, work in concert to increase efflux of xenobiotic compounds. Efflux transporters must be taken into consideration when discussing therapeutics since the presence of efflux transporters can reduce the penetration and efficacy of drugs (160) or potential interventions designed to deliver nutrients to the brain.

Sites of Nutrient Transport

The functional differences between the BBB and the BCSFB lie in the types of TJ proteins as well as the types of transport proteins and ion channels that are expressed in the cells that comprise each brain barrier (108). Specific claudin and TJ proteins are expressed differentially in the BBB and BCSFB (108), and this results in unique transport processes at each barrier. In addition, SLC and ABC transporters are differentially expressed on the BBB and BCSFB. Certain transporters may be present at both interfaces, while others are site specific (Table 1). Transporters can also be present on the apical membrane, basolateral membrane, or both. For example, at the BBB, P-glycoprotein is present at blood-facing endothelial membranes, while in the CP, P-glycoprotein is expressed on the CSF-facing side. The expression levels of transporters can also differ dramatically; for example, P-glycoprotein is the most abundant transporter at the BBB, while in the CP, it is expressed at 0.5% of the levels in BMECs (108).

EVIDENCE FOR VITAMIN CONCENTRATION IN THE BRAIN AND ASSOCIATED TRANSPORT MECHANISMS

Under normal conditions, the BBB regulates the exchange of nutrients and other constituents between the blood and the CSF of the CNS (70), creating a microenvironment that supports neurological functions (21). BBB and BCSFB functions are essential to maintain these nutrient concentration gradients. The roles of the BBB and BCSFB in nutrient transport and brain nutrition have been recently reviewed (21); here, we focus on vitamins that are either concentrated in the brain (relative to other tissues) or concentrated in the CSF (relative to the serum). Vitamins known to be concentrated across the BBB include niacin (vitamin B₃), pyridoxine (vitamin B₆), inositol, folate, and AA (121). However, a full inventory of nutrients that are concentrated across the BBB has not been established, and it is possible that other vitamins are actively concentrated within brain tissue. It is also worth noting that the concentration or accumulation of nutrients, or both, within the brain depends on the integrity of the transport systems for each nutrient and on vitamin turnover in both the CSF and the brain (121). In addition, the process of concentrating nutrients against such steep gradients often requires energy (i.e., ATP). As such, levels of vitamins in the brain can be depleted during reductions in energetic potential, even when whole-body nutrient status is adequate. Therefore, blood or serum biomarkers of vitamin status do not always report adequately on brain levels of vitamins. For example, brain folate levels remain constant even when whole-body folate levels are depleted (36, 121). Similarly, brain levels of vitamin C resist depletion in several animal models (46). There is evidence that vitamin E in the brain also resists depletion (29), although it is not concentrated in the CSF relative to the serum (121)—an apparent paradox that remains a gap in our knowledge of the mechanisms that maintain brain nutrient homeostasis.

Niacin (Vitamin B₃)

Niacin exists in several chemical forms within the body (niacin, nicotinamide, nicotinic acid). Niacin is a precursor of nicotinamide adenine dinucleotide (NAD) and nicotinamide adenine dinucleotide phosphate (NADP). NAD(H) and NADP(H) coenzymes serve in hydrogen transfer reactions that are critical for energy metabolism and for many biosynthetic reactions. In the plasma and the CSF, niacin exists primarily as nicotinamide (121). The primary site of transport of niacin into the brain (in the form of nicotinamide) is across the BMEC capillaries of the BBB (121) (Table 1). The transport of nicotinamide through the BBB is accomplished using low-affinity, high-capacity facilitated diffusion (121), which is similar to the nicotinamide transport (influx and efflux) systems in red blood cells (110, 121). Therefore, in practice, plasma levels of nicotinamide are a reasonable approximation of what the brain cells are exposed to (121). Conversely, the CP takes up nicotinamide using a high-affinity active transport system. However, much of the nicotinamide that is taken up by the CP is for use within CP epithelial cells and not for transport through the CP to the CSF (121).

While niacin and nicotinamide are not necessarily concentrated in the CSF relative to plasma, brain niacin and NAD levels resist depletion relative to liver tissue in niacin-deficient animal models. Similarly, elevated plasma niacin levels do not necessarily lead to increased CSF levels (121). The specific effects of brain niacin deficiency, which frequently occurs secondary to whole-body niacin deficiency, are difficult to characterize because they almost always occur in conjunction with other nutrient deficiencies (63). In animal studies, nicotinamide supplementation limited BBB break down following traumatic brain injury and also prevented neuronal apoptosis and necrosis (49). There has been considerable interest in the neuroprotective effects of niacin, or more specifically of NAD, with respect to several neurodegenerative disorders, as has been reviewed elsewhere (16, 146), although results are conflicting, and the mechanisms remain unclear. More recently, there is evidence that some of these discrepancies may be due to age at the time of injury, with younger animals subjected to cortical contusion injury exhibiting a better response to nicotinamide supplementation than older animals subjected to the same injury (130, 146).

