ReviewUridine and cytidine in the brain: Their transport and utilization
Introduction
The circulating pyrimidines uridine and cytidine, besides being incorporated into nucleic acids, can serve as substrates for the salvage pathway of pyrimidine nucleotide synthesis; as precursors of the cytidine triphosphate (CTP) needed in the phosphatidylcholine (PC) and phosphatidylethanolamine (PE) biosynthetic pathway (Kennedy and Weiss, 1956); and as precursors for the UDP (uridine diphosphate) and UTP (uridine triphosphate) that activate brain P2Y receptors (von Kugelgen, 2006) and that promote brain glycogen synthesis via UDP-glucose (Brown, 2004). In humans, the predominant circulating pyrimidine is uridine (Wurtman et al., 2000); in rats, it is cytidine (Traut, 1994); these variations probably reflect the species differences in cytidine deaminase, the enzyme that converts cytidine to uridine in the body. The transports of these pyrimidines into the brain's extracellular fluid, and then into neurons and glia, are essential prerequisites for these nucleosides to be utilized in brain. This review describes the mechanisms currently believed to mediate their passage across the blood–brain barrier (BBB) and then into brain cells. It also considers the biochemical consequences of changing brain uridine and cytidine levels, and related therapeutic implications.
CDP-choline is an endogenous intermediate which is produced in the rate-limiting step of PC biosynthesis via the Kennedy pathway (Kennedy and Weiss, 1956) (Fig. 1). Thus, CDP-choline has been extensively studied for its role in neuronal membrane synthesis. Exogenously administered CDP-choline is metabolized to cytidine and choline in rats (Weiss, 1995), while it is metabolized to uridine and choline in humans (Wurtman et al., 2000). Thus, besides choline, cytidine and uridine have attracted attention as precursors for endogenous CDP-choline production. An efficient mechanism mediating the brain uptake of circulating cytidine has not yet been demonstrated. When double-labeled CDP-choline was administered orally to rats, brain cytidine radioactivity peaking 4–6 h after the labeled CDP-choline was very low compared with that in liver (i.e., 0.2% vs. 60% of administered dose for brain and liver, respectively [Galletti et al., 1991]). Cytidine administered intracerebroventricularly (i.c.v.) does enter the brain (Trovarelli et al., 1982), but uptake via this route bypasses BBB transport.
On the other hand, the brain is known to take up circulating uridine. This uptake was initially demonstrated in the early 1970s (Hogans et al., 1971). Subsequently, Cornford and Oldendorf (1975), using a single injection method, showed that adenine, adenosine, guanosine, inosine and uridine all are able to cross the rat BBB, while cytidine, thymidine and their respective bases are not. Further studies on nucleoside transport suggested that nucleoside transporters could be classified based on their sensitivity to the nucleoside analog S-(4-nitrobenzyl)-6-thioinosine (NBTI) into two groups as NBTI-sensitive (es) or NBTI-insensitive (ei) (Belt, 1983). A few years later, the finding of a totally different type of nucleoside transporter family in mouse intestinal epithelial cells (Vijayalakshmi and Belt, 1988) gave rise to a new classification of nucleoside transporters, i.e., into equilibrative (Na+-independent, low-affinity) and concentrative (Na+-dependent, high-affinity) transporter families. The properties and substrate specificity of each family have been reviewed in Table 1. Detailed information can be found elsewhere (Baldwin et al., 2004, Gray et al., 2004, Kong et al., 2004, Podgorska et al., 2005).
Recent studies employing expression cloning and reverse transcriptase-PCR (RT-PCR) methods revealed the expression of a high-affinity nucleoside transporter (CNT2) at the rat BBB for purines like adenosine, and the pyrimidine uridine (Li et al., 2001, Redzic et al., 2005), but not for cytidine. Since BBB transport is the major determinant of brain uptake of most circulating compounds (Pardridge, 2001), this recent finding may open new avenues for exploring the possible effects of cytidine or uridine sources in neurodegenerative disorders like Alzheimer's disease.
Section snippets
Transport of cytidine and uridine across the BBB
Circulating cytidine and uridine can both be transported across the BBB by equilibrative transporters. Moreover, a concentrative system, as mentioned above, has also recently been shown to mediate BBB uridine transport. Equilibrative transport proteins (ENT, SLC29 family) exhibit characteristics of low-affinity nucleoside transport with Km values for their substrates in the high micromolar range (100–800 μM; Pastor-Anglada et al., 1998). Two such transporters, ENT1 and ENT2, have been cloned in
Transport of cytidine and uridine from brain extracellular fluid into cells
Uptake of uridine and cytidine from the extracellular fluid (ECF) into brain cells also is mediated by members of the two nucleoside transport families. Both the low- and high-affinity nucleoside transport proteins have been isolated in whole rat brain homogenates (Lu et al., 2004, Redzic et al., 2005). None of these transporters has been shown to be localized specifically on a particular type of brain cell (e.g., neuron, glia). Available data indicate similar or somewhat higher affinities for
Changes in brain levels of cytidine and uridine
In rat brain, de novo pyrimidine synthesis, although at lower rates than those of liver, has been described (Bourget and Tremblay, 1972). It is still unknown to what extent brain pyrimidine levels depend on de novo synthesis, however, it is well documented that the circulating cytidine and uridine are essential for maintaining various brain functions, i.e., electrophysiological activity and restoration of carbohydrate and phospholipid content (Geiger and Yamasaki, 1956, Benzi et al., 1984).
Consequences of changing brain cytidine and uridine levels
Cytidine and uridine exert important effects on a variety of brain functions by, for example, being converted to their respective nucleotides. One of these functions is neuronal membrane phospholipid synthesis. The biosynthesis of PC, the most abundant phosphatide in the brain, via the Kennedy pathway (Fig. 1) requires phosphocholine and cytidine triphosphate (CTP), a cytidine nucleotide, which is involved in the rate-limiting step (Kennedy and Weiss, 1956). The enzyme that converts CTP to
Implications for therapeutic uses of cytidine or uridine sources
Neuroprotective effects of exogenously administered CDP-choline (citicoline), a source of plasma uridine in humans (Wurtman et al., 2000), are being evaluated in treatment of neurological disorders such as acute or chronic ischemic stroke, traumatic brain injury and cognitive impairment in both experimental and clinical studies. Beneficial effects have been observed with CDP-choline administration in experimental models of cerebral ischemia (Aronowski et al., 1996, Schabitz et al., 1999,
Conclusions
It can be concluded that uridine, by crossing the rat BBB via the CNT2 transporter, is taken up by the rat brain more efficiently than is cytidine under physiological conditions. Although low-affinity ENT transporters have been shown to be localized at the rat or mouse BBB, no direct evidence has been presented to date regarding the presence of a high-affinity transporter for cytidine in this location. Whether or not human BBB exhibits similar characteristics to those of rat BBB is unknown,
Acknowledgments
I thank Drs. Richard J. Wurtman, Ismail H. Ulus and Ingrid U. Richardson for critical review of the manuscript. Studies in this report were supported in part by funds from the National Institutes of Health (Grant MH-28783) and Center for Brain Sciences and Metabolism Charitable Trust.
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