Expression of CTL1 in myelinating structures of Torpedo marmorata

Nectin-like 4 Complexes with Choline Transporter-like Protein-1 and Regulates Schwann Cell Choline Homeostasis and Lipid Biogenesis in Vitro

Nectin-like 4 (NECL4, CADM4) is a Schwann cell-specific cell adhesion molecule that promotes axo-glial interactions. In vitro and in vivo studies have shown that NECL4 is necessary for proper peripheral nerve myelination. However, the molecular mechanisms that are regulated by NECL4 and affect peripheral myelination currently remain unclear. We used an in vitro approach to begin identifying some of the mechanisms that could explain NECL4 function. Using mass spectrometry and Western blotting techniques, we have identified choline transporter-like 1 (CTL1) as a putative complexing partner with NECL4. We show that intracellular choline levels are significantly elevated in NECL4-deficient Schwann cells. The analysis of extracellular d₉-choline uptake revealed a deficit in the amount of d₉-choline found inside NECL4-deficient Schwann cells, suggestive of either reduced transport capabilities or increased metabolization of transported choline. An extensive lipidomic screen of choline derivatives showed that total phosphatidylcholine and phosphatidylinositol (but not diacylglycerol or sphingomyelin) are significantly elevated in NECL4-deficient Schwann cells, particularly specific subspecies of phosphatidylcholine carrying very long polyunsaturated fatty acid chains. Finally, CTL1-deficient Schwann cells are significantly impaired in their ability to myelinate neurites in vitro To our knowledge, this is the first demonstration of a bona fide cell adhesion molecule, NECL4, regulating choline homeostasis and lipid biogenesis. Phosphatidylcholines are major myelin phospholipids, and several phosphorylated phosphatidylinositol species are known to regulate key aspects of peripheral myelination. Furthermore, the biophysical properties imparted to plasma membranes are regulated by fatty acid chain profiles. Therefore, it will be important to translate these in vitro observations to in vivo studies of NECL4 and CTL1-deficient mice.

The choline transporter-like family SLC44: properties and roles in human diseases

The Na(+)-independent, high affinity choline carrier system proposed to supply choline for the synthesis of cell membrane phospholipids was recently associated with SLC44 family members (SLC44A1-5) also called choline-like transporter family. SLC44A1 is widely expressed throughout the nervous system in both neurons and oligodendrocytes, while SLC44A2-4 are mainly detected in peripheral tissues. The subcellular localization of the proteins was mainly addressed for SLC44A1 through the development of specific antibodies. SLC44A1 is detected in both the plasma and mitochondrial membranes where the protein is able to transport choline at high affinity and in a Na(+)-independent manner. The physiological relevance of SLC44A1 as a choline carrier is indicated by its likely involvement in membrane synthesis for cell growth or repair, and also by its role in phospholipid production for the generation of lung surfactant. Moreover, an autoimmune disease has been related to the blockade of SLC44A2 function, which results in the alteration of hair cells in the inner ear and leads to autoimmune hearing loss. In the alloimmune syndrome called transfusion-related acute lung injury, antibodies to SLC44A2 cause a deleterious aggregation of granulocytes. Therefore transporters of the SLC44 family represent attractive and promising targets for therapeutic and diagnostic applications regarding both immune and degenerative diseases.

Isoforms, expression, glycosylation, and tissue distribution of CTL2/SLC44A2

Antibodies to the solute carrier protein, CTL2/SLC44A2, cause hearing loss in animals, are frequently found in autoimmune hearing loss patients, and are implicated in transfusion-related acute lung injury. We cloned a novel CTL2/SLC44A2 isoform (CTL2 P1) from inner ear and identified an alternate upstream promoter and exon 1a encoding a protein of 704 amino acids which differs in the first 10-12 amino acids from the known exon 1b isoform (CTL2 P2; 706 amino acids). The expression of these CTL2/SLC44A2 isoforms, their posttranslational modifications in tissues and their localization in HEK293 cells expressing rHuCTL2/SLC44A2 were assessed. P1 and P2 isoforms with differing glycosylation are variably expressed in cochlea, tongue, heart, colon, lung, kidney, liver and spleen suggesting tissue specific differences that may influence function in each tissue. Because antibodies to CTL2/SLC44A2 have serious pathologic consequences, it is important to understand its distribution and modifications. Heterologous expression in X. laevis oocytes shows that while human CTL2-P1 does not transport choline, human CTL2-P2 exhibits detectable choline transport activity.