Pyridoxine (Vitamin B₆)

Pyridoxine (PN; vitamin B₆) is a water-soluble B vitamin that is obtained through the diet in nonphosphorylated forms [PN, pyridoxal (PL), pyridoxamine] that are readily phosphorylated by pyridoxal kinase in the liver and then, ultimately, converted to the active form of the vitamin, pyridoxal 5′ phosphate (PLP), by pyridoxamine phosphate oxidase (PNPO) (126). The major form of vitamin B₆ in circulation is PLP, and it is bound to either albumin or hemoglobin; the major form of B₆ in CSF is PL (5, 30). Once transported into the brain, PL is converted back to PLP (5, 30, 121). Inborn errors of metabolism in PNPO lead to impaired PLP synthesis and metabolism, and they are almost always associated with seizures and other neurological impairments that are frequently fatal if undiagnosed (30, 83, 84). Many PNPO patients respond to PLP, and some respond to PN, treatment (30, 83). Studies in the rat brain suggest that total vitamin B₆ concentrations in the CSF and brain are more than 25-fold higher than in blood (48, 119). Studies in children reveal that PL is the most abundant vitamer in the CSF and that PL concentrations in the CSF are about 1.3-fold higher than in plasma, but this is not the case for PLP and PA (5). This concentration gradient was not observed in infants in a study of PL levels in CSF compared with serum levels (48). Although transport of all nonphosphorylated forms of B₆ is believed to occur by facilitated diffusion at both the BBB and BCSFB, the exact mechanisms are not fully elucidated (121). The phosphorylation of PL to PLP is believed to account for the accumulation, retention, and slow turnover of PLP in brain tissue (121). Increased oxidative stress associated with inflammation has been shown to cause increased catabolism of vitamin B₆ (141). However, the effect of increased whole-body vitamin B₆ catabolism or brain inflammation, or both, on brain and CNS B₆ levels remains unclear.

Folate (Vitamin B₉)

Folate is a water-soluble B vitamin that carries and chemically activates one-carbon units for biosynthetic reactions, including de novo synthesis of purine nucleotides, de novo synthesis of thymidylate, and remethylation of homocysteine to methionine. Methionine is a precursor of S-adenosylmethionine, which serves as a methyl donor for more than 100 biological methylation reactions, such as DNA methylation, histone methylation, and neurotransmitter synthesis. Folate transport is mediated by three distinct receptors and transporters, with differing expression levels in specific tissues: folate receptors (FRα and FRβ), RFC1 (SLC19A1), and the proton-coupled folate transporter (PCFT). PCFT acts as a proton–folate cotransporter and is responsible for intestinal folate uptake, operating optimally at pH 5.5. RFC1 uses an antiport mechanism to transport reduced folates in many other cell types. Folate is four times more concentrated in the CSF than in serum and plasma, and the brain resists depletion compared with the liver and other tissues in animal models (36, 120). The transport of folate from the blood to the CSF occurs at the BCSFB and is mediated by FRα. FRα is anchored to the plasma membrane through a glycosylphosphatidyli-nositol linkage and is taken into cells via receptor-mediated endocytosis after binding folate. Experiments in rat CP have demonstrated that FRα-bound folate is endocytosed at the basolateral membrane of the cells of the CP and transcytosed in endosomes to the apical membrane, where the folate-bound receptor is released into the CSF in exosomes (44). PCFT is expressed throughout the human brain (60), and there is some evidence to suggest that it localizes to endosomes, where it serves to release folate from the endosome to the cytoplasm (44, 60). FRα has a high affinity (transport affinity <10 nM) for reduced folates, such as 5-methyltetrahydrofolate (5-methylTHF), the form of folate found in plasma, indicating that at normal plasma folate concentrations, FRα at the BCSFB is likely always saturated with folate. This, in part, contributes to the ability of the BCSFB to concentrate folate within the CSF. Because receptor-mediated endocytosis is an energy-requiring process, inborn errors of metabolism that affect either FRα expression or function or mitochondrial function (and, ultimately, energy production) are associated with cerebral folate deficiency (125). FRα has a high affinity for reduced folates, such as 5-methylTHF and 5-formylTHF (folinic acid), and treating individuals with inborn errors of metabolism with high levels of reduced folates often leads to some clinical improvement (125). Folic acid is not interchangeable with reduced folates such as 5-methylTHF. Folic acid is a non-natural provitamin that first must be converted to a reduced folate form before it can serve a biological function in the cell, and this bioconversion rate is very slow (98). High-dose folic acid is not effective and not recommended in cases of cerebral folate deficiency due to low rates of folic acid transport across the BCSFB (60) and low brain levels of dihydrofolate reductase, the enzyme required to convert folic acid into a biological and functional reduced folate cofactor. Furthermore, experimental studies have shown that folic acid in physiological doses can inhibit the transport of 5-methylTHF across the BCSFB (60, 153).

