Identification and expression of a mouse muscle-specific CTL1 gene

Unbiased insights into the multiplicity of the CYP46A1 brain effects in 5XFAD mice treated with low dose-efavirenz

Cytochrome P450 46A1 (CYP46A1) is the CNS-specific cholesterol 24-hydroxylase that controls cholesterol elimination and turnover in the brain. In mouse models, pharmacologic CYP46A1 activation with low-dose efavirenz or by gene therapy mitigates the manifestations of various brain disorders, neurologic, and nonneurologic, by affecting numerous, apparently unlinked biological processes. Accordingly, CYP46A1 is emerging as a promising therapeutic target; however, the mechanisms underlying the multiplicity of the brain CYP46A1 activity effects are currently not understood. We proposed the chain reaction hypothesis, according to which CYP46A1 is important for the three primary (unifying) processes in the brain (sterol flux through the plasma membranes, acetyl-CoA, and isoprenoid production), which in turn affect a variety of secondary processes. We already identified several processes secondary to changes in sterol flux and herein undertook a multiomics approach to compare the brain proteome, acetylproteome, and metabolome of 5XFAD mice (an Alzheimer's disease model), control and treated with low-dose efavirenz. We found that the latter had increased production of phospholipids from the corresponding lysophospholipids and a globally increased protein acetylation (including histone acetylation). Apparently, these effects were secondary to increased acetyl-CoA production. Signaling of small GTPases due to their altered abundance or abundance of their regulators could be affected as well, potentially via isoprenoid biosynthesis. In addition, the omics data related differentially abundant molecules to other biological processes either reported previously or new. Thus, we obtained unbiased mechanistic insights and identified potential players mediating the multiplicity of the CYP46A1 brain effects and further detailed our chain reaction hypothesis.

Mitochondrial choline import regulates purine nucleotide pools via SLC25A48

Choline is an essential nutrient for cellular metabolism, including the biosynthesis of phospholipids, neurotransmitters, and one-carbon metabolism. A critical step of choline catabolism is the mitochondrial import and synthesis of chorine-derived methyl donors, such as betaine. However, the underlying mechanisms and the biological significance of mitochondrial choline catabolism remain insufficiently understood. Here, we report that a mitochondrial inner-membrane protein SLC25A48 controls mitochondrial choline transport and catabolism in vivo. We demonstrate that SLC25A48 is highly expressed in brown adipose tissue and required for whole-body cold tolerance, thermogenesis, and mitochondrial respiration. Mechanistically, choline uptake into the mitochondrial matrix via SLC25A48 facilitates betaine synthesis and one-carbon metabolism. Importantly, cells lacking SLC25A48 exhibited reduced synthesis of purine nucleotides and failed to initiate the G1-to-S phase transition, thereby leading to cell death. Taken together, the present study identified SLC25A48 as a mitochondrial carrier that mediates choline import and plays a critical role in mitochondrial respiratory capacity, purine nucleotide synthesis, and cell survival.

Carrier-Mediated Process of Putrescine Elimination at the Rat Blood-Retinal Barrier

Putrescine is a bioactive polyamine. Its retinal concentration is strictly controlled to maintain a healthy sense of vision. The present study investigated putrescine transport at the blood-retinal barrier (BRB) to gain a better understanding of the mechanisms of putrescine regulation in the retina. Our microdialysis study showed that the elimination rate constant during the terminal phase was significantly greater (1.90-fold) than that of [¹⁴C]D-mannitol, which is a bulk flow marker. The difference in the apparent elimination rate constants of [³H]putrescine and [¹⁴C]D-mannitol was significantly decreased by unlabeled putrescine and spermine, suggesting active putrescine transport from the retina to the blood across the BRB. Our study using model cell lines of the inner and outer BRB showed that [³H]putrescine transport was time-, temperature-, and concentration-dependent, suggesting the involvement of carrier-mediated processes in putrescine transport at the inner and outer BRB. [³H]Putrescine transport was significantly reduced under Na⁺-free, Cl^--free, and K⁺-replacement conditions, and attenuated by polyamines or organic cations such as choline, a choline transporter-like protein (CTL) substrate. Rat CTL1 cRNA-injected oocytes exhibited marked alterations in [³H]putrescine uptake, and CTL1 knockdown significantly reduced [³H]putrescine uptake in model cell lines, suggesting the possible participation of CTL1 in putrescine transport at the BRB.

Transporter-Mediated Drug Delivery

Transmembrane transport of small organic and inorganic molecules is one of the cornerstones of cellular metabolism. Among transmembrane transporters, solute carrier (SLC) proteins form the largest, albeit very diverse, superfamily with over 400 members. It was recognized early on that xenobiotics can directly interact with SLCs and that this interaction can fundamentally determine their efficacy, including bioavailability and intertissue distribution. Apart from the well-established prodrug strategy, the chemical ligation of transporter substrates to nanoparticles of various chemical compositions has recently been used as a means to enhance their targeting and absorption. In this review, we summarize efforts in drug design exploiting interactions with specific SLC transporters to optimize their therapeutic effects. Furthermore, we describe current and future challenges as well as new directions for the advanced development of therapeutics that target SLC transporters.

