Choline metabolism in human term placenta--studies on de novo synthesis and the effects of some drugs on the metabolic fate of [N-methyl 3H]choline

Dietary choline and betaine intakes vary in an adult multiethnic population

Choline and betaine are important nutrients for human health, but reference food composition databases for these nutrients became available only recently. We tested the feasibility of using these databases to estimate dietary choline and betaine intakes among ethnically diverse adults who participated in the Multiethnic Cohort (MEC) Study. Of the food items (n = 965) used to quantify intakes for the MEC FFQ, 189 items were exactly matched with items in the USDA Database for the Choline Content of Common Foods for total choline, choline-containing compounds, and betaine, and 547 items were matched to the USDA National Nutrient Database for Standard Reference for total choline (n = 547) and 148 for betaine. When a match was not found, choline and betaine values were imputed based on the same food with a different form (124 food items for choline, 300 for choline compounds, 236 for betaine), a similar food (n = 98, 284, and 227, respectively) or the closest item in the same food category (n = 6, 191, and 157, respectively), or the values were assumed to be zero (n = 1, 1, and 8, respectively). The resulting mean intake estimates for choline and betaine among 188,147 MEC participants (aged 45-75) varied by sex (372 and 154 mg/d in men, 304 and 128 mg/d in women, respectively; P-heterogeneity < 0.0001) and by race/ethnicity among Caucasians, African Americans, Japanese Americans, Latinos, and Native Hawaiians (P-heterogeneity < 0.0001), largely due to the variation in energy intake. Our findings demonstrate the feasibility of assessing choline and betaine intake and characterize the variation in intake that exists in a multiethnic population.

Pre- and postnatal health: evidence of increased choline needs

Choline, a micronutrient found in food, serves as the starting material for several important metabolites that play key roles in fetal development, particularly the brain. Although human beings' requirement for choline is unknown, an Adequate Intake level of 425 mg/day was established for women with upward adjustments to 450 and 550 mg/day during pregnancy and lactation, respectively. The importance of choline in human development is supported by observations that a human fetus receives a large supply of choline during gestation; pregnancy causes depletion of hepatic choline pools in rats consuming a normal diet; human neonates are born with blood levels that are three times higher than maternal blood concentrations; and large amounts of choline are present in human milk. The development of the central nervous system is particularly sensitive to choline availability with evidence of effects on neural tube closure and cognition. Existing data show that the majority of pregnant (and presumably lactating) women are not achieving the target intake levels and that certain common genetic variants may increase requirements for choline beyond current recommendations. Because choline is not found in most varieties of prenatal vitamins (or regular multivitamins), increased consumption of choline-rich foods may be needed to meet the high pre- and postnatal demands for choline.

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.

Free and phospholipid-bound choline concentrations in serum during pregnancy, after delivery and in newborns

The aims of this study were to determine whether serum free choline and phospholipid-bound choline concentrations change during the pregnancy or after childbirth and to determine if the serum choline concentrations of the mother and newborn are correlated. Serum free and bound choline concentrations were 10.7 +/- 0.5 microM and 2780 +/- 95 microM in control, non-pregnant women, and rose significantly (p < 0.001) to 14.5 +/- 0.6 microM and 3370 +/- 50 microM or to 16.5 +/- 0.7 microM and 3520 +/- 150 microM after 16-20 weeks or 36-40 weeks of pregnancy, respectively. Serum free and phospholipid-bound choline fell by 14-22% (p < 0.05-01) after either vaginal delivery or caesarian section, and remained low (by 15-42%; p < 0.05-0.001) for 12 h and then rose toward the baseline within 24 h. In amniotic fluid, free choline and phospholipid-bound choline concentrations were 22.8 +/- 1.0 and 19.6 +/- 0.8 microM or 24.0 +/- 1.5 and 516 +/- 43 microM at 16-20 weeks of gestational age or at term, respectively. In newborns, serum free choline concentrations were higher (p < 0.001) and phospholipid-bound choline concentrations were lower (p < 0.001) than in their mothers. These results show that serum free choline and phospholipid-bound choline concentrations are elevated during the pregnancy, which may be required for an adequate maternal supply of choline to the fetus. These observations are clinically important to determine the ideal dietary intake of choline during the pregnancy.