Vitamin C (Ascorbic Acid)

Oxidative stress can be toxic to all cell types, and neuronal cells can be particularly vulnerable to oxidative stress, as has been reviewed extensively (148). Antioxidants such a vitamin E and vitamin C play critical roles in preventing oxidative stress in the brain (29, 46, 47). Vitamin C is concentrated in the CSF relative to the serum. In addition, one of the earliest clinical descriptions of human scurvy (whole-body vitamin C deficiency) noted that brain tissue appeared remarkably unaffected by the deficiency (3), suggesting that brain tissue resists vitamin C depletion. Rodents, unlike humans, can synthesize vitamin C. Mice that lack the enzyme responsible for the final step of AA synthesis (gulonolactone oxidase) develop vitamin C deficiency and oxidative stress in the brain within days after birth (47). Vitamin C also plays critical roles in neural maturation and neurotransmission, and there is evidence that vitamin C helps to maintain intracellular vitamin E levels in neuronal tissue (46), although vitamin E itself is not concentrated in the CSF relative to the serum (121). In mammals, AA levels are maintained at 0.05 mM in plasma, but are increased to 0.2 and 1.2 mM in the CSF and brain, respectively (121, 144). SVCT2 is a sodium- and energy-dependent transporter that concentrates AA into the CSF against the concentration gradient (46). In fact, mice lacking this brain vitamin C transporter (Svct2^−/−) have no detectable AA in the brain and die at birth (90). Mice with reduced SVCT2 levels (Svct2^+/−) exhibit decreased AA levels in the brain and other tissues compared with wild-type littermates (117). Evidence suggests that neurons and epithelial cells associated with the CP account for most of the total ascorbate within the brain and this accumulation is SVCT2 dependent (90). DHA, an oxidized form of vitamin C, is also transported by the GLUT1 transporter in astrocytes of the endothelial BBB (3). GLUT1 was first identified as a facilitative glucose transporter, and GLUT1-mediated transport of DHA is inhibited by D-glucose (3). DHA is reduced back to biologically active AA within the BBB (i.e., in astrocytes) (3). Clearly, this transport mechanism cannot compensate for the loss of SVCT2. However, the uptake of DHA by GLUT1 and conversion back to AA within the astrocytes has been proposed as a mechanism whereby the brain resists vitamin C depletion, as astrocytes can release AA for subsequent uptake by neurons (90).

Nutrients that Resist Depletion in Brain Tissue

Some nutrients are not actively concentrated in the CSF relative to the serum, but nutrient levels within the brain resist depletion when faced with whole-body nutrient deficiency. These include thiamine, PA, and cobalamin. Thiamine is a water-soluble B vitamin that, unlike other B vitamins discussed herein, is not concentrated within the CSF. However, depletion of brain levels by as little as 20% leads to severe neurological complications (121). In comparison to other water-soluble B vitamins that accumulate in the brain relative to other tissues, brain thiamine turnover approaches 100% per day as compared with less than 20% per day for other B vitamins. In most cases, thiamine deficiency in the brain is secondary to whole-body deficiency associated with nutrient malabsorption disorders, as is frequently observed in chronic alcoholics. Chronic alcoholics are often plagued by a series of conditions known as Wernicke’s encephalopathy, Korsakoff’s syndrome, or Wernicke–Korsakoff syndrome (2). Because of the rapid turnover of thiamine in the brain and the propensity of alcoholic patients to regress, even thiamine supplementation therapy is often ineffective in treating neurological symptoms. This is due, in part, to the fact that large oral doses of thiamine do not increase CSF thiamine levels (121).

Vitamin B₁₂ is also a water-soluble B vitamin that serves as a cofactor for two enzymatic reactions: the methionine synthase (MTR)-catalyzed conversion of homocysteine to methionine and the methylmalonyl–coenzyme A (CoA) mutase (MCM)-catalyzed conversion of succinyl–CoA to methylmalonyl-CoA. The MTR-catalyzed reaction occurs in the cytosol and relies on methylcobalamin, whereas the MCM-catalyzed reaction occurs in the mitochondria and uses adenosylcobalamin. Vitamin B₁₂ deficiency leads to hematological pathology, including megaloblastic anemia and several neurological pathologies (60). Vitamin B₁₂ absorption and transport throughout the body have been relatively well characterized, as recently reviewed (45). In addition, vitamin B₁₂ deficiency is common in the elderly and strict vegans (45). Considerably less is known regarding vitamin B₁₂ transport into the brain and CSF. The normal range for vitamin B₁₂ levels in the CSF (6–28 pmol/L) is considerably lower than in the serum (200–517 pmol/L) (72), and other studies have indicated that there is no correlation between serum and CSF vitamin B₁₂ levels in individuals (88). Interestingly, more recent studies have indicated that brain vitamin B₁₂ levels decrease with age in humans and that individuals with schizophrenia or autism exhibit lower brain vitamin B₁₂ levels than those without these disorders (157).