Protective Effects of Choline against Inflammatory Cytokines and Characterization of Transport in Motor Neuron-like Cell Lines (NSC-34)

Choline, a component of the neurotransmitter acetylcholine, is essential for nervous system functions, brain development, and gene expression. In our study, we investigated the protective effect and transport characteristics of choline in amyotrophic lateral sclerosis (ALS) model cell lines. We used the wild-type (WT) motor neuron-like hybrid cell line (NSC-34/hSOD1WT) as a control and the mutant-type (MT; NSC-34/hSOD1G93A) as a disease model. The uptake of [3H]choline was time-, pH-, and concentration-dependent. [3H]Choline transport was sodium-dependent, and, upon pretreatment with valinomycin, induced membrane depolarization. Gene knockdown of Slc44a1 revealed that choline-like transporter 1 (CTL1) mediates the transport of choline. In NSC-34 cell lines, the specific choline transporter inhibitor, hemicholinium-3 demonstrated significant inhibition. Donepezil and nifedipine caused dose-dependent inhibition of [3H]choline uptake by the MT cell line with minimal half inhibitory concentration (IC50) values of 0.14 mM and 3.06 mM, respectively. Four-day pretreatment with nerve growth factor (NGF) resulted in an inhibitory effect on [3H]choline uptake. Choline exerted protective and compensatory effects against cytokines mediators. Hence, the choline transport system CLT1 may act as a potential target for the delivery of novel pharmacological drugs, and the combination of drugs with choline can help treat symptoms related to ALS.

Blood-Brain Barrier Solute Carrier Transporters and Motor Neuron Disease

Defective solute carrier (SLC) transporters are responsible for neurotransmitter dysregulation, resulting in neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS). We provided the role and kinetic parameters of transporters such as ASCTs, Taut, LAT1, CAT1, MCTs, OCTNs, CHT, and CTL1, which are mainly responsible for the transport of essential nutrients, acidic, and basic drugs in blood-brain barrier (BBB) and motor neuron disease. The affinity for LAT1 was higher in the BBB than in the ALS model cell line, whereas the capacity was higher in the NSC-34 cell lines than in the BBB. Affinity for MCTs was lower in the BBB than in the NSC-34 cell lines. CHT in BBB showed two affinity sites, whereas no expression was observed in ALS cell lines. CTL1 was the main transporter for choline in ALS cell lines. The half maximal inhibitory concentration (IC50) analysis of [3H]choline uptake indicated that choline is sensitive in TR-BBB cells, whereas amiloride is most sensitive in ALS cell lines. Knowledge of the transport systems in the BBB and motor neurons will help to deliver drugs to the brain and develop the therapeutic strategy for treating CNS and neurological diseases.

Rational exploration of fold atlas for human solute carrier proteins

The solute carrier (SLC) superfamily is the largest group of proteins responsible for the transmembrane transport of substances in human cells. It includes more than 400 members that are organized into 65 families according to their physiological function and sequence similarity. Different families of SLCs can adopt the same or different folds that determine the mechanism and reflect the evolutionary relationship between SLC members. Analysis of structural data in the literature before this work showed 13 different folds in the SLC superfamily covering 40 families and 343 members. To further study their mechanism, we systematically explored the SLC superfamily to look for more folds. Based on our results, at least three new folds are found for the SLC superfamily, one of which is in the choline-like transporter family (SLC44) and has been experimentally verified. Our work has laid a foundation and provided important insights for the systematic and comprehensive study of the structure and function of SLC.

Prenatal choline supplementation during mouse pregnancy has differential effects in alcohol-exposed fetal organs

BACKGROUND

Fetal alcohol spectrum disorders (FASD) are preventable adverse outcomes consequent to prenatal alcohol exposure. Supplemental choline confers neuroprotection to the alcohol-exposed offspring, but its actions outside the brain are unclear. We previously reported that prenatal exposure of mice to 4.5 g/kg of alcohol decreased placental weight in females only, but decreased body weight and liver-to-body weight ratio and increased brain-to-body weight ratio in both sexes. Here we test the hypotheses that a lower alcohol dose will elicit similar outcomes, and that concurrent choline treatment will mitigate these outcomes.

METHODS

Pregnant C57BL/6J mice were gavaged with alcohol (3 g/kg; Alc) or maltodextrin (MD) from embryonic day (E) 8.5-17.5. Some also received a subcutaneous injection of 100 mg/kg choline chloride (Alc + Cho, MD + Cho). Outcomes were evaluated on E17.5.

RESULTS

Alc dams had lower gestational weight gain than MD; this was normalized by choline. In males, Alc decreased placental weight whereas choline increased placental efficiency, and Alc + Cho (vs. MD) tended to further reduce placental weight and increase efficiency. Despite no significant alcohol effects on these measures, choline increased fetal body weight but not brain weight, thus reducing brain-to-body weight ratio in both sexes. This ratio was also lower in the Alc + Cho (vs. MD) fetuses. Alc reduced liver weight and the liver-to-body weight ratio; choline did not improve these. Placental weight and efficiency correlated with litter size, whereas placental efficiency correlated with fetal morphometric measurements.

