- Email: [email protected]
Why is folate effective in preventing neural tube closure defects?
Medical Hypotheses 134 (2020) 109429
Contents lists available at ScienceDirect
Medical Hypotheses journal homepage: www.elsevier.com/locate/mehy
Why is folate effective in preventing neural tube closure defects? Kohji Sato
⁎
T
Department of Organ & Tissue Anatomy, Hamamatsu University School of Medicine, 1-20-1 Handayama, Higashiku, Hamamatsu, Shizuoka 431-3192, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Neural tube defects Folate Glycine cleavage system Nucleic acids synthesis Neuroepithelium
Neural tube defects (NTDs) originate from a failure of the embryonic neural tube to close. The pathogenesis of NTDs is largely unknown. Fortunately, adequate maternal folate application is known to reduce the risk of human NTDs. However, why folate reduces NTDs is largely unknown. The main cause for NTDs is the disturbance of the cell growth in the neuroepithelium. Of course, rapid cell growth needs enough synthesis of nuclei acids. Interestingly, folate is used as a source for the synthesis of nucleic acids. Furthermore, glycine cleavage system (GCS) is essential for the synthesis of nucleic acids from folate, and very strongly expressed in neuroepithelial cells, suggesting that these highly proliferating cells need enough synthesis of nuclei acids and high amounts of folate. Taken together, I speculate the following hypothesis; (1) The closure of the neural tube requires rapid growth of neuroepithelial cells. (2) High rates of nuclei acids synthesis are needed for the rapid growth. (3) GCS, which is requisite in nucleic acid synthesis from folate, is expressed very strongly and functions robustly in neuroepithelial cells. (4) Pregnant women require 5–10-fold higher amounts of folate compared to non-pregnant women. (5) So, folate-deficient situations are easy to occur in neuroepithelial cells, resulting in NTDs. (6) Thus, folate is effective to prevent NTDs.
Introduction The process of neurulation occurs during early embryogenesis, starting with the formation of the neural plate from specialized ectodermal cells and the neural plate develops bilateral neural folds, which elevate and fuse at the midline to create the neural tube [1]. Neural tube defects (NTDs), such as spina bifida and anencephaly, originate from a failure of the embryonic neural tube to close [2] with a worldwide incidence ranging from 1.0 to 10.0 per 1000 births [3]. The pathogenesis of NTD is genetically complex and diverse, and the pathogenetic mechanisms are largely unknown [2]. Fortunately, adequate maternal folate application is known to reduce the risk of human NTDs [1]. However, why folate reduces NTDs is largely unknown. Thus, in this manuscript, I try to find the missing link between them. Hypothesis NTDs originate from a failure of the embryonic neural tube to close. The main cause is the disturbance of the cell growth in the neuroepithelium of the neural plate and neural fold. Of course, rapid cell growth needs enough synthesis of nuclei acids. Interestingly, folate is very deeply involved in the synthesis of nucleic acids. Folate one-
carbon metabolism comprises a network of enzymatic reactions required for synthesis of purines and thymidylate for DNA synthesis [4]. Furthermore, glycine cleavage system (GCS), a key enzyme complex in folate one-carbon metabolism and very important for DNA synthesis, is very strongly expressed and functions in the neuroepithelium [5], suggesting that these highly proliferating cells need enough synthesis of nuclei acids and high amounts of folate. On the other hand, adequate folate intake is particularly important for pregnant women, in whom folate requirements are 5–10-fold higher compared to non-pregnant women [6]. So, folate-deficient situations are easy to occur in neuroepithelial cells of the neural plate and neural fold, resulting in NTDs. Taken together, I speculate the following hypothesis (Figs. 1 and 2). (1) The closure of the neural tube requires rapid growth of neuroepithelial cells. (2) High rates of nuclei acids synthesis are needed for the rapid growth. (3) GCS, which is requisite in nucleic acid synthesis from folate, is expressed very strongly and functions robustly in neuroepithelial cells. (4) Pregnant women require 5–10-fold higher amounts of folate compared to non-pregnant women. (5) So, folate-deficient situations are easy to occur in neuroepithelial
⁎ Address: Department of Anatomy & Neuroscience, Hamamatsu University School of Medicine, 1-20-1 Handayama, Higashiku, Hamamatsu, Shizuoka 431-3192, Japan. E-mail address: [email protected].
https://doi.org/10.1016/j.mehy.2019.109429 Received 24 June 2019; Received in revised form 3 October 2019; Accepted 10 October 2019 0306-9877/ © 2019 Elsevier Ltd. All rights reserved.
