Fueling Brain Development: Nutrient Effects on Neurulation
Abstract
Defects in neurulation, a process in vertebrate embryonic development that forms the neural tube and eventually the central nervous system, lead to a range of disorders characterized by malformation of the brain and spinal cord (e.g., anencephaly, spina bifida). Subsequent deformation of stem-like neural crest cells migrating from the neural tube leads to malformation of neural crest-derived structures (e.g., cleft palate, albinism). Past studies have shown that folate, a nutrient involved in one-carbon metabolism, plays a vital role in reducing the occurrence of these congenital malformations. One-carbon metabolism generates methyl groups for the synthesis of biomolecules including DNA, proteins, and lipids essential to early development. Further investigation of lesser-known nutrients involved in one-carbon metabolism, including Vitamin B12, choline, and betaine, may provide new insight on preventative measures. In this review, we highlight the current understanding of nutrients implicated in the embryonic process of neurulation and how they are interrelated in one-carbon metabolism.
Introduction
Neurulation is a process during vertebrate embryonic development in which an embryonic forerunner of the nervous system called the neural plate folds to form the neural tube, eventually differentiating into the central nervous system. Neural crest cells that arise at the back side of the neural tube migrate throughout the body to form facial bone, cartilage, melanocytes, and many other tissues (Fig. 1). Congenital malformations (birth defects) arise from one of two ways: neural tube defects (NTDs) or defects in neural crest formation. NTDs result in brain or spinal cord abnormalities while defects in neural crest development disrupt tissue development which causes several disorders classified as neurocristopathies (e.g. cleft palate, albinism) [1]. Since congenital malformations arise from abnormal embryonic development, it is essential to understand how one can prevent these anomalies before and during pregnancy.
Since 1992, the Centers for Disease Control and Prevention (CDC) has advised daily consumption of 0.4 mg of folic acid, a synthetic form of Vitamin B9, to all women of reproductive age to reduce the risk of a fetus affected by NTDs [2]. This recommendation came about after several studies showed a correlation between increased folic acid consumption and reduced occurrences of NTDs, such as spina bifida and anencephaly [2]. More recent research also attributes folic acid to a reduction in neurocristopathies [3]. While B9 is a well-established nutrient that is necessary for developing embryos, there is less information on whether other nutrients have a similar effect. B9 is involved in one-carbon metabolism, a series of metabolic pathways including the B9 cycle (also known as the “folate cycle”) and methionine cycle. Together, they generate methyl groups for the synthesis of biomolecules including DNA, proteins, and lipids. Given that these biomolecules are necessary for growth and early development, it is likely that other nutrients involved in one-carbon metabolism, including Vitamin B12, choline, and betaine, may reveal new treatment options for NTDs [4]. Understanding protective or detrimental effects of other nutrients on neurulation is imperative for reducing the occurrence of such congenital malformations. This review discusses the current literature on B9 and explores information on other nutrients that have emerged over the decades since the early studies that led to the CDC’s recommendation.
Vitamin B9
Folic acid is not the only form of B9. There exists several naturally-occurring variants of the vitamin, which the CDC classifies under the general term “folate.” Though many studies report the protective effect of folate for neural tube development, the exact mechanism of how folate prevents such defects remains unclear [5]. NTDs may result from abnormal DNA repair or methylation typically regulated by folate. After folate is converted into its biologically active form as 5-methyltetrahydrofolate (5-mTHF), a methyl donor for the methylation of homocysteine to methionine, it assists in the biochemical reactions of one-carbon metabolism to generate methyl groups necessary for the synthesis and regulation of DNA and other biomolecules (Fig. 2) [6]. Thus, among the consequences of low folate levels is insufficient DNA methylation, an epigenetic modification essential to mammalian development that uses methyl groups to regulate inappropriate gene expression patterns that lead to NTDs [5]. A study on chick embryos investigated the impact of poor folate levels by inducing a loss of function on folate transporters, thereby preventing folate from accessing the neural tubes and neural crest cells during neurulation. This led to reduced levels of DNA methylation and histone H3 (another essential epigenetic modification) that eventually led to severe defects in facial development [3]. Folate occurs in small amounts in beans, fruits, and leafy vegetables [7]. Since it is challenging to consume the recommended amount of B9 naturally, many countries, including the United States, mandate the adding of folic acid into grain products (“fortification”) [8]. Due to folate and folic acid’s shared identities as B9 variants, scientists have often conflated the two as the same type of biomolecule [9]. However, folic acid enters the one-carbon metabolism through the opposite end of the B9 cycle by conversion into tetrahydrofolate (THF) [6, 9]. This suggests that folic acid acts on folate-binding enzymes differently and therefore may have different effects on the consumer than folate [10]. Nevertheless, the effectiveness of folic acid in NTD prevention has been intensely studied and supported since the 1980s. For example, in one study of 1817 women with a previous NTD-affected pregnancy, those who consumed a daily 4 mg capsule of folic acid saw a 72% reduction in NTD incidence in subsequent pregnancies [11]. Another study observed only two NTD recurrences out of 234 births from a cohort of mothers taking a daily supplement containing folic acid, compared to eleven NTD recurrences observed in the cohort of 219 unsupplemented mothers [12]. Therefore, while folate and folic acid may act on consumers very differently, both nutrients have proven themselves to be useful for preventing NTDs.
Vitamin B12
Vitamin B12 is found in animal products such as fish, milk, meat, and eggs [13]. B9 and B12 deficiency are often studied together due to their interconnecting roles in the methionine and B9 cycles (Fig. 2). In the latter, THF is eventually converted to 5-mTHF, which requires B12 as a cofactor [13]. Since B9 and B12 deficiencies share similar effects, such as decreased histone and DNA methylation, B12 deficiency is also implicated as a contributor to NTDs. B12 and B9 have also been studied together in relation to pathologies of cardiovascular disease and dementia [14].