CONCLUSIONS

Choline prevents an alcohol-induced reduction in gestational weight gain and fetal body weight and corrects fetal brain sparing, consistent with clinical findings of improvements in alcohol-exposed children born to mothers receiving choline supplementation. Importantly, we show that choline enhances placental efficiency in the alcohol-exposed offspring but does not normalize fetal liver growth. Our findings support choline supplementation during pregnancy to mitigate the severity of FASD and emphasize the need to examine choline's actions in different organ systems.

Transcriptomics analysis revealing candidate genes and networks for sex differentiation of yesso scallop (Patinopecten yessoensis)

Background

The Yesso scallop, Patinopecten (Mizuhopecten) yessoensis, is a commercially important bivalve in the coastal countries of Northeast Asia. It has complex modes of sex differentiation, but knowledge of the mechanisms underlying this sex determination and differentiation is limited.

Results

In this study, the gonad tissues from females and males at three developmental stages were used to investigate candidate genes and networks for sex differentiation via RNA-Req. A total of 901,980,606 high quality clean reads were obtained from 18 libraries, of which 417 expressed male-specific genes and 754 expressed female-specific genes. Totally, 10,074 genes differentially expressed in females and males were identified. Weighted gene co-expression network analysis (WGCNA) revealed that turquoise and green gene modules were significantly positively correlated with male gonads, while coral1 and black modules were significantly associated with female gonads. The most important gene for sex determination and differentiation was Pydmrt 1, which was the only gene discovered that determined the male sex phenotype during early gonadal differentiation. Enrichment analyses of GO terms and KEGG pathways revealed that genes involved in metabolism, genetic and environmental information processes or pathways are sex-biased. Forty-nine genes in the five modules involved in sex differentiation or determination were identified and selected to construct a gene co-expression network and a hypothesized sex differentiation pathway.

Conclusions

The current study focused on screening genes of sex differentiation in Yesso scallop, highlighting the potential regulatory mechanisms of gonadal development in P. yessoensis. Our data suggested that WCGNA can facilitate identification of key genes for sex differentiation and determination. Using this method, a hypothesized P. yessoensis sex determination and differentiation pathway was constructed. In this pathway, Pydmrt 1 may have a leading function.

Electronic supplementary material

The online version of this article (10.1186/s12864-019-6021-6) contains supplementary material, which is available to authorized users.

Choline transport for phospholipid synthesis: An emerging role of choline transporter-like protein 1

IMPACT STATEMENT

This review will provide a summary of recent advances in choline transport research and highlight important novel areas of focus in the field.

Adaptations to excess choline in insulin resistant and Pcyt2 deficient skeletal muscle

It was hypothesized that choline supplementation in insulin resistant (IR) CTP:phosphoethanolamine cytidylyltransferase deficient (Pcyt2^+/-) mice would ameliorate muscle function by remodeling glucose and fatty acid (FA) metabolism. Pcyt2^+/- mice either received no treatment or were allowed access to 2 mg/mL choline in drinking water for 4 weeks. Skeletal muscle was harvested from choline treated and untreated mice. Lipid analysis and metabolic gene expression and signaling pathways were compared between untreated Pcyt2^+/- mice, treated Pcyt2^+/- mice, and Pcyt2^+/+ mice. The major positive effect of choline supplementation on IR muscle was the reduction of glucose utilization for FA and triglyceride (TAG) synthesis and increased muscle glucose storage as glycogen. Choline reduced the expression of genes for FA and TAG formation (Scd1, Fas, Srebp1c, Dgat1/2), upregulated the genes for FA oxidation (Cpt1, Pparα, Pgc1α), and had minor effects on phospholipid and lipolysis genes. Pcyt2^+/- muscle had reduced insulin signaling (IRS1), autophagy (LC3), and choline transport (CTL1) proteins that were restored by choline treatment. Additionally, choline activated AMPK and Akt while inhibiting mTORC1 phosphorylation. These data established that choline supplementation could restore muscle glucose metabolism by reducing lipogenesis and improving mitochondrial and intracellular signaling for protein and energy metabolism in insulin resistant Pcyt2 deficient mice.

Choline on the Move: Perspectives on the Molecular Physiology and Pharmacology of the Presynaptic Choline Transporter

Genetic, biochemical, physiological, and pharmacological approaches have advanced our understanding of cholinergic biology for over 100 years. High-affinity choline uptake (HACU) was one of the last features of cholinergic signaling to be defined at a molecular level, achieved through the cloning of the choline transporter (CHT, SLC5A7). In retrospect, the molecular era of CHT studies initiated with the identification of hemicholinium-3 (HC-3), a potent, competitive CHT antagonist, though it would take another 30 years before HC-3, in radiolabeled form, was used by Joseph Coyle's laboratory to identify and monitor the dynamics of CHT proteins. Though HC-3 studies provided important insights into CHT distribution and regulation, another 15 years would pass before the structure of CHT genes and proteins were identified, a full decade after the cloning of most other neurotransmitter-associated transporters. The availability of CHT gene and protein probes propelled the development of cell and animal models as well as efforts to gain insights into how human CHT gene variation affects the risk for brain and neuromuscular disorders. Most recently, our group has pursued a broadening of CHT pharmacology, elucidating novel chemical structures that may serve to advance cholinergic diagnostics and medication development. Here we provide a short review of the transformation that has occurred in HACU research and how such advances may promote the development of novel therapeutics.