Detection of choline transporter-like 1 protein CTL1 in neuroblastoma x glioma cells and in the CNS, and its role in choline uptake

Choline is an essential nutrient necessary for synthesis of membrane phospholipids, cell signalling molecules and acetylcholine. The aim of this study was to detect and characterize the choline transporter-like 1 (CTL1/SLC44A1) protein in CNS tissues and the hybrid neuroblastoma x glioma cell line NG108-15, which synthesizes acetylcholine and has high affinity choline transport but does not express the cholinergic high affinity choline transporter 1. The presence of CTL1 protein in NG108-15 cells was confirmed using our antibody G103 which recognizes the C-terminal domain of human CTL1. Three different cognate small interfering RNAs were used to decrease CTL1 mRNA in NG108-15 cells, causing lowered CTL1 protein expression, choline uptake and cell growth. None of the small interfering RNAs influenced carnitine transport, demonstrating the absence of major non-specific effects. In parental C6 cells knockdown of CTL1 also reduced high affinity choline transport. Our results support the concept that CTL1 protein is necessary for the high affinity choline transport which supplies choline for cell growth. The presence of CTL1 protein in rat and human CNS regions, where it is found in neuronal, glial and endothelial cells, suggests that malfunction of this transporter could have important implications in nervous system development and repair following injury, and in neurodegenerative diseases.

Choline transport for phospholipid synthesis

Choline is an essential nutrient for all cells because it plays a role in the synthesis of the membrane phospholipid components of the cell membranes, as a methyl-group donor in methionine metabolism as well as in the synthesis of the neurotransmitter acetylcholine. Choline deficiency affects the expression of genes involved in cell proliferation, differentiation, and apoptosis, and it has been associated with liver dysfunction and cancer. Abnormal choline transport and metabolism have been implicated in a number of neurodegenerative disorders such as Alzheimer's and Parkinson's disease. Therefore, the study of choline transport and the characteristics of choline transporters are of central importance to understanding the mechanisms that underlie membrane integrity and cell signaling in such disorders. Kinetic studies with radiolabeled choline and inhibitors distinguish three systems for choline transport: (i) low-affinity facilitated diffusion, (ii) high-affinity, Na+-dependent transport, and (iii) intermediate-affinity, Na+-independent transport. It is only recently, however, that the proteins having transport characteristics of at least one of these systems have been identified. They include (i) polyspecific organic cation transporters (OCTs) with low affinity for choline, (ii) high-affinity choline transporters (CHT1s), and (iii) intermediate-affinity choline transporter-like (CTL1) proteins. CHT1 and CTL1 but not OCT transporters are selectively inhibited with hemicholinium-3 and essentially display characteristics of specialized transporters for targeted choline metabolism. CHT1 is abundant in neurons and almost exclusively supplies choline for acetyl-choline synthesis. The focus here is more on newly-discovered CTL1 choline transporters. They are expressed in different organisms and cell types, apparently not for the biosynthesis of acetylcholine but for the production of the most abundant metabolite of choline, the membrane lipid phosphatidylcholine.