In a study where liver samples were collected from fourteen NTD-affected fetuses, a 33.3% decrease in B12 concentration was observed in comparison to the sixteen non-NTD liver tissues [15]. A decrease in the expression and activity of methionine synthase, the enzyme linking the B9 and methionine cycles, was also observed. B12 deficiency is more prevalent in pregnant mothers carrying an NTD-affected fetus compared to unaffected mothers, with 62.9% of the NTD group being B12-deficient compared to 36.4% in the control group [16]. Since the functionality of B9 and its progression through the B9 cycle depends partly on B12, B12 deficiency is often correlated with B9 deficiency. Impaired methionine synthase causes the accumulation of homocysteine and B9 as 5-mTHF [17]. With B9 “trapped” as 5-mTHF, cells become functionally deficient in active B9 [18]. Abnormally high levels of homocysteine contribute to issues in pregnancy, such as abruptio placentae (premature separation of the placenta from the uterus), while lack of non-methylated B9, THF, and methionine impair DNA synthesis during embryonic development [19].
B12 deficiency is a common issue affecting various populations due to unintentional low intake, personal choices such as vegetarianism, or malnutrition [13]. For this reason, it is common for women to enter pregnancy with B12 levels too low to support proper B9 function [18]. While mandatory B12 fortification is uncommon, there exists treatments for B12 deficiency including supplementation through oral dosing and intramuscular injections [13]. Further research is required to determine the appropriate dosage and how that should be adjusted for populations with different levels of B12 deficiency.
Choline & Betaine
Choline is a nutrient found in animal products as well as some fruits and vegetables. Once choline is oxidized into betaine, it serves as an enzyme in one-carbon metabolism by converting homocysteine to methionine. Betaine is also obtained from food sources such as grains and beets [20]. Betaine intake has been shown to decrease homocysteine levels, which in turn decreases cardiovascular disease and stroke [21]. Choline is also a precursor to phosphatidylcholine, a major phospholipid in the eukaryotic cell membrane [22]. Thus, choline and betaine have been nutrients of interest in relation to neurulation.
One-carbon metabolism consists of a series of metabolic pathways that generate methyl groups for the synthesis of a variety of molecules including DNA and amino acids. These methyl groups also contribute to important processes such as redox and methylation reactions. Since the cofactors have interconnected roles, a change in the intake of one nutrient is likely to affect the functionality of another nutrient.
Results on whether choline reduces the chance of NTDs or neurocristopathies vary across studies. In a study where choline concentrations were measured mid-pregnancy, NTD-affected cases exhibited lower total choline values compared to control cases [22]. However, research by other groups show conflicting results on the protective effect of choline [23]. Similar trends are seen in research surrounding betaine. While some studies have found no difference in betaine concentrations between NTD-affected cases and non-affected cases, other studies have linked betaine to a decreased risk of anencephaly [20, 24]. More research is needed before determining whether food fortification of choline or betaine would be beneficial.
Conclusion
While the relationship between Vitamin B9 and defects in neurulation has been extensively studied, more information is needed to determine if low concentrations of other nutrients have a similar effect. Many preliminary studies, such as the ones previously referenced, involve comparing the concentration of a nutrient between NTD-affected pregnancies and non-affected pregnancies. At present, Vitamin B12 and B9 are shown to prevent NTDs, while there are conflicting reports on whether choline and betaine help reduce NTDs. Though the interconnected relationships of folic acid, folate, Vitamin B12, choline, and betaine in one-carbon metabolism are understood, the exact mechanism by which these nutrients affect neurulation and cause congenital abnormalities is not fully clear. Further research is required to understand the effects of dosage, how nutrient intake and deficiency vary among diets of different populations, and how nutrients are metabolized by people of different populations. Answers to these questions will help us determine whether specific measures, such as food fortification or supplementation, are necessary.
About the Author: Briani Zhang
Briani is a class of Spring 2023 Neurobiology, Physiology, and Behavior major.
Author’s Note
Soon after I joined the Rogers’ Lab, I learned that NSAIDs, when taken during pregnancy, lead to increased risk of developmental defects in the fetus. This was shocking to learn given how commonly used NSAIDs are. This information also raised the question of what other factors may affect a developing embryo, such as a pregnant mother’s health conditions, environment, or daily habits. One such habit that stuck out to me was diet. I learned that the nutrient folate had consistently shown a protective effect on reducing the chance of defects during neurulation, to the extent that the CDC actively recommends certain doses of folic acid for women well in advance of getting pregnant. As an avid food-lover, I felt that diet would be an interesting topic to research as there may be other nutrients that have either a protective or negative effect on neurulation. If so, it would be imperative to understand how one could change their diet as even a small change could potentially make a significant difference on a fetus’ development. At the same time, as a student pursuing a career in healthcare, I felt that researching this topic would help educate me on how nutrients participate in essential chemical pathways and how these pathways subsequently contribute to different pathologies that I could one day diagnose. Given how essential food is to everyday life, I think it is important to understand the impact of the nutrients we consume. For this reason, I wrote this review with the goal of not only learning more about an interesting topic, but also to inform others of a significant example of the impact of nutrition choices. I would like readers to take away the idea that dietary changes from taking a supplement to nation-wide nutrient fortification have impactful results on one’s health. At the same time, research is ever evolving so that every demographic may learn more about how to optimize their health.
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