Cholinergic activation of the murine trachealis muscle via non-vesicular acetylcholine release involving low-affinity choline transporters

In addition to quantal, vesicular release of acetylcholine (ACh), there is also non-quantal release at the motor endplate which is insufficient to evoke postsynaptic responses unless acetylcholinesterase (AChE) is inhibited. We here addressed potential non-quantal release in the mouse trachea by organ bath experiments and (immuno)histochemical methods. Electrical field stimulation (EFS) of nerve terminals elicited tracheal constriction that is largely due to ACh release. Classical enzyme histochemistry demonstrated acetylcholinesterase (AChE) activity in nerve fibers in the muscle and butyrylcholinesterase (BChE) activity in the smooth muscle cells. Acute inhibition of both esterases by eserine significantly raised tracheal tone which was fully sensitive to atropine. This effect was reduced, but not abolished, in AChE, but not in BChE gene-deficient mice. The eserine-induced increase in tracheal tone was unaffected by vesamicol (10(-5)M), an inhibitor of the vesicular acetylcholine transporter, and by corticosterone (10(-4)M), an inhibitor of organic cation transporters. Hemicholinium-3, in low concentrations an inhibitor of the high-affinity choline transporter-1 (CHT1), completely abrogated the eserine effects when applied in high concentrations (10(-4)M) pointing towards an involvement of low-affinity choline transporters. To evaluate the cellular sources of non-quantal ACh release in the trachea, expression of low-affinity choline transporter-like family (CTL1-5) was evaluated by RT-PCR analysis. Even though these transporters were largely abundant in the epithelium, denudation of airway epithelial cells had no effect on eserine-induced tracheal contraction, indicating a non-quantal release of ACh from non-epithelial sources in the airways. These data provide evidence for an epithelium-independent non-vesicular, non-quantal ACh release in the mouse trachea involving low-affinity choline transporters.

Choline supplementation restores substrate balance and alleviates complications of Pcyt2 deficiency

Choline plays a critical role in systemic lipid metabolism and hepatic function. Here we conducted a series of experiments to investigate the effect of choline supplementation on metabolically altered Pcyt2(+/-) mice. In Pcyt2(+/-) mice, the membrane phosphatidylethanolamine (PE) turnover is reduced and the formation of fatty acids (FA) and triglycerides (TAG) increased, resulting in hypertriglyceridemia, liver steatosis and obesity. One month of choline supplementation reduced the incorporation of FA into TAG and facilitated TAG degradation in Pcyt2(+/-) adipocytes, plasma and liver. Choline particularly stimulated adipocyte and liver TAG lipolysis by specific lipases (ATGL, LPL and HSL) and inhibited TAG formation by DGAT1 and DGAT2. Choline also activated the liver AMPK and mitochondrial FA oxidation gene PPARα and reduced the FA synthesis genes SREBP1, SCD1 and FAS. Liver (HPLC) and plasma (tandem mass spectroscopy and (1)H-NMR) metabolite profiling established that Pcyt2(+/-) mice have reduced membrane cholesterol/sphingomyelin ratio and the homocysteine/methionine cycle that were improved by choline supplementation. These data suggest that supplementary choline is beneficial for restoring FA and TAG homeostasis under conditions of obesity caused by impaired PE synthesis.

Molecular properties of the high-affinity choline transporter CHT1

This article summarizes molecular properties of the high-affinity choline transporter (CHT1) with reference to the historical background focusing studies performed in laboratories of the author. CHT1 is present on the presynaptic terminal of cholinergic neurons, and takes up choline which is the precursor of acetylcholine. The Na(+)-dependent uptake of choline by CHT1 is the rate-limiting step for synthesis of acetylcholine. CHT1 is the integral membrane protein with 13 transmembrane segments, belongs to the Na(+)/glucose co-transporter family (SLC5), and has 20-25% homology with members of this family. A single nucleotide polymorphism (SNP) for human CHT1 has been identified, which has a replacement from isoleucine to valine in the third transmembrane segment and shows the choline uptake activity of 50-60% as much as that of wild-type CHT1. The proportion of this SNP is high among Asians. Possible importance of choline diet for those with this SNP was discussed.