Medical Hypotheses 134 (2020) 109429
K. Sato
Fig. 2. Cartoon for the theory. The neural plate develops bilateral neural folds (arrows in A). The neural folds elevate and make the neural groove (arrows in B). The neural groove fuses at the midline to create the neural tube (C). NTDs originate from a failure of the embryonic neural tube to close. In every stage, neuroepithelium expresses abundant GCS (red areas), and GCS functions to make enough nucleic acids for cell growth from abundant up-taken folate (blue circles). Thus, folate-deficient situations are easy to occur in neuroepithelial cells, resulting in NTDs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
brain [5]. Interestingly, GCS is very strongly expressed and functions robustly in neuroepithelial cells during the neural tube formation [5]. As shown in Fig. 2, the neural plate develops bilateral neural folds (arrows in Fig. 2A), and the neural folds elevate and make the neural groove (arrows in Fig. 2B), and finally the neural groove fuses at the midline to create the neural tube (Fig. 2C). NTDs originate from a failure of the embryonic neural tube to close. In every stage, neuroepithelium expresses abundant GCS (red areas in Fig. 2), further supporting pivotal roles of GCS in the neural tube formation.
Fig. 1. Basic concept.
cells, resulting in NTDs. (6) Thus, folate is effective to prevent NTDs. Evaluation of the hypothesis GCS is highly expressed in neuroepithelium
Glycine cleavage system is requisite in nucleic acid synthesis from folate in the neuroepithelium
GCS is the main system to degrade glycine in mammalians. The defect of this pathway causes nonketotic-hyperglycinemia (NKH), which is an autosomal recessive disorder associated with very severe clinical symptoms [7] and also grave brain malformations including NTDs [8–10]. These severe symptoms and grave brain malformations are not normally observed in the other amino acid metabolic disorders, suggesting that GCS and glycine should have unknown pivotal roles in brain development. The distribution of GCS has been investigated in the developing
GCS is indispensable in supplying proliferating cells with 5,10-methylenetetrahydrofolate as a one-carbon donor, which is essential for the synthesis of nucleic acids in cell proliferation [11]. Because neuroepithelial cells proliferate very rapidly, a large amount of 5,10-methylenetetrahydrofolate should be supplied via GCS in these cells. As shown in Fig. 3, in the neuroepithelium, up-taken folate is converted to 2
Medical Hypotheses 134 (2020) 109429
K. Sato
supplementary maternal peri-conceptional intake of folate has been demonstrated to reduce the occurrence of human NTD by as much as 70% [26] and has led to government policies of fortification of various foods with folate in some countries [27]. Pregnant women require 5–10-fold higher amounts of folate compared to non-pregnant women [6]. So, folate-deficient situations are easy to occur in neuroepithelial cells, where large amounts of folate are needed to make nuclei acids, resulting in NTDs. Thus, folate is effective to prevent NTDs. Consequences of the hypothesis and discussion Neural tube defects (NTDs) originate from a failure of the embryonic neural tube to close. In this report, I speculate the following hypothesis; (1) The closure of the neural tube requires rapid growth of neuroepithelial cells. (2) High rates of nuclei acids synthesis are needed for the rapid growth. (3) GCS, which is requisite in nucleic acid synthesis from folate, is expressed very strongly and functions robustly in neuroepithelial cells. (4) Pregnant women require 5–10-fold higher amounts of folate compared to non-pregnant women. (5) So, folatedeficient situations are easy to occur in neuroepithelial cells, resulting in NTDs. (6) Thus, folate is effective to prevent NTDs. Although supplementary maternal peri-conceptional intake of folate has led to government policies of fortification of various foods with folate in some countries [27]. Further promotion of mandatory folate fortification is needed to prevent NTDs.