PA is a water-soluble B vitamin that serves as the precursor of CoA. CoA and CoA derivatives, especially acetyl-CoA, function in numerous metabolic and catabolic processes that are essential for energy metabolism, neurotransmitter synthesis, and cholesterol synthesis, among others (77). PA deficiency is associated with neurological complications that affect both the central and peripheral nervous systems (58). PA is not concentrated in the CSF relative to the serum, but studies have shown that brain PA resists depletion when faced with whole-body deficiency (121). In animal models and human BMECs, PA is transported through the BBB and BCSFB by SMVT, which also transports biotin, as discussed above (121, 138). It is believed that PA resists depletion in the brain due to slow rates of CoA turnover (121).

EVIDENCE FOR BARRIER BREAK DOWN AND BRAIN-SPECIFIC VITAMIN DEFICIENCIES IN DISEASE STATES

It is increasingly recognized that the nutritional needs of the CNS change as a result of chronic disease or other adverse states. Chronic diseases can be associated with brain-specific nutritional deficiencies that manifest in the absence of whole-body nutritional deficiencies. These brain-specific deficiencies are caused by BBB dysfunction or inflammation, or both, and result in an increased risk for disease-associated comorbidities (e.g., depression). Certain vitamins are concentrated many-fold in an energy-dependent process as they move from the serum to the CSF. The import, retention, and stability of vitamins determine individual vitamin levels in the brain. Vitamin depletion in the CSF can occur via impaired nutrient transport due to BBB or BCSFB dysfunction or through elevated rates of catabolism, or both, as a result of pathogenic processes such as inflammation. BBB dysfunction and increased nutrient catabolism can be caused by disease- and age-related symptoms, such as chronic inflammation, mitochondrial dysfunction or depletion, stroke, hypoxia, hypertension, diabetes mellitus, Alzheimer’s disease (AD), and multiple sclerosis (70, 86, 133, 160). These pathologies, as well as natural aging, can lead to BBB break down through several mechanisms: the opening of TJs, dysfunction of transporters and receptors, and decreased levels or function of mitochondria, which lead to a loss of energetic potential (160). When break down of the BBB occurs, plasma proteins and immune cells can enter the brain parenchyma (101); environmental exposures as well as genetic determinants of BBB function also influence the development of brain nutritional deficiencies (100, 125).

Inflammation and Brain Vitamin Accumulation

The immune inflammatory response is a normal clinical occurrence resulting from disease or injury. The incidence of chronic inflammation is on the rise, due partially to an increase in global rates of obesity and metabolic disorders. Chronic inflammation due to systemic infection or disease can lead to disruptive BBB alterations and an inability to concentrate required nutrients across brain barriers. More specifically, the loosening of TJs due to aging and chronic inflammation can increase the permeability of the barrier and lead to impairments in concentrating nutrients into the CSF, as well as infiltration of immune cells into the brain parenchyma (101). The formation of lesions in multiple sclerosis has been associated with focal BBB dysfunction due to atypical TJ protein localization, which allows for the penetration of inflammation-promoting cells (101). During systemic inflammation, disruptive and nondisruptive changes in the BBB can occur (143). Disruptive histological changes in BBB structure result from increased TJ permeability, increased vesicular activity, astrocytopathy, a break down of the inner BBB layer (also known as the glia limitans), and BMEC damage. The functional changes in BMECs can be induced in vitro and in vivo (with effects varying by age, sex, and health of the animal) using lipopolysaccharide injections as a model of systemic inflammation (143). Other models of systemic inflammation include cytokine injection and bacterial infection (143). Nondisruptive or molecular changes in BBB architecture can also occur due to inflammation, although it is not clear how these changes affect nutrient transport across the barrier.

Disease states and inflammation are also known to affect rates of nutrient catabolism (128, 140). While this accelerated catabolism primarily lowers whole-body (i.e., serum and plasma) nutrient levels, it may also exacerbate brain and CSF nutrient depletion for vitamins that are concentrated from the serum and plasma to the CSF, such as vitamins B₆ and B₉. Chronic systemic inflammation can lead to differential regulation of specific nutrient transporters, such as amino acid and B vitamin transporters (143), which contributes to the pathology of chronic, inflammation-associated cognitive decline (112). The causal role of inflammatory processes in inducing brain-specific vitamin deficiencies has not been systematically assessed.