Palmitic acid and oleic acid differentially regulate choline transporter-like 1 levels and glycerolipid metabolism in skeletal muscle cells

Choline is an essential nutrient required for the biosynthesis of membrane lipid phosphatidylcholine (PtdCho). Here we elucidate the mechanism of how palmitic acid (PAM) and oleic acid (OLA) regulate choline transporter-like protein 1 (CTL1/SLC44A1) function. We evaluated the mechanism of extracellular and intracellular transport of choline, and their contribution to PtdCho and other glycerolipid-diacylglycerol (DAG) and triacylglycerol (TAG) homeostasis in differentiated skeletal muscle cells. PAM reduces total and plasma membrane CTL1/SLC44A1 protein by lysosomal degradation, and limits the choline uptake while increasing DAG and TAG synthesis. OLA maintains total and plasma membrane CTL1/SLC44A1, but increases PtdCho synthesis more than PAM. OLA does not increase the rate of DAG synthesis, but does increase TAG content. Thus, the CTL1/SLC44A1 presence at the plasma membrane regulates choline requirements in accordance with the type of fatty acid. The increased PtdCho and TAG turnover by OLA stimulates cell growth and offers a specific protection mechanism from the excess of intracellular DAG and autophagy. This protection was present after OLA treatments, but not after PAM treatments. The mitochondrial choline uptake was reduced by both FA; however, the regulation is complex and guided not only by the presence of the mitochondrial CTL1/SLC44A1 protein but also by the membrane potential and general mitochondrial function.

Functional expression of α7-nicotinic acetylcholine receptors by muscle afferent neurons

The exercise pressor reflex (EPR) is generated by group III and IV muscle afferents during exercise to increase cardiovascular function. Muscle contraction is triggered by ACh, which is metabolized into choline that could serve as a signal of exercise-induced activity. We demonstrate that ACh can induce current in muscle afferents neurons isolated from male Sprague-Dawley rats. The nicotinic ACh receptors (nAChRs) appear to be expressed by some group III-IV neurons since capsaicin (TRPV1) and/or ATP (P2X) induced current in 56% of ACh-responsive neurons. α7- And α4β2-nAChRs have been shown to be expressed in sensory neurons. An α7-nAChR antibody stained 83% of muscle afferent neurons. Functional expression was demonstrated by using the specific α7-nAChR blockers α-conotoxin ImI (IMI) and methyllycaconitine (MLA). MLA inhibited ACh responses in 100% of muscle afferent neurons, whereas IMI inhibited ACh responses in 54% of neurons. Dihydro-β-erythroidine, an α4β2-nAChR blocker, inhibited ACh responses in 50% of muscle afferent neurons, but recovery from block was not observed. Choline, an α7-nAChR agonist, elicited a response in 60% of ACh-responsive neurons. Finally, we demonstrated the expression of α7-nAChR by peripherin labeled (group IV) afferent fibers within gastrocnemius muscles. Some of these α7-nAChR-positive fibers were also positive for P2X3 receptors. Thus choline could serve as an activator of the EPR by opening α7-nAChR expressed by group IV (and possible group III) afferents. nAChRs could become pharmacological targets for suppressing the excessive EPR activation in patients with peripheral vascular disease.

Choline transporter-like proteins CTLs/SLC44 family as a novel molecular target for cancer therapy

Choline is essential for the synthesis of the major membrane phospholipid phosphatidylcholine (PC), the methyl donor betaine and the neurotransmitter acetylcholine (ACh). Elevated levels of choline and up-regulated choline kinase activity have been detected in various cancers. Thus, the intracellular accumulation of choline through choline transporters is the rate-limiting step in phospholipid metabolism and a prerequisite for cancer cell proliferation. Previous studies have demonstrated abnormalities in choline uptake and choline phospholipid metabolism in cancer cells using the imaging of cancer with positron emission tomography (PET) and magnetic resonance spectroscopy (MRS). The aberrant choline metabolism in cancer cells is strongly correlated with their malignant progression. Using quantitative real-time PCR, the mRNA expression of choline transporters was measured, and it was found that choline transporter-like proteins CTLs/SLC44 family are highly expressed in various cancer cell lines. Choline uptake through CTLs is associated with cell viability, and the functional inhibition of CTLs could promote apoptotic cell death. Furthermore, non-neuronal cholinergic systems that include CTLs-mediated choline transport are associated with cell proliferation and their inhibition promotes apoptotic cell death in colon cancer, small cell lung cancer and human leukemic T-cells. The identification of this new CTLs-mediated choline transport system provides a potential new target for cancer therapy.

Formation and regulation of mitochondrial membranes

Mitochondrial membrane phospholipids are essential for the mitochondrial architecture, the activity of respiratory proteins, and the transport of proteins into the mitochondria. The accumulation of phospholipids within mitochondria depends on a coordinate synthesis, degradation, and trafficking of phospholipids between the endoplasmic reticulum (ER) and mitochondria as well as intramitochondrial lipid trafficking. Several studies highlight the contribution of dietary fatty acids to the remodeling of phospholipids and mitochondrial membrane homeostasis. Understanding the role of phospholipids in the mitochondrial membrane and their metabolism will shed light on the molecular mechanisms involved in the regulation of mitochondrial function and in the mitochondrial-related diseases.