Genomic organization, promoter activity, and expression of the human choline transporter-like protein 1

Choline transporter-like (CTL) proteins of the CTL1 family are novel transmembrane proteins implicated in choline transport for phospholipid synthesis. In this study, we characterized the 5'-flanking region of the human (h)CTL1 gene and examined some of the possible mechanisms of its regulation, including promoter activity, splicing, and expression. The transcription start site of the hCTL1 gene was mapped by 5'-rapid amplification of cDNA ends (RACE), and the presence of two splice variants, hCTL1a and hCTL1b, was investigated using isoform-specific PCR and 3'-RACE. The hCTL1 promoter region of approximately 900 bp was isolated from MCF-7 human breast cancer cells. The promoter was TATA-less and driven by a long stretch of GC-rich sequence in accordance with widespread expression of hCTL1 at both mRNA and protein levels. Deletion analyses demonstrated that a very strong promoter is contained within 500 bp of the transcription start site, and more upstream regions did not increase its activity. The core promoter that conferred the minimal transcription is within the -188/+27-bp region, and its activity varied in human breast cancer and mouse skeletal muscle cells. Multiple motifs within the promoter regulatory region bound nuclear factors from both cultured cells and normal human skeletal muscle. The motifs within the three regions [S1 (-92/-61 bp), S2 (-174/-145 bp), and S3 (-289/-260 bp)] contained overlapping binding sites for hematopoietic transcription factors and ubiquitous transcription factors, in line with the expected gene function. Genomic analyses demonstrated a high conservation of hCTL1 and mouse CTL1 proximal promoters. Accordingly, mRNA profiles demonstrated that human splice variants were expressed ubiquitously, as demonstrated for the mouse transcripts; however, they differed from the profiles of rat CTL1 transcripts, which were more restricted to neurons and intestinal tissues. The shorter hCTL1b variant contained the cytosolic COOH-terminal motif L651KKR654 for endoplasmic reticulum retrieval/retention. This retention signal was conserved in hCTL1b and rat and mouse CTL1b and is typical for transmembrane proteins of type 1 topology.

Chronic treatment with amyloid β1–42 inhibits non-cholinergic high-affinity choline transport in NG108-15 cells through protein kinase C signaling

We investigated the influence of the amyloid-beta-peptide(1-42) on hemicholinum-3-sensitive high-affinity choline uptake in NG108-15 cells. RT-PCR analysis revealed the presence of mRNA for a choline transporter-like protein but not for cholinergic high-affinity choline transporter. Differentiation of cells increased both hemicholinum-3-sensitive choline uptake and high-affinity hemicholinium-3 binding. This transport was not influenced by tenfold excess of carnitine. Continuous presence of submicromolar concentrations of amyloid-beta-peptide(1-42) during differentiation resulted in a decrease of both choline uptake and hemicholinium-3 binding. These effects were not present when amyloid-beta-peptide(1-42) was added 5 min prior to measurements. Neither differentiation nor amyloid-beta-peptide(1-42) treatment changed levels of choline transporter-like protein mRNA. Protein kinase C inhibition by staurosporine or its inactivation by continuous presence of tetradecanoyl phorbol acetate prevented the inhibitory effect of amyloid-beta-peptide(1-42) treatment on choline uptake. Activation of protein kinase C by tetradecanoyl phorbol acetate during measurement had inhibitory effect on choline uptake in control but not amyloid-beta-peptide(1-42)-treated cells. The concentration of amyloid-beta-peptide(1-42) maximally effective on hemicholinium-3-sensitive choline uptake had no effect on cell growth, oxidative activity, membrane integrity, number of surface muscarinic receptors, caspase-3 and -8 activities, or uptake of deoxyglucose. Results demonstrate that long-term treatment with non-toxic concentrations of amyloid-beta-peptide(1-42) downregulates choline uptake presumably mediated by a choline transporter-like protein through activation of protein kinase C signaling. The decrease of choline uptake may have relevance to the pathogenesis of Alzheimer's disease.