Diabetes

Type 1 and type 2 diabetes are associated with an array of neurological disorders that occur as a result of BBB breakdown (112). Possible mechanisms leading to brain microvascular dysfunction are diabetic hyperglycemia, which causes increased oxidative stress; the accumulation of advanced glycation end products; and activation of the polyol pathway and protein kinase C (6). The prevalences of ischemic stroke and dementia are increased in diabetic patients (11, 112), and managing these comorbidities is an increasing medical concern. An in vitro study of a BBB co-culture model showed that there was an increase in BBB permeability in a hyperglycemic environment and that this permeabilization was attenuated by the addition of individual antioxidants, including vitamin C (6).

Inborn Errors Affecting Nutrient Transport

Inborn errors of metabolism affecting nutrient transporters can alter the nutritional status of the brain. FRα was shown to be localized, in large part, to the apical BCSFB of the CP epithelium for the transcytosis and endocytosis of folate (44). FRα mutations lead to cerebral folate transport impairment and have been shown to be associated with severe developmental disorders, motor dysfunction, and epilepsy. Treating patients with folinic acid, a reduced form of folate, leads to improvements in myelination and associated cognitive impairments (124). As discussed in the section titled Folate and Depression, the development of autoantibodies to FRα can lead to adult-onset cerebral folate deficiency. Although the etiologies of cerebral folate deficiency due to inborn errors of metabolism in childhood and the development of autoantibodies in adulthood are distinct, effective treatments of the resulting cerebral folate deficiency are the same (125).

Mitochondrial Dysfunction

Mitochondrial dysfunction has been implicated in neurodegeneration and epilepsy. BMECs have a high concentration of mitochondria compared with nonneural endothelial cells (92). Mitochondrial DNA depletion syndrome is commonly associated with neurological pathology and brain abnormalities, as assessed by magnetic resonance imaging (102). Additionally, the neurovascular unit shows decreased BMEC mitochondrial density in aging and AD (76). Because the BBB is an energetic barrier, mitochondrial dysfunction is hypothesized to result in a secondary decrease in ATP-dependent nutrient transport and the decreased ability of the brain to concentrate required nutrients, such as folates (85).

Alpers–Huttenlocher syndrome, a type of mitochondrial DNA depletion syndrome, occurs as a result of a mutation in the POLG gene (105, 137). POLG encodes for the catalytic subunit of DNA polymerase-γ, which is the only DNA polymerase present in mitochondria. Mitochondrial DNA mutations increase with age, making a mouse model of this disease ideal for studying the effects of age-related disease on brain nutrient deficiencies and cognitive function (137).

Pharmacological Agents and Drugs of Abuse Cause Barrier Dysfunction

Pharmacological agents and drugs of abuse have also been known to cause BBB dysfunction. Alcohol abuse is one of the most common factors in brain thiamine deficiency and associated pathologies, such as Wernicke’s encephalopathy. Lack of thiamine can lead to decreased mitochondrial function, increased oxidative stress, BBB break down, and neuronal dysfunction and degeneration (2). The use of drugs of abuse may also influence BBB function acutely or chronically (70). Methamphetamine and cocaine use have been associated with altered expression of TJ proteins (34, 73) and with increased BBB permeability in the hippocampus (78). However, drugs may also be used therapeutically to induce opening of the BBB for site-specific and overall drug targeting to the CNS. Pharmacological permeabilization has been used to deliver drugs that could not typically cross the BBB (69); however, the efficacy of these types of treatments has not been thoroughly investigated for nutritional therapies.

Stroke

Stroke is the leading cause of long-term disability in the United States, with approximately 800,000 cases occurring each year (56). Neurological symptoms following a stroke are a major health problem, as a stroke is more often debilitating than lethal. Stroke causes massive permeabilization of the BBB and also causes CNS nutrient depletion (22). Following ischemia or hypoxia, matrix metalloproteinases are released that cause a reversible breakdown in TJ proteins and increased paracellular transport of molecules (154). More than 80% of all strokes are ischemic strokes, which block the flow of blood to the brain, leading to a lack of oxygen and glucose and long-term disability or death (133). Hypoxia (due to decreased blood flow) and subsequent reoxygenation-induced oxidative stress have been shown to alter BBB permeability in vivo, potentially due to decreased occludin localization to TJs (71). A hemorrhagic stroke can occur secondary to an ischemic stroke or spontaneously as a result of multiple etiologies (56). In a hemorrhagic stroke, components of blood enter the brain parenchyma, leading to cell damage, inflammation, immune cell infiltration, and microglial activation, all of which can result in BBB dysfunction (56). The role of stroke in affecting the short- and long-term nutritional status of the brain is largely unknown.