A mechanism for suppression of the CDP-choline pathway during apoptosis

Inhibition of the CDP-choline pathway during apoptosis restricts the availability of phosphatidylcholine (PtdCho) for assembly of membranes and synthesis of signaling factors. The N-terminal nuclear localization signal (NLS) in CTP:phosphocholine cytidylyltransferase (CCT)α is removed during apoptosis but the caspase(s) involved and the contribution to suppression of the CDP-choline pathway is unresolved. In this study we utilized siRNA silencing of caspases in HEK293 cells and caspase 3-deficient MCF7 cells to show that caspase 3 is required for CCTα proteolysis and release from the nucleus during apoptosis. CCTα-Δ28 (a caspase-cleaved mimic) expressed in CCTα-deficient Chinese hamster ovary cells was cytosolic and had increased in vitro activity. However, [³H]choline labeling experiments in camptothecin-treated MCF7 cells and MCF7 cells expressing caspase 3 (MCF7-C3) revealed a global suppression of the CDP-choline pathway that was consistent with inhibition of a step prior to CCTα. In camptothecin-treated MCF7 and MCF7-C3 cells, choline kinase activity was unaffected; however, choline transport into cells was reduced by 30 and 60%, respectively. We conclude that caspase 3-mediated removal of the CCTα NLS contributes minimally to the inhibition of PtdCho synthesis during DNA damage-induced apoptosis. Rather, the CDP-choline pathway is inhibited by caspase 3-independent and -dependent suppression of choline transport into cells.

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.

The molecular effects of a polymorphism in the 5'UTR of solute carrier family 44, member 5 that is associated with birth weight in Holsteins

Dystocia is a major problem for the dairy cattle industry, and the observed high rates of this condition stem from genetic selection to increase subsequent milk production of the calving female. Because smaller birth size does not adversely affect subsequent milk production, selecting for cows with a smaller birth size would reduce dystocia rates and be beneficial for both the cattle and the farmers. To identify genes that regulate birth weight, we conducted a genome-wide association study using 1151 microsatellite markers and identified a single nucleotide polymorphism (SNP) associated with birth weight: A-326G in the 5' untranslated region (UTR) of solute carrier family 44, member 5 (SLC44A5). Cows with higher birth weights carried the A polymorphism in the SLC44A5 5' UTR, and the presence of the A polymorphism correlated with a high rate of dystocia. Luciferase assays and quantitative polymerase chain reaction (QPCR) assays revealed that SLC44A5 transcripts with the A polymorphism are expressed at lower levels than those carrying the G polymorphism. SLC44A5 encodes a choline transporter-like protein, and choline is a component of the major phospholipids of cell membranes. Uptake studies in HeLa cells demonstrated that SLC44A5 knockdown reduces choline efflux, whereas SLC44A5 overexpression resulted in the opposite effect. Furthermore, cell viability assays indicated that SLC44A5 knockdown increased cell proliferation, whereas SLC44A5 overexpression repressed proliferation. Taken together, our results suggest that calves with reduced SLC44A5 expression are larger due to enhanced cell proliferation. This study provides novel insights into the molecular mechanisms that control birth weight in Holsteins and suggests that SLC44A5 may serve as a potential target for preventing dystocia.

CTP:phosphocholine cytidylyltransferase α (CCTα) and lamins alter nuclear membrane structure without affecting phosphatidylcholine synthesis

CTP:phosphocholine cytidylyltransferase α (CCTα) is a nuclear enzyme that catalyzes the rate-limiting step in the CDP-choline pathway for phosphatidylcholine (PC) synthesis. Lipid activation of CCTα results in its translocation to the nuclear envelope and expansion of an intranuclear membrane network termed the nucleoplasmic reticulum (NR) by a mechanism involving membrane deformation. Nuclear lamins are also required for stability and proliferation of the NR, but whether this unique structure, or the nuclear lamina in general, is required for PC synthesis is not known. To examine this relationship, the nuclear lamina was depleted by RNAi or disrupted by expression of the Hutchinson-Gilford progeria syndrome (HGPS) mutant lamin A (progerin), and the effect on CCTα and choline metabolism was analyzed. siRNA-mediated silencing of lamin A/C or lamin B1 in CHO cells to diminish the NR had no effect on PC synthesis, while double knockdown non-specifically inhibited the pathway. Confirming this minor role in PC synthesis, only 10% of transiently overexpressed choline/ethanolamine phosphotransferase was detected in the NR. In CHO cells, CCTα was nucleoplasmic and co-localized with GFP-progerin in nuclear folds and invaginations; however, HGPS fibroblasts displayed an abnormal distribution of CCTα in the cytoplasm and nuclear envelope that was accompanied by a 2-fold reduction in PC synthesis. In spite of its altered localization, choline-labeling experiments showed that CCT activity was unaffected, and inhibition of PC synthesis was traced to reduced activity of a hemicholinium-sensitive choline transporter. We conclude that CCTα and lamins specifically cooperate to form the NR, but the overall structure of the nuclear envelope has a minimal impact on CCT activity and PC synthesis.