Fig. 3. GCS in the synthesis of nucleic acids from folate. In the neuroepithelium, folate is up-taken from the blood, and folate is converted to tetrahydrofolate (THF). GCS breaks down glycine and generates 5,10-methylaneTHF from THF. And, 5,10-methylane-THF is used to make nucleic acids.
tetrahydrofolate (THF), and GCS generates 5,10-methylane-THF from THF. And, 5,10-methylane-THF is used to make nucleic acids [12]. Interestingly, the neuroepithelium also expresses very high levels of folate-binding proteins, which are needed to take up folate as a source of 5,10-methylenetetrahydrofolate [13,14]. In addition, it has been demonstrated that folate deficiency affects proliferation of neural stem cells [15] and the association between neural tube defects and inadequate folate intake has already been established [11,16]. Thus, I speculate that embryonic neuroepithelial cells transport folate for de novo synthesis of sufficient nucleic acids for proliferation, and that GCS may be indispensable for this process. Therefore, I think that the lack of folate greatly impairs proliferation of neural stem cells, leading to NTDs. Actually, GCS-encoding genes represent candidates for involvement in NTDs [12]. In addition, loss-of-function mutation in GCS genes predisposes to NTDs in mice and humans [12], supporting my theory.
Grant Sponsor The Ministry of Education, Science and Culture of Japan; Shintenkai. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Additional possibilities about the involvement of folate-depletion in the pathogenesis of NTDs Besides the functions in nucleic acid synthesis, folate has many other roles in various biological events. Thus, folate-depletion may also cause NTDs via the disturbance of many other biological events in the development of the neuroepithelium. For example, maternal folate deficiency has been shown to affect DNA repair and DNA methylation [17], synthesis and transport of fatty acids [18] and choline metabolism [19]. Since these events are also very important in the development of the neuroepithelium, the disturbance of these events may impair proliferation of neural stem cells, leading to NTDs. Furthermore, cell–cell adhesion molecules, such as cadherins, are reported to be very important for the complete closure of neural tube [20]. For example, during neural tube formation, E-cadherin and Ncadherin play pivotal roles [21]. Interestingly, neural tube formation is impaired in N-cadherin knockout mice [22]. In addition, Su et al. have reported that folate deficiency causes the down-regulation of E-cadherin [23]. Taken together, there is a possibility that folate-depletion induces aberrant functions of cell–cell adhesion molecules, such as cadherins, leading to NTDs. In addition, it has been reported that the number of apoptotic cells was almost doubled in the septum and hippocampus of offspring of folate-depleted mothers [24], suggesting that the increase of apoptosis might be also involved in the pathogenesis of NTDs.