Biological Markers of Blood–Brain Barrier Integrity and Function

In aging, neurodegenerative disorders, and traumatic brain injury, the BBB can become compromised, leading to the leakage of CNS-localized proteins into the periphery (14, 18). Several biomarkers have been identified that report on BBB integrity, with the most common biomarkers including various interleukins, S100 calcium-binding protein B (S100B), glial fibrillary acidic protein (GFAP), and the CSF:albumin quotient (131). Elevated levels of these biomarkers in the CSF or serum can reveal the presence or cause, or both, of neurological dysfunction. Interleukins in the serum and CSF are used to measure neuroinflammation and can indicate whether there is chronic or acute inflammation (18, 55, 75). Transthyretin, a transporter of vitamin A, and thyroid hormones found in CSF have both been implicated as markers of BBB integrity in schizophrenia and AD (51). Transthyretin in serum has been proposed to be a biomarker of BCSFB dysfunction in a study examining serum from patients undergoing BBB disruption for treatment of primary brain lymphoma (75). Elevated serum S100B, an astrocytic protein, is indicative of BBB dysfunction due to acute brain injury, but it can also be found in patients with neurodegenerative disorders. Elevated CSF neurofilament light (NFL) protein is used to diagnose neuronal damage, even in HIV patients asymptomatic for neurocognitive disorders (4). In addition, CSF biomarkers can be used to diagnose neurometabolic diseases, such as glucose transporter deficiency and vitamin transporter and receptor defects including in folate receptor 1 (FOLR1) and thiamine transporter 2 (THTR2, or SLC19A3) (14). However, several challenges and inadequacies exist with current biomarkers of BBB integrity. Time sensitivity, confounding sites of release, sensitivity of detection, and contamination of CSF by blood (especially in small model organisms) all present issues for establishing validated and robust biomarkers of BBB integrity.

DISTINCT NUTRITIONAL REQUIREMENTS FOR MAINTAINING BRAIN NUTRITION IN DISEASE STATES

The BBB can be compromised by age, chronic inflammation, hypoxia, diabetes mellitus, and other neurological diseases (70), which can lead to the brain undernutrition observed in disease (21). As a result, patients may have specific nutritional requirements to prevent disease comorbidities that result from these brain-specific nutritional deficiencies.

Folate and Depression

In the Institute of Medicine’s 1998 report on the DRI levels of various nutrients, it addressed the link between folate deficiency and psychiatric disorders (52). This report concluded that “the database linking folate to altered mental function is not large but appears sufficient to suggest the likelihood of a causative association” (52, p. 267). This was confirmed by the World Health Organization’s report from 2008 on folate and vitamin B₁₂ deficiencies that also recognized the evidence “that low serum or red blood cell folate concentrations are associated with either a higher prevalence or a longer duration of depression” (31, p. S240). More than 30 neurological and psychiatric diseases have been associated with cerebral folate deficiency. The brain is susceptible to folate deficiency due to impairments in both folate metabolism and energy metabolism. The body concentrates folate across the BCSFB by about fourfold in the CSF relative to the plasma, which is an energy-requiring process; patients with inborn errors of metabolism affecting folate utilization, energy metabolism, or folate transport across the barrier are at increased risk for cerebral folate deficiency and subsequent neurological and neuropsychiatric conditions, even in the absence of whole-body folate deficiency.

In addition, decreased folate transport activity resulting from autoimmunity or compromised BBB function can result in biochemical and clinical sequelae in adults that are similar to what is observed in individuals (often children) who present with cerebral folate deficiency, as has been recently reviewed (125). The administration of high-dose reduced folate, which includes 5-methylTHF, to patients with folate receptor autoimmunity and to populations with folate-related inborn errors of metabolism restores CSF folate levels and is associated with improvements in clinical conditions, as evidenced by seizure control as well as normalization of brain metabolites, white matter lesions, and hypomyelination. Interestingly, mouse models of cerebral folate deficiency exhibit increased endothelial BCSFB permeability and mislocalized TJ proteins (147). Perhaps more interestingly, supplementation with reduced folate restored BCSFB integrity in this model (147).

Although cerebral folate deficiency can occur in the absence of whole-body deficiency, decreased serum folate may affect CSF folate levels. Folate- and vitamin-B₁₂-associated pathologies are common and increase with age. They are complex, polygenic traits that result from delete-rious gene–nutrient interactions (38). Folate-mediated one-carbon metabolism (FOCM) can be impaired by deficiencies of folate and other B vitamins and genetic mutations and single nucleotide polymorphisms (10, 81, 113, 142). Nutritional and genetic epidemiological studies and randomized controlled trials have implicated impaired folate metabolism in several pathologies, including neural tube defects (41), neurodegenerative and neuropsychiatric diseases (62, 129), and cancer (8, 17, 23, 59, 103).