The impact of choline availability on muscle lipid metabolism

Consumption of choline-rich foods is essential to ensure membrane integrity, neurotransmission and genomic methylation pathways. Insufficient dietary choline supply can cause choline deficiency (CD) which manifests in the development of non-alcoholic fatty liver disease. There is very limited information regarding the effect of CD on non-hepatic tissues such as muscle. In this study, we induced CD in muscle cells and investigated the effect on choline transport, phosphatidylcholine (PC), fatty acid and triacylglycerol (TAG, fat) metabolism. Choline transport was stable across the plasma membrane of CD cells but significantly impaired in mitochondria. The main choline-transporter SLC44A1 was down-regulated by CD at the mRNA level, and SLC44A1 protein was reduced in total cell lysates and isolated mitochondria. CD significantly reduced PC synthesis but PC degradation was unaffected. PC from CD muscle was modified and contained more monounsaturated fatty acids at the expense of saturated fatty acids. Surprisingly, CD muscle cells also accumulated TAG in the form of large lipid droplets. Those droplets were formed from endogenous fatty acids and by slower TAG metabolism. This study established for the first time that choline availability affects muscle membrane lipid composition and intracellular lipid metabolism, and underlines the significance of choline-rich foods for proper muscle function.

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.

The solute carrier 44A1 is a mitochondrial protein and mediates choline transport

Choline oxidation to betaine takes place in the mitochondria; however, a protein regulating mitochondrial choline transport was never identified. The purpose of this study was to analyze subcellular localization of the solute carrier 44A1 (SLC44A1), a plasma membrane choline transporter sensitive to inhibition by hemicholinium-3. We generated N- and C-terminal-SLC44A1-specific antibodies and analyzed localization of endogenous and overexpressed SLC44A1 in C2C12 mouse muscle cells, MCF7 human breast cancer cells, and mouse tissues using confocal microscopy, differential centrifugation, and Western blotting. We further performed choline uptake competition studies on isolated mitochondria using the specific inhibitor hemicholinium-3 and SLC44A1 antibodies, and analyzed mitochondria of FL83B hepatocytes after the targeted knock-down of SLC44A1 using siRNA technology. In addition, we analyzed SLC44A1 expression during choline deficiency. Localization studies revealed plasma membrane, cytosolic, microsomal, and mitochondrial localization of endogenous and His-tagged SLC44A1. Uptake studies in isolated mitochondria show an accumulation of (3)H-choline, which is strongly inhibited by hemicholinium-3 (60%), by an excess of unlabeled choline (97%), and by both SLC44A1 antibodies. SLC44A1 mRNA and protein expression were down-regulated during choline deficiency. These data clearly establish SLC44A1 as an important mediator of choline transport across both the plasma membrane and the mitochondrial membrane.

Choline transport via choline transporter-like protein 1 in conditionally immortalized rat syncytiotrophoblast cell lines TR-TBT

Choline is an essential nutrient for phospholipids and acetylcholine biosynthesis in normal development of fetus. In the present study, we investigated the functional characteristics of choline transport system and inhibitory effect of cationic drugs on choline transport in rat conditionally immortalized syncytiotrophoblast cell line (TR-TBT). Choline transport was weakly Na(+) dependent and significantly influenced by extracellular pH and by membrane depolarization. The transport process of choline is saturable with Michaelis-Menten constants (K(m)) of 68microM and 130microM in TR-TBT 18d-1 and TR-TBT 18d-2 respectively. Choline uptake in the cells was inhibited by unlabeled choline and hemicholinium-3 as well as various organic cations including guanidine, amiloride and acetylcholine. However, the prototypical organic cation tetraethylammonium and cimetidine showed very little inhibitory effect of choline uptake in TR-TBT cells. RT-PCR revealed that choline transporter-like protein 1 (CTL1) and organic cation transporter 2 (OCT2) are expressed in TR-TBT cells. The transport properties of choline in TR-TBT cells were similar or identical to that of CTL1 but not OCT2. CTL1 was also detected in human placenta. In addition, several cationic drugs such as diphenhydramine and verapamil competitively inhibited choline uptake in TR-TBT 18d-1 with K(i) of 115microM and 55microM, respectively. Our results suggest that choline transport system, which has intermediate affinity and weakly Na(+) dependent, in TR-TBT seems to occur through a CTL1 and this system may have relevance with the uptake of pharmacologically important organic cation drugs.

Na(+)-independent choline transport in rat retinal capillary endothelial cells

The purpose of this study was to clarify the mechanism of the inner blood-retinal barrier (inner BRB) transport of choline and examine the choline uptake ability of rat choline transporter-like protein (CTL) 1. The transcript level of CTL1 in a conditionally immortalized rat retinal capillary endothelial cell line (TR-iBRB2) is more than 100-fold greater than that of CTL3 and CTL4, and no expression of organic cation transporter (OCT) mRNA was detected. The apparent influx permeability clearance of [(3)H]choline in the rat retina was found to be 271 microl/(min x g retina). The [(3)H]choline uptake by TR-iBRB2 cells was Na(+)-independent, potential-dependent, and concentration-dependent with Michaelis-Menten constants of 6.44 microM and 99.7 microM, and inhibited by several organic cations but not tetraethylammonium. The inhibition of CTL1 mRNA by small interfering RNA had little effect on the [(3)H]choline uptake by TR-iBRB2 cells. Rat CTL1-expressing Xenopus laevis oocytes exhibited an increase in the [(3)H]choline uptake by 45% compared with a control. In conclusion, our findings are consistent with Na(+)-independent choline transport being the mechanism for blood-to-retina transport of choline at the inner BRB. Although rat CTL1 expression is associated with the choline uptake, CTL1 does not play a major role in the choline uptake at the inner BRB.

