Importance of Choline as Essential Nutrient and Its Role in Prevention of Various Toxicities

Choline is a water-soluble essential nutrient included as a member of the vitamin B12 group owing to its structural similarities with that of the other members of the group. Its roles and functions, however, extend much wider than that of the vitamins with which it is grouped. Choline is vital for maintenance of various key metabolic processes which play a role in the prevention or progression of various health impairments. The occurrence of diseases like neural tube defect (NTD) and Alzheimer’s is prevented by the metabolic role of choline. It is also indispensable for mitigation of various forms of toxic contamination. While adequate level of choline in the body is essential, an excess of choline can result in various forms of disorder. To maintain the optimal level of choline in the body can be a challenge. The vital roles played by choline together with the range of contradictions and problems that choline presents make choline an interesting area of study. This paper attempts to summarize and review some recent publications on choline that have opened up new prospect in understanding the multiple role played by choline and in throwing light on the role played by this wonder essential nutrient in mitigating various forms of toxic contamination. http://dx.doi.org/10.14712/23362936.2015.40 © Charles University in Prague – Karolinum Press, 2015 Mailing Address: Prof. Sarbani Giri, Department of Life Science and Bioinformatics, Assam University, Silchar-788011, India; e-mail: girisarbani@yahoo.com


Introduction
Choline (trimethyl-beta-hydroxyethylammonium) (Figure 1) is a dietary component that is crucial for normal functioning of all cells (Zeisel and Blusztajn, 1994).It was discovered by Andreas Strecker in 1862, but was officially recognized as an essential nutrient by the US Institute of Medicine's Food and Nutrition Board (Food and Nutrition Board, 1998).Choline is a quaternary ammonium compound that lacks ester bond and contains three methyl groups which are a vital requirement for an array of metabolic reactions.
Though choline has often been clubbed with vitamin B12 group, its functions however, suggest that it is more than just another vitamin.Choline in the diet is available as free choline or is bound as esters such as phosphocholine (Pho), glycerophosphocholine (GPCho), sphingomyelin (SM) or phosphatidylcholine (PtdCho).From these choline esters, choline is freed by pancreatic enzymes.Dietary choline from a variety of choline containing foods is absorbed by the intestine and uptake is mediated by choline transporters.The fate of choline is conversion into PtdCho (also known as lecithin), which occurs in all nucleated cells (Li and Vance, 2008).PtdCho is the predominant phospholipid (>50%) in most mammalian membranes (Zeisel, 2006a).Choline is absorbed in small intestine.Free choline enters the portal circulation and is mostly taken up by the liver (Le Kim and Betzing, 1976).Lipid soluble PtdCho and SM enter via lymph and bypass the liver.Therefore, different forms of choline could have different bioavailability (Cheng et al., 1996).Betaine, a choline derivative, plays an important role in donation of methyl groups to homocysteine to form the essential amino acid methionine (Zeisel et al., 2003).Choline uptake by liver, kidney, mammary gland, placenta and brain is of special importance.Choline and choline containing compounds are crucial for normal sustenance of life.Choline or its metabolites are important for the structural integrity of cell membranes, methyl-metabolism, transmembrane signalling, lipid and cholesterol transport, metabolism and cholinergic neurotransmission and therefore it is vital during critical periods in brain development.
Choline was added to the list of essential nutrients only in recently.In 1998, based on the contemporary research studies, the US Institute of Medicine's Food and Nutrition Board, recognized that for the maintenance of normal health, Choline and Its Role against Toxicities Prague Medical Report / Vol. 116 (2015) No. 1, p. 5-15