Regulation of the choline transport system in superfused microcarrier cultures of BeWo cells

BeWo choriocarcinoma cells were cultured onto solid microcarrier beads, packed into syringe barrels and superfused. The unidirectional choline uptake across the microvillous membrane of the cells was measured by a rapid single-circulation paired-tracer dilution procedure using methyl[3H]choline with D-[14C]mannitol as the extracellular reference molecule. Choline influx was saturable with a K(t) of 214+/-15 microM and a V(max) of 45.29+/-0.94 nmol/min/mg of cell protein. Uptake of labelled choline was partially inhibited by nicotine, strongly inhibited by hemicolinium-3, and was reduced by about 50 per cent in sodium-free perfusates. A range of agents was added to the stirrer flasks 24 h prior to the experiments to determine if intracellular or extracellular levels of choline or its metabolic product, acetylcholine, regulated choline uptake. Pre-incubation with 2 mM choline reduced the choline maximal uptake by half, while pre-incubation with 100 microM alpha-NETA [2-(alpha-naphthoyl)ethyltrimethyl-ammonium] reduced the influx by 77 per cent. Choline influx was also reduced to about half in the presence of 100 microM vesamicol, bethanecol or neostigmine. It is concluded that BeWo cells possess a choline transporter similar to that described in isolated cytotrophoblasts and syncytiotrophoblast microvillous membrane preparations, and that uptake appeared to be regulated by both intracellular and extracellular concentrations of choline and acetylcholine. Therefore, these cells provide a novel model for studying the role of acetylcholine in human placenta.

Human placental cholinergic system

The occurrence of acetylcholine (ACh)-like activity in human placenta, a tissue without innervation, has been known for more than 60 years. However, the non-neuronal functions of ACh in human placenta are not clearly understood. The components of the cholinergic system-ACh, choline acetyltransferase, acetylcholinesterase, butyrylcholinesterase, muscarinic receptors, and nicotinic receptors--in human placenta have been demonstrated by unequivocal methods. Primate placentae store and release ACh by mechanisms similar to those of nervous tissue. However, there are many gaps in our knowledge, which include: (a) endogenous quaternary ammonium compounds other than ACh in human placental extracts; (b) the specificity of placental enzymes; (c) the subtypes and structures of placental muscarinic and nicotinic receptors; and (d) the significance of placental alpha-bungarotoxin binding proteins, ACh receptor stimulation-cellular signaling by second messengers, and activation of immediate early target genes (C-fos, C-jun) encoding transcription factors. Several hypothetical non-neuronal functions of ACh in placenta have been postulated based upon available experimental evidence. These include: (a) regulation of blood flow and fluid volume in placental vessels; (b) opening and closing of trophoblastic channels; (c) induction of contractile properties to myofibroblasts; (d) facilitation of amino acid transport necessary for fetal growth across placenta; (e) release of placental hormones; and (f) modulation of the formation of myometrial and placental prostaglandins in human parturition. All of these roles are reasonable, and some of these roles mav turn out to be linked to one another to influence or maintain placental function.

Isolation of syncytial microvillous membrane vesicles from human term placenta and their application in drug-nutrient interaction studies

The initial step in placental uptake of nutrients occurs across the syncytial microvillous membrane of the trophoblast. This study was designed to isolate syncytial microvillous membrane vesicles (SMMV) of human term placenta, to validate their purity and viability, and to investigate the interaction of several commonly used drugs with the transport of two essential nutrients: alanine and choline. SMMV were isolated according to an established procedure, but instead of homogenization the initial preparation step was replaced by mincing of placental tissue followed by gently stirring to loosen the microvilli. These modifications doubled the protein recovery and increased the enrichment in alkaline phosphatase, whereas no substantial contamination with basal membranes nor interfering subcellular organelles was found. The functional viability of the vesicles was evaluated through the transport of alanine. In accordance with literature, uptake was sodium-dependent, inhibitable by structural analogues, and saturable. A number of cationic drugs were were able to able to inhibit choline uptake, whereas no effect on alanine transport was observed. Anionic drugs, drugs of abuse, and catecholamines did not interfere with alanine transport either. In conclusion, our isolated SMMV provide a suitable tool for screening drug-nutrient interactions at the level of membrane transport. In view of the very low susceptibility of the alanine transporter to drug inhibition and the relatively high drug concentrations necessary to inhibit choline transport, it seems unlikely that clinically important drug interactions may occur with these nutrients.