Sex and menopausal status influence human dietary requirements for the nutrient choline

BACKGROUND

Although humans require dietary choline for methyl donation, membrane function, and neurotransmission, choline can also be derived from the de novo synthesis of phosphatidylcholine, which is up-regulated by estrogen. A recommended Adequate Intake (AI) exists for choline; however, an Estimated Average Requirement has not been set because of a lack of sufficient human data.

OBJECTIVE

The objective of the study was to evaluate the dietary requirements for choline in healthy men and women and to investigate the clinical sequelae of choline deficiency.

DESIGN

Fifty-seven adult subjects (26 men, 16 premenopausal women, 15 postmenopausal women) were fed a diet containing 550 mg choline x 70 kg(-1) x d(-1) for 10 d followed by <50 mg choline x 70 kg(-1) x d(-1) with or without a folic acid supplement (400 microg/d per randomization) for up to 42 d. Subjects who developed organ dysfunction during this diet had normal organ function restored after incremental amounts of choline were added back to the diet. Blood and urine were monitored for signs of toxicity and metabolite concentrations, and liver fat was assessed by using magnetic resonance imaging.

RESULTS

When deprived of dietary choline, 77% of men and 80% of postmenopausal women developed fatty liver or muscle damage, whereas only 44% of premenopausal women developed such signs of organ dysfunction. Moreover, 6 men developed these signs while consuming 550 mg choline x 70 kg(-1) x d(-1), the AI for choline. Folic acid supplementation did not alter the subjects' response.

CONCLUSION

Subject characteristics (eg, menopausal status) modulated the dietary requirement for choline, and a daily intake at the current AI was not sufficient to prevent organ dysfunction in 19 of the subjects.

Guar gum consumption increases hepatic nuclear SREBP2 and LDL receptor expression in pigs fed an atherogenic diet

To gain insight into the regulation of hepatic sterol-responsive genes that are thought to mediate the hypocholesterolemic effects of guar gum (GG) consumption, the mRNA and protein expression of sterol regulatory element binding protein 2 (SREBP2), LDL receptor (LDLr), and scavenger receptor class B, type 1 (SR-B1) were examined in pigs consuming an atherogenic control diet or the control diet supplemented with 10% GG. Compared with the control group, GG consumption reduced (P < 0.05) plasma total cholesterol and LDL cholesterol concentrations by 27 and 37%, respectively. Furthermore, hepatic free cholesterol concentration was lower (P < 0.05) in the GG-fed pigs in comparison with the control group. GG consumption increased hepatic LDLr mRNA (1.5-fold of the control, P = 0.09) and protein (2-fold of the control, P < 0.05) expression in comparison with the control group. However, GG consumption reduced hepatic SR-B1 mRNA to 36% of the control (P < 0.05) expression but did not affect (P = 0.19) SR-B1 protein abundance in comparison with the control group. Although SREBP2 mRNA expression was similar (P = 0.89) in the 2 groups, GG consumption increased (P < 0.05) the expression of the cytoplasmic precursor (3-fold of the control) and nuclear active forms (1.5-fold of the control) of SREBP2. We conclude that the hypocholesterolemic effects of GG consumption are related to a reduction in hepatic free cholesterol concentration and associated increases in nuclear active SREBP2 expression and hepatic LDLr abundance.

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.

Impaired trafficking of choline transporter-like protein-1 at plasma membrane and inhibition of choline transport in THP-1 monocyte-derived macrophages

The present study investigates choline transport processes and regulation of choline transporter-like protein-1 (CTL1) in human THP-1 monocytic cells and phorbol myristate 13-acetate (PMA)-differentiated macrophages. Choline uptake is saturable and therefore protein-mediated in both cell types, but its transport characteristics change soon after treatments with PMA. The maximal rate of choline uptake intrinsic to monocytic cells is greatly diminished in differentiated macrophages as demonstrated by alterations in Vmax values from 1,973 ± 118 to 380 ± 18 nmol·mg−1·min−1, when the binding affinity did not change significantly ( Km values 56 ± 8 and 53 ± 6 μM, respectively). Treatments with hemicholinim-3 effectively inhibit most of the choline uptake, establishing that a choline-specific transport protein rather than a general transporter is responsible for the observed kinetic parameters. mRNA screening for the expression of various transporters reveals that CTL1 is the most plausible candidate that possesses the described kinetic and inhibitory properties. Fluorescence-activated cell sorting analyses at various times after PMA treatments further demonstrate that the disappearance of CTL1 protein from the cell surface follows the same trend as the reduction in choline uptake. Importantly, the loss of functional CTL1 from the cell surface occurs without significant changes in total CTL1 protein or its mRNA level indicating that an impaired CTL1 trafficking is the key contributing factor to the reduced choline uptake, subsequent to the PMA-induced THP-1 differentiation to macrophages.