Several genetic polymorphisms affect folate status. The best-characterized gene variant is the C677T methylenetetrahydrofolate reductase (MTHFR) polymorphism. It has biologically meaningful impacts on both the functioning of FOCM as well as on whole-body folate status. The dual effects of the MTHFR polymorphism on both folate status and the functional capacity of FOCM have challenged our ability to determine the mechanism underlying the association between the C677T MTHFR polymorphism and risk of disease. The association could be caused by the impact of the polymorphism on folate status or its impact on MTHFR catalysis, including its role in providing 5-methylTHF for the homocysteine remethylation pathway and cellular methylation reactions. In the case of the relationship between folate and neural tube defects, there is increasing evidence that the primary contribution of the MTHFR variant is likely due to decreased folate status as opposed to effects on homocysteine remethylation and cellular methylation capacity (136).

Major depressive disorder (MDD) is projected to be the second leading global disease by 2020. Standard pharmacological treatments, such as selective serotonin reuptake inhibitors (SSRIs), yield a response to medication in less than 50% of patients. The etiology of MDD is poorly understood, but it is likely to involve a complex interplay of sensitivities to psychosocial stressors and alterations in metabolic pathways. Peripheral biomarkers suggest that BBB integrity is compromised in depressed patients (87, 89). Patients with depression or schizophrenia are more likely to have common genetic variants that contribute to folate deficiency, and many of these polymorphisms are also associated with neuropsychiatric disease (41, 111). The MTHFR C677T polymorphism is also associated with an increased risk of depression and schizophrenia, with up to 70% percent of patients with either depression or schizophrenia having some form of the polymorphism (57). Patients with depression or schizophrenia are also more likely to have autoimmunity to folate receptors (107).

Using folic acid, a synthetic form of folate, as a dietary supplement is not sufficient to reduce or eliminate the risk of all pathologies associated with impaired FOCM (79), especially pathologies resulting from secondary nutrient deficiencies (e.g., of vitamin B₁₂) or those associated with genetic subpopulations involving gene–nutrient interactions (50) or autoimmunity (123). Overall, folic acid supplementation, even at doses exceeding the tolerable upper intake level, has not resulted in symptom improvement in depressed patients (7, 15, 24, 28, 109, 145). In contrast, two small studies have shown that 15–30 mg/day of 5-methylTHF, but not 7.5 mg/day, resulted in significant and clinically meaningful improvement in depression symptoms in individuals with MDD also taking SSRIs (95, 96). These studies demonstrate that reduced forms of folate, not folic acid, are most effective in addressing CNS-specific folate deficiencies and that the dose is an important modifier of the effect (125). However, there exists a need for large, well-controlled trials to elucidate the efficacy of reduced folates for treating depression symptoms and augmenting pharmaceutical approaches.

It is increasingly recognized that vitamin B₁₂ deficiency and genetic variants in the transcobalamin II receptor, which is required to transport vitamin B₁₂ into peripheral tissues (94), interact to increase the risk for folate-associated pathologies (149), and this may also be true for cerebral pathologies. Brain vitamin B₁₂ levels decrease with age and are also decreased in individuals with schizophrenia or autism (157). Although CSF vitamin B₁₂ levels are not concentrated relative to serum levels, the relationship between brain vitamin B₁₂ status and the functioning of FOCM within the brain has not been rigorously investigated.

Barrier Disruption as a Result of HIV Infection

Before the discovery of antiretroviral treatments, as many as 30% of HIV-positive patients developed dementia. Although the rates of HIV-associated dementia are decreased with antiretroviral treatment, neurocognitive decline remains a common comorbidity in HIV infection (4). Several in vitro and in vivo models demonstrate functional impairment of the BBB as a result of HIV infection (80). The nature of the relationship between BBB function and brain nutrient levels as a result of HIV infection remains largely unknown. One recent cross-sectional observational study assessed the association of plasma homocysteine, vitamin B₁₂, and folate levels with NFL levels in the CSF of HIV-positive patients (4). The release of NFL into the CSF is a sensitive biomarker of neuronal injury that may be a predictive biomarker of dementia onset (4, 42), and homocysteine is a functional biomarker of decreased folate and/or vitamin B₁₂ status (37). Consistent with previous studies, about 25% of HIV-positive patients exhibited increased plasma homocysteine (4). Interestingly, there was an independent correlation of plasma homocysteine with CSF NFL levels, suggesting an association between functional impairments in folate or vitamin B₁₂ metabolism, or both, and the extent of neuronal damage in HIV infection (4). However, it is important to note that plasma homocysteine levels do not necessarily reflect brain vitamin status (folate or B₁₂). Additionally, although elevated plasma homocysteine is associated with several pathologies, including cardiovascular disease and stroke, lowering homocysteine with folate or vitamin B₁₂ supplementation, or both, does not affect disease progression (27, 158).