Choline transport in human placental brush-border membrane vesicles

Pathways for transport of choline by human placental epithelia were investigated using brush border membrane vesicles isolated by divalent cation precipitation. The presence of choline transport mechanisms mediating Na(+)-choline cotransport, choline/H+ exchange and facilitated diffusion were assessed from [3H]choline tracer flux measurements. The rate and magnitude of intravesicular choline accumulation was unaffected by the imposition of an inwardly directed Na+ gradient suggesting an absence of a mechanism mediating brush border membrane Na(+)-choline cotransport. The imposition of inside-acid or inside-alkaline pH gradients was observed to have no significant effect on choline uptake suggesting choline is not a substrate for placental epithelial organic cation/H+ exchange. Conditions favoring the development of an inside-negative K+ diffusion potential was observed to induce a concentrative accumulation of choline to levels exceeding equilibrium suggesting the presence of a conductive uptake pathway for choline in placental brush border membrane. Evidence to suggest conductive choline uptake resulted from a mediated transport process includes a demonstration of the counterflow phenomena, the concentration-dependent inhibition by hemicholinium-3 (IC50 approximately equal to 100 microM) and the saturable rate of conductive choline uptake (Km approximately equal to 300 microM, Vmax approximately equal to 30 nmol/mg per min). Substrate specificity studies of the mechanism mediating conductive choline uptake suggest the interaction of choline with the transport protein occurs at a minimum of two sites: a site of negativity with the positively charged nitrogen group and a site of hydrogen bonding to the primary alcohol. Several commonly prescribed pharmaceuticals known to cross the placental barrier including imipramine, verapamil, propranolol, quinine, flurazepam, amiloride and ritodrin were observed to inhibit conductive choline uptake suggesting an interaction with the mechanism mediating conductive choline transport. Conductive choline uptake was unaffected by the presence of the basic amino acids lysine, arginine and histidine; the neurotransmitters serotonin, dopamine and histamine and the vitamins thiamine and carnitine which suggests the mechanism mediating conductive choline transport is not a pathway for placental uptake of these compounds.

Uptake of choline into syncytial microvillus membrane vesicles of human term placenta

The uptake of the quaternary ammonium compound choline was studied in syncytial microvillus membrane vesicles of human term placenta. Uptake was stimulated by an inside negative membrane potential and by loading the vesicles with unlabeled choline. Imposition of an inwardly directed Na+ or outwardly directed H+ gradient did not stimulate choline uptake. Several organic cations were able to inhibit choline transport in the following order: hemicholinium-3 > or = choline > or = mepiperphenidol > cimetidine > or = famotidine. The kinetics of uptake involved a saturable process for choline with high affinity (Km = 550 microM). Our results confirm the presence of a carrier mediated transport system in human placental syncytial microvillus membranes. The system appears to be electrogenic, and able to transport choline efficiently from the maternal circulation into the placenta.

Transtrophoblast and microvillus membrane potential difference in mature intermediate human placental villi

Single mature intermediate villi from term placentas after normal gestation and vaginal or cesarian delivery were identified microscopically and mounted in a bath for conventional micro-electrode studies. With the application of strict selection criteria, the following observations on electrical potential difference (PD) were made. 1) With the tissue bathed in Earle's medium (37 degrees C) the PD across the syncytiotrophoblast microvillus membrane with respect to the bath was not normally distributed. The median PD was -22 mV (range -12 to -60 mV, n = 200). This fell to -6 mV after prior incubation for 4 h with cyanide (3 mM) or iodoacetate (2 mM) but was not altered by short-term application of these agents or by 0.1 mM ouabain. Substituting Na+ in Earle's medium with choline had no effect on PD, but replacing Cl- with gluconate caused a depolarization of 6 mV (P < 0.002). Increasing KCl in the bath fluid revealed an apparently low microvillus membrane K+ conductance. The low microvillus membrane PD may reflect a low Na(+)-K(+)-ATPase activity and/or a low membrane permeability to K+. 2) The transtrophoblast PD measured by insertion and withdrawal of the electrode was significantly different from zero [P < 0.003, median -3 mV (range 0 to -15 mV), n = 11]; and PD measured by insertion of the electrode into the villus core and beyond was -6 mV (significantly different from zero P < 0.003, range -2.5 to -10 mV, n = 6). If a similar PD were to exist in vivo, it could have a significant influence on ion transport across the placenta at term.