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humans needed to obtain choline from the diet and issued guidelines for its daily intake.Choline is found in a wide variety of foods, mainly in the form of phosphatidylcholine, which is often called lecithin.Among the most concentrated sources of dietary choline are egg yolk and offal, beef, nuts, leafy greens, legumes, seed oils, grain germs, and dairy products (Shronts, 1997).However, choline can also be synthesized de novo.
The liver is the primary site for endogenous synthesis of choline.The only other source of choline apart from normal diet is from the de novo biosynthesis of phosphatidylcholine (PtdCho).Phosphatidylethanolamine N-methyltransferase (PEMT) activity catalyses the synthesis of PtdCho by the sequential methylation of phosphatidylethanolamine (PtdEtn), using S-adenosylmethionine (AdoMet) as a methyl donor and forms a new choline moiety (Blusztajn et al., 1985).
Several factors such as gender, menopausal status, pregnancy, lactation and genetic mutation affect choline requirement of an individual and de novo synthesis of choline alone fails to meet all human requirements for choline; as a result the recommended adequate intake (AI) for choline has been set at 425 mg/day for women, 450 mg/day for pregnant women, 550 mg/day for men and lactating women as well (Food and Nutrition Board, 1998).
Choline requirement is diminished in premenopausal women because estrogen induces PEMT (Resseguie et al., 2007), the gene in liver enabling endogenous biosynthesis of choline moiety.But many women have single nucleotide polymorphism (SNPs) in the PEMT gene that repeals estrogen-induction of endogenous synthesis (Resseguie et al., 2011) and these women, therefore, require dietary choline just as men do.Thus genetic variance can have effects on choline requirement.Notably, the adequate intake of choline is increased for pregnant and breastfeeding women to satisfy the needs of the fetus and the baby whose choline is supplied via placenta and milk (Zeisel et al., 1986).Also, many of the foods that have high choline content are also high in fats or cholesterol (e.g.eggs).As a result, many people are decreasing their intake of these foods leading to a situation where only a few people today adhere to a diet that meets the recommended choline levels (Food and Nutrition Board, 1998;Jensen et al., 2007).
The reasons mentioned above have resulted in an increasing population with a choline deficient diet which ultimately impedes many normal physiological processes as well as causes a diverse group of pathological processes.In most mammals, prolonged (weeks to months) ingestion of a diet deficient in choline leads to consequences that include hepatic, renal, pancreatic, memory and growth disorders.
However, excess choline intake has detrimental effect in human too.Choline doses that are of magnitude greater than estimated intake from food have been associated with body odor, sweating, salivation, vomiting, gastrointestinal effects, hypotension and hepatotoxicity in humans (LSRO/FASEB, 1981).These apparently contradictory facts about choline, makes its maintenance in the body a challenge.

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Prague Medical Report / Vol. 116 (2015) No. 1, p. 5-15 In view of the importance of choline as a nutrient it is of utmost importance to review the protective action of choline against genotoxicity, neurotoxicity, fetotoxicity and antioxidative potentials.This review summarizes some current literature findings on the effects of choline in mitigating various toxicity targeting DNA and repair system, neurons, development and fetotoxicity.

Methodology of literature sources selection
The Tables in this review contain the summary of original research articles, casecontrol and cohort or cross sectional studies published between 2003 and 2013.A literature search was undertaken using Science Direct, PubMed and Google using specific key words.Title and abstracts were read and inclusion/exclusion being decided according to the key words which included choline along with toxicity, genotoxicity, DNA, DNA damage, apoptosis, teratogenicity, maternal health, fetal alcohol syndrome, pregnancy, antioxidant properties, neurotoxicity, neural tube defect, neuroprotection.All the abstracts were read to decide on the inclusion criteria.Studies were included based on the following parameters: i) if the focus was on relevant outcomes i.e. role of choline in mitigating genotoxicity, neurotoxicity, fetotoxicity and antioxidative potentials etc., ii) if the articles were in English.
Studies were excluded if: i) only abstract was available, ii) primary emphasis were on other one carbon nutrients like folate, vitamin B6, vitamin B12, methionine and betaine, iii) focus laid on role of choline in methionine formation via folate metabolism.

Review
Choline: its role in DNA damage and repair Choline deficiency has been associated with DNA damage (Table 1).DNA methylation is influenced by choline that ultimately influences genomic stability (Loughery et al., 2011) by altering gene expression for critical genes involved in DNA mismatch repair, resulting in increased mutation rates.Steven H. Zeisel (2012) summarized that choline deficiency increases leakage of reactive oxygen species from mitochondria which is due to altered mitochondrial membrane composition and enhanced fatty acid oxidation.Choline deficiency impairs folate metabolism since the metabolic pathways of these two have a closely knit pathway of metabolism, resulting in decreased thymidylate synthesis and increased uracil misincorporation into DNA, with strand breaks resulting during error-prone repair attempts.