Acknowledgement The author would like to thank Ms Chiemi Kawamoto for her excellent technical help. References [1] Blom HJ, Shaw GM, den Heijer M, Finnell RH. Neural tube defects and folate: case far from closed. Nat Rev Neurosci 2006;7:724–31. [2] Juriloff DM, Harris MJ. Insights into the etiology of mammalian neural tube closure defects from developmental, genetic and evolutionary. Studies J Dev Biol 2018;6:E22. [3] Au KS, Ashley-Koch A, Northrup H. Epidemiologic and genetic aspects of spina bifida and other neural tube defects. Dev Disabil Res Rev 2010;16:6–15. [4] Beaudin AE, Stover PJ. Insights into metabolic mechanisms underlying folate-responsive neural tube defects: a minireview. Birth Defects Res A Clin Mol Teratol 2009;85:274–84. [5] Ichinohe A, Kure S, Mikawa S, Ueki T, Kojima K, Fujiwara K, et al. Glycine cleavage system in neurogenic regions. Eur J Neurosci 2004;19:2365–70. [6] Tamura T, Picciano MF. Folate and human reproduction. Am J Clin Nutr 2006;83:993–1016. [7] Hoover-Fong JE, Shah S, Van Hove JL, Applegarth D, Toone J, Hamosh A. Natural history of nonketotic hyperglycinemia in 65 patients. Neurology 2004;63:1847–53. [8] Press GA, Barshop BA, Haas RH, Nyhan WL, Glass RF, Hesselink JR. Abnormalities of the brain in nonketotic hyperglycinemia: MR manifestations. AJNR Am J Neuroradiol 1989;10:315–21. [9] Dobyns WB. Agenesis of the corpus callosum and gyral malformations are frequent manifestations of nonketotic hyperglycinemia. Neurology 1989;39:817–20. [10] Nissenkorn A, Michelson M, Ben-Zeev B, Lerman-Sagie T. Inborn errors of metabolism: a cause of abnormal brain development. Neurology 2001;56:1265–72. [11] Fleming A, Copp AJ. Embryonic folate metabolism and mouse neural tube defects. Science 1998;280:2107–9. [12] Narisawa A, Komatsuzaki S, Kikuchi A, Niihori T, Aoki Y, Fujiwara K, et al. Mutations in genes encoding the glycine cleavage system predispose to neural tube defects in mice and humans. Hum Mol Genet 2012;21:1496–503.
Folate is effective in preventing neural tube defects The importance of an adequate periconceptional maternal folate status to prevent fetal neural tube defects has been well demonstrated and resulted in the recommendation for women to use folate supplements during the periconceptual period [25]. For example, 3
Medical Hypotheses 134 (2020) 109429
K. Sato
[20] Nishimura T, Honda H, Takeichi M. Planar cell polarity links axes of spatial dynamics in neural-tube closure. Cell 2012;149:1084–97. [21] Hatta K, Takeichi M. Expression of N-cadherin adhesion molecules associated with early morphogenetic events in chick development. Nature 1986;320:447–9. [22] Radice GL, Rayburn H, Matsunami H, Knudsen KA, Takeichi M, Hynes RO. Developmental defects in mouse embryos lacking N-cadherin. Dev Biol 1997;181:64–78. [23] Su YH, Huang WC, Huang TH, Huang YJ, Sue YK, Huynh TT, et al. Folate deficient tumor microenvironment promotes epithelial-to-mesenchymal transition and cancer stem-like phenotypes. Oncotarget 2016;7:33246–56. [24] Craciunescu CN, Brown EC, Mar MH, Albright CD, Nadeau MR, Zeisel SH. Folic acid deficiency during late gestation decreases progenitor cell proliferation and increases apoptosis in fetal mouse brain. J Nutr 2004;134:162–6. [25] Naninck EFG, Stijger PC, Brouwer-Brolsma EM. The importance of maternal folate status for brain development and function of offspring. Adv Nutr 2019;10:502–19. [26] MRC Vitamin Study Research Group. Prevention of neural tube defects: results of the Medical Research Council Vitamin Study. MRC Vitamin Study Research Group Lancet 1991;338:131–7. [27] Garrett GS, Bailey LB. A public health approach for preventing neural tube defects: folic acid fortification and beyond. Ann N Y Acad Sci 2018;1414:47–58.
