Functional Expression of Choline Transporters in the Blood-Brain Barrier

Abstract

Cholinergic neurons in the central nervous system play a vital role in higher brain functions, such as learning and memory. Choline is essential for the synthesis of the neurotransmitter acetylcholine by cholinergic neurons. The synthesis and metabolism of acetylcholine are important mechanisms for regulating neuronal activity. Choline is a positively charged quaternary ammonium compound that requires transporters to pass through the plasma membrane. Currently, there are three groups of choline transporters with different characteristics, such as affinity for choline, tissue distribution, and sodium dependence. They include (I) polyspecific organic cation transporters (OCT1-3: SLC22A1-3) with a low affinity for choline, (II) high-affinity choline transporter 1 (CHT1: SLC5A7), and (III) choline transporter-like proteins (CTL1-5: SLC44A1-5). Brain microvascular endothelial cells, which comprise part of the blood-brain barrier, take up extracellular choline via intermediate-affinity choline transporter-like protein 1 (CTL1) and low-affinity CTL2 transporters. CTL2 is responsible for excreting a high concentration of choline taken up by the brain microvascular endothelial cells on the brain side of the blood-brain barrier. CTL2 is also highly expressed in mitochondria and may be involved in the oxidative pathway of choline metabolism. Therefore, CTL1- and CTL2-mediated choline transport to the brain through the blood-brain barrier plays an essential role in various functions of the central nervous system by acting as the rate-limiting step of cholinergic neuronal activity.

Keywords: acetylcholine; blood–brain barrier; brain microvascular endothelial cells; central nervous system; choline; transporter.

Figure 1

Schematic illustration of the blood–brain barrier and transporters. The blood–brain barrier is composed of brain microvascular endothelial cells, astrocytes, and pericytes. Diffusion between cells is limited by the mutual binding of brain microvascular endothelial cells by tight junctions. Many of the soluble carrier (SLC) transporters expressed in brain microvascular endothelial cells allow substances, such as nutrients (e.g., glucose, amino acids, peptides, and nucleotides), to selectively cross the blood–brain barrier. In addition, ATP binding cassette (ABC) transporters that are expressed in cerebral microvascular endothelial cells play a role in preventing the entry of toxic substances and drugs into the brain by releasing them into the blood.

Figure 2

Choline metabolic pathway. Choline is an essential biological molecule for all cells and is required for the synthesis of phosphatidylcholine and sphingomyelin, which are the major components of the plasma membrane. New cell membrane synthesis requires the rate-limiting step of choline uptake, followed by phospholipid biosynthesis. Choline is also a precursor for the neurotransmitter acetylcholine and the methyl donor betaine, which are involved in several important biological functions. Betaine, an oxidized metabolite of choline, is a source of methyl groups for the production of S-adenosylmethionine (SAM), which serves as a substrate for DNA and histone methyltransferases, and is thus required for the establishment and maintenance of the epigenome. Epigenetic mechanisms play important roles in biology and human diseases. ADP, adenosine diphosphate; ATP, adenosine triphosphate; BHMT, betaine-homocysteine methyltransferase; VB12, vitamin B12; CDP, cytidine diphosphate; CK, choline kinase; CMP, cytidine monophosphate; CO, choline oxidase; CPT, choline phosphotransferase; DAG, diacylglycerol; CTP, cytidine triphosphate; methyl-THF, 5-methyltetrahydrofolate; MTHF, 5,10-methylene-tetrahydrofolate; PCP, phosphatidylcholine:ceramide choline phosphotransferase; Pcyt1, CTP:phosphocholine cytidyltransferase; PEMT, phosphatidylethanolamine N-methyltransferase; PLA, phospholipase A2; PPi, pyrophosphate; SAH, S-adenosylhomocysteine; SAM, S-adenosylmethionine; THF, tetrahydrofolate.

Figure 3

The localization of choline transporters in the blood–brain barrier. CTL1 and CTL2 are expressed in brain microvascular endothelial cells in the blood–brain barrier. Their localization on the luminal side of these cells suggests that they are responsible for the uptake of choline into the brain. CTL2 is also expressed on the apical side of brain microvascular endothelial cells, where choline is excreted into the brain. CTL1 is also expressed in astrocytes, one of the cell types that comprise the blood–brain barrier, and is thought to be linked to phospholipid synthesis. The high-affinity CHT1 and intermediate-affinity CTL1 are functionally expressed in neurons, where they may be involved in the synthesis of acetylcholine and phospholipids, respectively.