Barrier Function and Alzheimer’s Disease

BCSFB break down as a result of aging and AD is widely recognized, and the mechanisms whereby the barrier deteriorates during normal aging and AD have been recently reviewed (82). This senescence of the CP affects the cellular morphology of the epithelial cells of the CP and also affects the ability of the cells to secrete and renew the CSF. This senescence is due, in part, to the accumulation of amyloid-β peptide that is a hallmark of AD. Amyloid-β peptide accumulation disrupts the TJ proteins that comprise the barrier and also leads to mitochondrial dysfunction in CP cells, affecting energy-requiring processes (82). The relationship between AD-induced BCSFB break down and brain nutrient levels, however, is complex. A recent meta-analysis demonstrated that patients with AD exhibit lower CSF or brain levels, or both, of several vitamins, many of which have been shown to be concentrated in the CSF relative to the serum, but these decreased CNS levels are also accompanied by lower serum nutrient levels (32). Therefore, it seems unlikely that compromised nutrient transport through the BBB and BCSFB is the sole reason for lowered brain nutrient levels, although it could be a contributing factor. In addition, oxidative stress in the AD brain has also been well documented (104), and there is evidence that the CSF:serum ratio of vitamin C (an antioxidant) is increased in individuals with AD or other neurological conditions, suggesting compensatory mechanisms (104). In addition, vitamin E has been shown to increase the clearance of amyloid-β peptide. However, vitamin supplementation has generally not proved efficacious in treating AD (23, and references therein). This is true even when CSF vitamin levels (vitamin C and vitamin E) are increased and oxidative stress is decreased by the intervention (9). Several observational studies have demonstrated an association between elevated homocysteine and cognitive decline or AD. As is the case with several other pathologies, although B vitamin supplementation trials have demonstrated the efficacy of folic acid and vitamin B₁₂ (and other B vitamins) in lowering serum homocysteine, supplementation did not affect cognitive aging or AD progression (26, 156). One could argue that these trials used folic acid, which does not effectively increase folate levels in the CSF or brain, and that CSF vitamin levels were not measured.

SUMMARY

The rates of chronic diseases that impact the CNS are increasing globally, and it is also increasingly recognized that disease-related pathological processes such as inflammation and oxidative stress impact nutrient accretion in the brain or increase rates of nutrient catabolism, leading to brain-specific nutrient deficiencies. There is limited evidence that restoring brain vitamin levels (i.e., brain folate levels) can lead to functional restoration of brain barrier break down, although this reciprocal relationship between brain vitamin levels and barrier integrity has not been systematically evaluated. There is also some evidence that restoring brain nutrient levels can restore BBB function and combat chronic diseases such as depression, but more rigorous, well-controlled human trials are needed. Although several biomarkers of brain barrier break down have been characterized, the identification of brain-specific vitamin deficiencies resulting from disease-induced barrier break down presents unique challenges, with respect to both diagnosis and management. In most cases, robust biomarkers reporting on tissue-specific (especially brain and CNS) vitamin deficiency are lacking. The discovery of such biomarkers will allow for identification of the causal relationships between disease-related processes and brain vitamin deficiencies, while also allowing for identification of nutritional interventions that restore and maintain brain vitamin levels following injury or disease.

Glossary

CNS

central nervous system

Blood–brain barrier (BBB)

composed of endothelial cells connected by tight junctions that filter harmful substances from the blood and transport nutrients from the blood to interstitial fluid

ISF

interstitial fluid

BMEC

brain microvascular endothelial cell

TJ

tight junction

Blood–cerebrospinal fluid barrier (BCSFB)

barrier between the blood and the cerebrospinal fluid; composed of epithelial cells of the choroid plexus

Cerebrospinal fluid (CSF)

fluid secreted by the cells of the choroid plexus that surrounds, protects, and nourishes the brain

CP

choroid plexus

SMVT

sodium-dependent multivitamin transporter

PA

pantothenic acid

SLC

solute carrier

SVCT2

sodium-dependent vitamin C transporter

DHA

dehydroascorbic acid

ABC

adenosine triphosphate (ATP)-binding cassette

NAD

nicotinamide adenine dinucleotide

NADP

nicotinamide adenine dinucleotide phosphate

PN

pyridoxine

PL

pyridoxal

PLP

pyridoxal 5′ phosphate

PNPO

pyridoxamine phosphate oxidase

FOCM

folate-mediated one-carbon metabolism