[13] Finnell RH, Wlodarczyk BC, Craig JC, Piedrahita JA, Bennett GD. Strain-dependent alterations in the expression of folate pathway genes following teratogenic exposure to valproic acid in a mouse model. Am J Med Genet 1997;70:303–11. [14] Piedrahita JA, Oetama B, Bennett GD, et al. Mice lacking the folic acid-binding protein Folbp1 are defective in early embryonic development. Nat Genet 1999;23:228–32. [15] Mattson MP, Shea TB. Folate and homocysteine metabolism in neural plasticity and neurodegenerative disorders. Trends Neurosci 2003;26:137–46. [16] Whitehead AS, Gallagher P, Mills JL, Kirke PN, Burke H, Molloy AM, et al. A genetic defect in 5,10 methylenetetrahydrofolate reductase in neural tube defects. QJM 1995;88:763–6. [17] Langie SA, Achterfeldt S, Gorniak JP, Halley-Hogg KJ, Oxley D, van Schooten FJ, et al. Maternal folate depletion and high-fat feeding from weaning affects DNA methylation and DNA repair in brain of adult offspring. FASEB J 2013;27:3323–34. [18] da Silva RP, Kelly KB, Al Rajabi A, Jacobs RL. Novel insights on interactions between folate and lipid metabolism. BioFactors 2014;40:277–83. [19] Jadavji NM, Deng L, Malysheva O, Caudill MA, Rozen R. MTHFR deficiency or reduced intake of folate or choline in pregnant mice results in impaired short-term memory and increased apoptosis in the hippocampus of wild-type offspring. Neuroscience 2015;300:1–9.
4
Contents lists available at ScienceDirect
Medical Hypotheses journal homepage: www.elsevier.com/locate/mehy
Why is folate effective in preventing neural tube closure defects? Kohji Sato
⁎
T
Department of Organ & Tissue Anatomy, Hamamatsu University School of Medicine, 1-20-1 Handayama, Higashiku, Hamamatsu, Shizuoka 431-3192, Japan
A R T I C LE I N FO
A B S T R A C T
Keywords: Neural tube defects Folate Glycine cleavage system Nucleic acids synthesis Neuroepithelium
Neural tube defects (NTDs) originate from a failure of the embryonic neural tube to close. The pathogenesis of NTDs is largely unknown. Fortunately, adequate maternal folate application is known to reduce the risk of human NTDs. However, why folate reduces NTDs is largely unknown. The main cause for NTDs is the disturbance of the cell growth in the neuroepithelium. Of course, rapid cell growth needs enough synthesis of nuclei acids. Interestingly, folate is used as a source for the synthesis of nucleic acids. Furthermore, glycine cleavage system (GCS) is essential for the synthesis of nucleic acids from folate, and very strongly expressed in neuroepithelial cells, suggesting that these highly proliferating cells need enough synthesis of nuclei acids and high amounts of folate. Taken together, I speculate the following hypothesis; (1) The closure of the neural tube requires rapid growth of neuroepithelial cells. (2) High rates of nuclei acids synthesis are needed for the rapid growth. (3) GCS, which is requisite in nucleic acid synthesis from folate, is expressed very strongly and functions robustly in neuroepithelial cells. (4) Pregnant women require 5–10-fold higher amounts of folate compared to non-pregnant women. (5) So, folate-deficient situations are easy to occur in neuroepithelial cells, resulting in NTDs. (6) Thus, folate is effective to prevent NTDs.
Introduction The process of neurulation occurs during early embryogenesis, starting with the formation of the neural plate from specialized ectodermal cells and the neural plate develops bilateral neural folds, which elevate and fuse at the midline to create the neural tube [1]. Neural tube defects (NTDs), such as spina bifida and anencephaly, originate from a failure of the embryonic neural tube to close [2] with a worldwide incidence ranging from 1.0 to 10.0 per 1000 births [3]. The pathogenesis of NTD is genetically complex and diverse, and the pathogenetic mechanisms are largely unknown [2]. Fortunately, adequate maternal folate application is known to reduce the risk of human NTDs [1]. However, why folate reduces NTDs is largely unknown. Thus, in this manuscript, I try to find the missing link between them. Hypothesis NTDs originate from a failure of the embryonic neural tube to close. The main cause is the disturbance of the cell growth in the neuroepithelium of the neural plate and neural fold. Of course, rapid cell growth needs enough synthesis of nuclei acids. Interestingly, folate is very deeply involved in the synthesis of nucleic acids. Folate one-
carbon metabolism comprises a network of enzymatic reactions required for synthesis of purines and thymidylate for DNA synthesis [4]. Furthermore, glycine cleavage system (GCS), a key enzyme complex in folate one-carbon metabolism and very important for DNA synthesis, is very strongly expressed and functions in the neuroepithelium [5], suggesting that these highly proliferating cells need enough synthesis of nuclei acids and high amounts of folate. On the other hand, adequate folate intake is particularly important for pregnant women, in whom folate requirements are 5–10-fold higher compared to non-pregnant women [6]. So, folate-deficient situations are easy to occur in neuroepithelial cells of the neural plate and neural fold, resulting in NTDs. Taken together, I speculate the following hypothesis (Figs. 1 and 2). (1) The closure of the neural tube requires rapid growth of neuroepithelial cells. (2) High rates of nuclei acids synthesis are needed for the rapid growth. (3) GCS, which is requisite in nucleic acid synthesis from folate, is expressed very strongly and functions robustly in neuroepithelial cells. (4) Pregnant women require 5–10-fold higher amounts of folate compared to non-pregnant women. (5) So, folate-deficient situations are easy to occur in neuroepithelial
⁎ Address: Department of Anatomy & Neuroscience, Hamamatsu University School of Medicine, 1-20-1 Handayama, Higashiku, Hamamatsu, Shizuoka 431-3192, Japan. E-mail address: [email protected].