Choline and neurotoxicity
A number of researches conducted since 1980s have proved that choline accelerates the synthesis and release of acetylcholine in nerve cells, which is one of the principal neurotransmitters (Haubrich et al., 1974;Cohen and Wurtman, 1975; 9)

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In ovo (chicken DT40 model system) Choline influences histone methylation, which in turn is important for the activation of DNA damage response pathways that consist of complex signalling networks that detect and repair DNA damage before the cell divides.Wecker, 1986;Zeisel, 2006a).From diminishing memory loss to preventing neural tube defects, which are a group of congenital malformations, choline plays a crucial role in combating neurotoxicity (Table 2).Though the etiology of Alzheimer's disease is unknown, postmortem studies of brain samples from Alzheimer's disease patients showed lower levels of acetylcholine (Nitsch et al., 1992).In a review, Zeisel (2006b) observes that dietary intake of choline by a pregnant mother and later by the infant directly influences brain development and results in permanent changes in brain function including memory enhancement and learning functions.
Experiments with animal models testify that choline supplementation during neonatal period prevents memory decline due to age (Meck and Williams, 2003) and decreases apoptosis rate in hippocampus of fetus whose mothers consumed high choline diets (Craciunescu et al., 2003).In a review Ziesel and da Costa (2009) highlights recent studies which show that choline supplementation during critical periods of neonatal development can have long-term beneficial effects on memory.The mechanism whereby choline supplementation in mothers results in a permanent change in memory of their offspring is not clear.It was assumed that increased brain choline results in subsequent increase in acetylcholine release.However, further investigations proved that choline supplementation to dams results in significantly greater accumulation of phosphocholine and betaine in fetal brain as compared to fetuses without choline exposure (Garner et al., 1995).
Although there are sufficient data with animal models indicating that choline is a necessary nutrient in reducing cognitive decline and it aids in brain hippocampal development and that choline supplementation during pregnancy results in multiple modifications in the patterns of gene expression known to influence learning and memory, yet there are insufficient human studies to confirm the same (Mellott et al., 2007).Also, several studies hypothesize that choline may play the protective role against neural tube defects (NTDS) by contributing methyl groups through betaine and lowering homocysteine concentration.

Choline and teratogenicity/fetotoxicity
Maternal nutrition is important for normal human development and in particular the supply of methyl groups is vital at all stages from conception to early infancy.Adequate maternal choline intake is vital to a healthy pregnancy.A number of studies have demonstrated the protective role of maternal choline supplementation (Table 3).Maternal choline intake is critical not only for proper fetal brain development, but also for maintaining normal maternal homocysteine levels.Elevated maternal homocysteine has been associated with an increased incidence of birth defects, such as neural tube defect, spontaneous abortions and low birth weight babies.

Choline as antioxidant
An undisturbed choline transport and distribution throughout the body essentially plays a vital role in multiple clinical manifestations.The methyl donation function of choline is of major importance in maintaining balanced cellular antioxidant defence systems thereby checking oxidative stress and apoptosis (Table 4).A recent review (Corbin and Zeisel, 2012) sites a battery of works which attempts to establish the intricate connection between choline deficiency and development of non-alcoholic fatty liver disease (NAFLD) which may ultimately progress to hepatocarcinogenesis.
Studies in human as well as in mouse, confirm that a deletion of choline-related genes, alteration of mitochondrial membrane composition owing to choline deficiency, chronic endoplasmic reticulum stress, levels of gut microbiome modulating the availability of choline may enhance the fatty liver disease.

Discussion and Conclusion
The recognition to choline as an essential nutrient is not a new concept.In the past choline has been recognized as an essential nutrient as being biologically Adverse effects of choline deficiency on hepatic mitochondrial structure and function could be linked to the unique signature of hepatic lipid accumulation, inflammation, and cellular and mitochondrial injury induced in mice maintained on a very high fat, protein-restricted, very low carbohydrate and ketogenic diet.Schugar et al. (2013) important, without a complete understanding about the underlying reasons.The earlier recognition was not backed up by studied evidence about its precise functions in the various complex biological processes.But there is a vital difference in the approach to the recognition following newer findings.The recent studies provide a clear insight into the molecular basis of various roles played by choline and its metabolites, leading to better understanding about the functions of these substances in health and disease development and control processes.This understanding based on the recent findings establishes choline as a vital component of our diet requirement and opens up newer areas of knowledge, ways in which many physiological conditions take place and the role played by choline in these processes.The understanding offers a clue into the mechanisms associated with diseases like Alzheimer's, non-alcoholic fatty liver disease, fetal alcohol syndrome and neural tube defects with the prospect to control, minimize and even to cure them.