Molecular and functional characterization of an Na+-independent choline transporter in rat astrocytes

In this study, we examined the molecular and functional characterization of choline uptake into cultured rat cortical astrocytes. Choline uptake into astrocytes showed little dependence on extracellular Na+. Na+-independent choline uptake was saturable and mediated by a single transport system, with an apparent Michaelis-Menten constant (Km) of 35.7 +/- 4.1 microm and a maximal velocity (Vmax) of 49.1 +/- 2.0 pmol/mg protein/min. Choline uptake was significantly decreased by acidification of the extracellular medium and by membrane depolarization. Na+-independent choline uptake was inhibited by unlabeled choline, acetylcholine and the choline analogue hemicholinium-3. The prototypical organic cation tetrahexylammonium (TEA), and other n-tetraalkylammonium compounds such as tetrabutylammonium (TBA) and tetrahexylammonium (THA), inhibited Na+-independent choline uptake, and their inhibitory potencies were in the order THA > TBA > TEA. Various organic cations, such as 1-methyl-4-tetrahydropyridinium (MPP+), clonidine, quinine, quinidine, guanidine, N-methylnicotinamide, cimetidine, desipramine, diphenhydramine and verapamil, also interacted with the Na+-independent choline transport system. Corticosterone and 17beta-estradiol, known inhibitors of organic cation transporter 3 (OCT3), did not cause any significant inhibition. However, decynium22, which inhibits OCTs, markedly inhibited Na+-independent choline uptake. RT-PCR demonstrated that astrocytes expressed low levels of OCT1, OCT2 and OCT3 mRNA, but the functional characteristics of choline uptake are very different from the known properties of these OCTs. The high-affinity Na+-dependent choline transporter, CHT1, is not expressed in astrocytes as evidenced by RT-PCR. Furthermore, mRNA for choline transporter-like protein 1 (CTL1), and its splice variants CTL1a and CTL1b, was expressed in rat astrocytes, and the inhibition of CTL1 expression by RNA interference completely inhibited Na+-independent choline uptake. We conclude that rat astrocytes express an intermediate-affinity Na+-independent choline transport system. This system seems to occur through a CTL1 and is responsible for the uptake of choline and organic cations in these cells.

Molecular characterization of the family of choline transporter-like proteins and their splice variants

We show here that the choline transporter-like (CTL) family is more extensive than initially described with five genes in humans and complex alternative splicing. In adult rat tissues, CTL2-4 mRNAs are mainly detected in peripheral tissues, while CTL1 is widely expressed throughout the nervous system. During rat post-natal development, CTL1 is expressed in several subpopulations of neurones and in the white matter, where its spatio-temporal distribution profile recalls that of myelin basic protein, an oligodendrocyte marker. We identified two major rat splice variants of CTL1 (CTL1a and CTL1b) differing in their carboxy-terminal tails with both able to increase choline transport after transfection in neuroblastoma cells. In the developing brain, CTL1a is expressed in both neurones and oligodendroglial cells, whereas CTL1b is restricted to oligodendroglial cells. These findings suggest specific roles for CTL1 splice variants in both neuronal and oligodendrocyte physiology.

Differential expression and regulation of the high-affinity choline transporter CHT1 and choline acetyltransferase in neurons of superior cervical ganglia

Previous studies revealed that leukemia inhibitory factor (LIF) and retinoic acid (RA) induce a noradrenergic to cholinergic switch in cultured sympathetic neurons of superior cervical ganglia (SCG) by up-regulating the coordinate expression of choline acetyltransferase (ChAT) and the vesicular acetylcholine transporter. Here, we examined the effect of both factors on high-affinity choline uptake (HACU) and on expression of the high-affinity choline transporter CHT1. We found that HACU and CHT1-mRNA levels are up-regulated by LIF and down-regulated by RA in these neurons. Thus, in contrast to LIF, RA differentially regulates the expression of the presynaptic cholinergic proteins. Moreover, we showed that untreated SCG neurons express HACU and CHT1-mRNAs at much higher levels than ChAT activity and transcripts. In intact SCG, CHT1-mRNAs are abundant and synthesized by the noradrenergic neurons themselves. This study provides the first example of CHT1 expression in neurons which do not use acetylcholine as neurotransmitter.