https://doi.org/10.1016/j.mehy.2019.109429 Received 24 June 2019; Received in revised form 3 October 2019; Accepted 10 October 2019 0306-9877/ © 2019 Elsevier Ltd. All rights reserved.
Medical Hypotheses 134 (2020) 109429
K. Sato
Fig. 2. Cartoon for the theory. The neural plate develops bilateral neural folds (arrows in A). The neural folds elevate and make the neural groove (arrows in B). The neural groove fuses at the midline to create the neural tube (C). NTDs originate from a failure of the embryonic neural tube to close. In every stage, neuroepithelium expresses abundant GCS (red areas), and GCS functions to make enough nucleic acids for cell growth from abundant up-taken folate (blue circles). Thus, folate-deficient situations are easy to occur in neuroepithelial cells, resulting in NTDs. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
brain [5]. Interestingly, GCS is very strongly expressed and functions robustly in neuroepithelial cells during the neural tube formation [5]. As shown in Fig. 2, the neural plate develops bilateral neural folds (arrows in Fig. 2A), and the neural folds elevate and make the neural groove (arrows in Fig. 2B), and finally the neural groove fuses at the midline to create the neural tube (Fig. 2C). NTDs originate from a failure of the embryonic neural tube to close. In every stage, neuroepithelium expresses abundant GCS (red areas in Fig. 2), further supporting pivotal roles of GCS in the neural tube formation.
Fig. 1. Basic concept.
cells, resulting in NTDs. (6) Thus, folate is effective to prevent NTDs. Evaluation of the hypothesis GCS is highly expressed in neuroepithelium
Glycine cleavage system is requisite in nucleic acid synthesis from folate in the neuroepithelium
GCS is the main system to degrade glycine in mammalians. The defect of this pathway causes nonketotic-hyperglycinemia (NKH), which is an autosomal recessive disorder associated with very severe clinical symptoms [7] and also grave brain malformations including NTDs [8–10]. These severe symptoms and grave brain malformations are not normally observed in the other amino acid metabolic disorders, suggesting that GCS and glycine should have unknown pivotal roles in brain development. The distribution of GCS has been investigated in the developing
GCS is indispensable in supplying proliferating cells with 5,10-methylenetetrahydrofolate as a one-carbon donor, which is essential for the synthesis of nucleic acids in cell proliferation [11]. Because neuroepithelial cells proliferate very rapidly, a large amount of 5,10-methylenetetrahydrofolate should be supplied via GCS in these cells. As shown in Fig. 3, in the neuroepithelium, up-taken folate is converted to 2
Medical Hypotheses 134 (2020) 109429
K. Sato
supplementary maternal peri-conceptional intake of folate has been demonstrated to reduce the occurrence of human NTD by as much as 70% [26] and has led to government policies of fortification of various foods with folate in some countries [27]. Pregnant women require 5–10-fold higher amounts of folate compared to non-pregnant women [6]. So, folate-deficient situations are easy to occur in neuroepithelial cells, where large amounts of folate are needed to make nuclei acids, resulting in NTDs. Thus, folate is effective to prevent NTDs. Consequences of the hypothesis and discussion Neural tube defects (NTDs) originate from a failure of the embryonic neural tube to close. In this report, I speculate the following hypothesis; (1) The closure of the neural tube requires rapid growth of neuroepithelial cells. (2) High rates of nuclei acids synthesis are needed for the rapid growth. (3) GCS, which is requisite in nucleic acid synthesis from folate, is expressed very strongly and functions robustly in neuroepithelial cells. (4) Pregnant women require 5–10-fold higher amounts of folate compared to non-pregnant women. (5) So, folatedeficient situations are easy to occur in neuroepithelial cells, resulting in NTDs. (6) Thus, folate is effective to prevent NTDs. Although supplementary maternal peri-conceptional intake of folate has led to government policies of fortification of various foods with folate in some countries [27]. Further promotion of mandatory folate fortification is needed to prevent NTDs.