Characterization of choline metabolism and secretion by human placental trophoblasts in culture

Choline is an essential nutrient for fetal development and may be utilized to form phospholipids such as phosphatidylcholine and sphingomyelin; to synthesize the neurotransmitter, acetylcholine; and to donate methyl groups after being oxidized to betaine. Since the majority of choline required for fetal growth must be transported by the placenta from the maternal circulation, we examined the ability of isolated human trophoblasts to metabolize choline and to release choline and its metabolites into culture medium. Cytotrophoblasts were isolated from normal, full-term human placentas and incubated with [14C]choline for 3 h; the cells were washed to remove extracellular radiolabel, and the changes in intracellular and medium choline pools were followed for an additional 24 h. During the incubation, choline rapidly reached steady state intracellularly and label was incorporated into betaine, phosphocholine, cytidylyldiphosphocholine, phosphatidylcholine, glycerophosphocholine, lysophosphatidylcholine, and sphingomyelin. All labeled choline metabolites in cells, except glycerophosphocholine, decreased at 6 and 27 h of incubation (3 and 24 h, respectively, after labeled choline was removed), and labeled metabolites appeared in media. By 24 h after labeled choline was removed, the major labeled metabolites in the media were choline (82%), betaine (11%), and glycerophosphocholine (5%). Small amounts of phosphatidylcholine (1%), and lysophosphatidylcholine (1%) were found. Acetylcholine was a very minor choline metabolite in these cells. When placental cells were incubated for 66 h after isolation, they formed syncytiotrophoblasts, which incorporated labeled choline into metabolites in a similar pattern to cytotrophoblasts. These data indicate that isolated trophoblast cells can metabolize choline to form all of its major metabolites and that several metabolites are released to the medium in significant amounts. Thus, our data suggest that the major metabolite supplied to the fetus may be choline, but that betaine and glycerophosphocholine may also be vehicles for transfer of choline equivalents from mother to fetus.

Characterization of choline transport at maternal and fetal interfaces of the perfused guinea-pig placenta

Unidirectional influx and efflux of choline into the syncytiotrophoblast were investigated from both maternal and fetal circulations of the perfused guinea-pig placenta by using a single-circulation paired-tracer (extracellular reference and test substrate) dilution technique. Cellular uptake of [3H]choline at 0.05 mM was (mean percentage +/- S.E. of mean, n = 14 placentae) 51 +/- 2 and 49 +/- 2, on maternal and fetal sides, respectively. Kinetics of unidirectional influx (0.05-4.0 mM-choline) indicated the existence of saturable and non-saturable components on both sides: on maternal and fetal interfaces the Km (mM) values were respectively, 0.12 and 0.13, the Vmax (mumol min-1 g-1) values, 0.08 and 0.07 and the apparent linear transfer constants (min-1 g-1) 0.11 and 0.12. Efflux of [3H]choline from the placenta back into the ipsilateral circulation (backflux) was generally fast (20-60% in 5-6 min) and asymmetric with the fetal: maternal ratio usually above unity. Transplacental specific choline transfer in the dually perfused placenta, when observed, was small (less than 10% of the injected dose) following tracer injections in either direction based on the 5-6 min collection of the contralateral circulation (at 0.05 mM-choline). Placental retention of [3H]choline at the end of the 5-6 min period was about 25% of the injected dose when the tracers were injected from either circulation. Analogues of choline such as hemicholinium-3, thiamine, ethanolamine and N,N-dimethylethanolamine inhibited choline unidirectional influx, whereas betaine and acetate had no effect. The absence of the normal sodium gradient (perfusate sodium was replaced by Tris or by lithium) did not inhibit choline transport. The metabolic inhibitors dinitrophenol (1.0 mM) and potassium cyanide (1.0 mM) were essentially ineffective (up to 40 min perfusion). The sulphydryl reagent N-ethylmaleimide did not appear to inhibit the influx, in comparison with its effect on [3H]choline backflux which was greatly accelerated, resulting in a dramatic reduction in placental net uptake of the label. Our findings show that choline transport into the placenta is a rapid carrier-mediated process occurring at both maternal and fetal sides of the trophoblast, at physiological blood concentrations. This cellular uptake is possibly related to the synthesis of acetylcholine, which is known to occur in human placental tissue. Specific transplacental transfer of choline was a very slow process under the conditions of our experiments and this contrasted with the observed fast and high uptake into the trophoblast.