Fig. 3. GCS in the synthesis of nucleic acids from folate. In the neuroepithelium, folate is up-taken from the blood, and folate is converted to tetrahydrofolate (THF). GCS breaks down glycine and generates 5,10-methylaneTHF from THF. And, 5,10-methylane-THF is used to make nucleic acids.
tetrahydrofolate (THF), and GCS generates 5,10-methylane-THF from THF. And, 5,10-methylane-THF is used to make nucleic acids [12]. Interestingly, the neuroepithelium also expresses very high levels of folate-binding proteins, which are needed to take up folate as a source of 5,10-methylenetetrahydrofolate [13,14]. In addition, it has been demonstrated that folate deficiency affects proliferation of neural stem cells [15] and the association between neural tube defects and inadequate folate intake has already been established [11,16]. Thus, I speculate that embryonic neuroepithelial cells transport folate for de novo synthesis of sufficient nucleic acids for proliferation, and that GCS may be indispensable for this process. Therefore, I think that the lack of folate greatly impairs proliferation of neural stem cells, leading to NTDs. Actually, GCS-encoding genes represent candidates for involvement in NTDs [12]. In addition, loss-of-function mutation in GCS genes predisposes to NTDs in mice and humans [12], supporting my theory.
Grant Sponsor The Ministry of Education, Science and Culture of Japan; Shintenkai. Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Additional possibilities about the involvement of folate-depletion in the pathogenesis of NTDs Besides the functions in nucleic acid synthesis, folate has many other roles in various biological events. Thus, folate-depletion may also cause NTDs via the disturbance of many other biological events in the development of the neuroepithelium. For example, maternal folate deficiency has been shown to affect DNA repair and DNA methylation [17], synthesis and transport of fatty acids [18] and choline metabolism [19]. Since these events are also very important in the development of the neuroepithelium, the disturbance of these events may impair proliferation of neural stem cells, leading to NTDs. Furthermore, cell–cell adhesion molecules, such as cadherins, are reported to be very important for the complete closure of neural tube [20]. For example, during neural tube formation, E-cadherin and Ncadherin play pivotal roles [21]. Interestingly, neural tube formation is impaired in N-cadherin knockout mice [22]. In addition, Su et al. have reported that folate deficiency causes the down-regulation of E-cadherin [23]. Taken together, there is a possibility that folate-depletion induces aberrant functions of cell–cell adhesion molecules, such as cadherins, leading to NTDs. In addition, it has been reported that the number of apoptotic cells was almost doubled in the septum and hippocampus of offspring of folate-depleted mothers [24], suggesting that the increase of apoptosis might be also involved in the pathogenesis of NTDs.
Acknowledgement The author would like to thank Ms Chiemi Kawamoto for her excellent technical help. References [1] Blom HJ, Shaw GM, den Heijer M, Finnell RH. Neural tube defects and folate: case far from closed. Nat Rev Neurosci 2006;7:724–31. [2] Juriloff DM, Harris MJ. Insights into the etiology of mammalian neural tube closure defects from developmental, genetic and evolutionary. Studies J Dev Biol 2018;6:E22. [3] Au KS, Ashley-Koch A, Northrup H. Epidemiologic and genetic aspects of spina bifida and other neural tube defects. Dev Disabil Res Rev 2010;16:6–15. [4] Beaudin AE, Stover PJ. Insights into metabolic mechanisms underlying folate-responsive neural tube defects: a minireview. Birth Defects Res A Clin Mol Teratol 2009;85:274–84. [5] Ichinohe A, Kure S, Mikawa S, Ueki T, Kojima K, Fujiwara K, et al. Glycine cleavage system in neurogenic regions. Eur J Neurosci 2004;19:2365–70. [6] Tamura T, Picciano MF. Folate and human reproduction. Am J Clin Nutr 2006;83:993–1016. [7] Hoover-Fong JE, Shah S, Van Hove JL, Applegarth D, Toone J, Hamosh A. Natural history of nonketotic hyperglycinemia in 65 patients. Neurology 2004;63:1847–53. [8] Press GA, Barshop BA, Haas RH, Nyhan WL, Glass RF, Hesselink JR. Abnormalities of the brain in nonketotic hyperglycinemia: MR manifestations. AJNR Am J Neuroradiol 1989;10:315–21. [9] Dobyns WB. Agenesis of the corpus callosum and gyral malformations are frequent manifestations of nonketotic hyperglycinemia. Neurology 1989;39:817–20. [10] Nissenkorn A, Michelson M, Ben-Zeev B, Lerman-Sagie T. Inborn errors of metabolism: a cause of abnormal brain development. Neurology 2001;56:1265–72. [11] Fleming A, Copp AJ. Embryonic folate metabolism and mouse neural tube defects. Science 1998;280:2107–9. [12] Narisawa A, Komatsuzaki S, Kikuchi A, Niihori T, Aoki Y, Fujiwara K, et al. Mutations in genes encoding the glycine cleavage system predispose to neural tube defects in mice and humans. Hum Mol Genet 2012;21:1496–503.
Folate is effective in preventing neural tube defects The importance of an adequate periconceptional maternal folate status to prevent fetal neural tube defects has been well demonstrated and resulted in the recommendation for women to use folate supplements during the periconceptual period [25]. For example, 3
Medical Hypotheses 134 (2020) 109429
K. Sato
[20] Nishimura T, Honda H, Takeichi M. Planar cell polarity links axes of spatial dynamics in neural-tube closure. Cell 2012;149:1084–97. [21] Hatta K, Takeichi M. Expression of N-cadherin adhesion molecules associated with early morphogenetic events in chick development. Nature 1986;320:447–9. [22] Radice GL, Rayburn H, Matsunami H, Knudsen KA, Takeichi M, Hynes RO. Developmental defects in mouse embryos lacking N-cadherin. Dev Biol 1997;181:64–78. [23] Su YH, Huang WC, Huang TH, Huang YJ, Sue YK, Huynh TT, et al. Folate deficient tumor microenvironment promotes epithelial-to-mesenchymal transition and cancer stem-like phenotypes. Oncotarget 2016;7:33246–56. [24] Craciunescu CN, Brown EC, Mar MH, Albright CD, Nadeau MR, Zeisel SH. Folic acid deficiency during late gestation decreases progenitor cell proliferation and increases apoptosis in fetal mouse brain. J Nutr 2004;134:162–6. [25] Naninck EFG, Stijger PC, Brouwer-Brolsma EM. The importance of maternal folate status for brain development and function of offspring. Adv Nutr 2019;10:502–19. [26] MRC Vitamin Study Research Group. Prevention of neural tube defects: results of the Medical Research Council Vitamin Study. MRC Vitamin Study Research Group Lancet 1991;338:131–7. [27] Garrett GS, Bailey LB. A public health approach for preventing neural tube defects: folic acid fortification and beyond. Ann N Y Acad Sci 2018;1414:47–58.
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