Investigation of the rate-limiting step in the synthesis of acetylcholine by the human placenta

The uptake of [3H]choline and its conversion to [3H]acetylcholine were investigated in term human placental tissue in vitro. Although the net uptake of [3H]choline increased throughout a 45 min incubation period, intracellular [3H]choline levels reach a plateau after 2 min. There was a constant increase in [3H]acetylcholine levels throughout the incubation period. After 45 min, 36.5 per cent of the total intracellular tritium was recovered as acetylcholine by high-voltage electrophoresis. The effects of the choline acetyltransferase inhibitors, 2-benzoylethyltrimethyl-ammonium chloride (BETA) and 4-naphthylvinyl pyridine (NVP), and an inhibitor of choline uptake, hemicholinium-3 (HC-3), were also investigated for their influence on the uptake and metabolism of [3H]choline. A significant depression in both [3H]choline uptake and [3H]acetylcholine synthesis could be demonstrated with all three compounds, although with somewhat different time courses and activities. An analysis of the accumulation of [3H]acetylcholine in relation to the uptake and intracellular levels of [3H]choline as well as the patterns of inhibition produced by the inhibitors indicates that, unlike nervous tissue, the rate-limiting step in the synthesis of acetylcholine in human placental tissue is the transacetylation reaction catalysed by choline acetyltransferase.

Choline metabolism in placenta: evidence for the biosynthesis of phosphatidylcholine in microsomes via the methylation pathway

Microsomes from human, mouse and rat placenta were found to contain enzymatic activity which methylates the phospholipids phosphatidylethanolamine (PE), phosphatidyl-N-monomethylethanolamine (PMME) and phosphatidyl-N,N-dimethylethanolamine (PDME) to form phosphatidylcholine (PCh) with 3H-methyl-S-adenosyl-l-methionine as the methyl donor. The three labelled reaction products were isolated by solvent extraction and separated on thin-layer chromatography (TLC) plates. The endogenous methyltransferase activity was low, indicating that the methylation pathway is quantitatively not important for the synthesis of free choline to meet the fetal needs. The distribution of 3H-methyl among PMME, PDME and PCh revealed fairly even labelling of all products when analysed by TLC. Addition of authentic PE, PMME and PDME to a level of approximately 2.5 mM stimulated the incorporation of 3H-methyl into the total lipid-soluble fraction with all three substrates, but was most pronounced with PMME. Present observations suggest that all three methylation steps were catalysed by one enzyme with a pH optimum of 9.0 in a reaction that does not require Mg++.

Acetylcholine in human placenta. Identification by pyrolysis gas chromatography/mass spectrometry and tissue levels following different modes of delivery

Extracts from human term placentae were analyzed by pyrolysis gas chromatography (GC) combined with mass spectrometry. The only choline ester of carbonic acid whose presence could be unequivocally established was acetylcholine (ACh). The mass spectrum of its demethylated tertiary derivative dimethylaminoethyl acetate (nor-ACh) agreed entirely with that of authentic pyrolyzed ACh. The concentrations of ACh were determined in placentae obtained after vaginal or Caesarean delivery to investigate the claim that the latter resulted in tissue with higher concentrations of ACh-like activity presumably because ACh was not expended during labor and delivery. No differences inACh content were found with GC analysis. The ACh concentrations were 102 +/- 17 (n=8) nmoles/g fresh tissue (vaginal delivery) vs. 105 +/- 16 (n=6) nmoles/g (Caesarean section). The fetal membranes (amnion and chorion) and the umbilical cord contained no ACh under the analytical conditions with a limit of sensitivity of 200 pmoles ACh.