Spatial and temporal expression of folate-binding protein 1 (Fbp1) is closely associated with anterior neural tube closure in mice

Folate receptor alpha for cancer therapy: an antibody and antibody-drug conjugate target coming of age

Folate receptor alpha (FRα) has long been the focus of therapeutics development in oncology across several solid tumors, notably ovarian, lung, and subsets of breast cancers. Its multiple roles in cellular metabolism and carcinogenesis and tumor-specific overexpression relative to normal tissues render FRα an attractive target for biological therapies. Here we review the biological significance, expression distribution, and characteristics of FRα as a highly promising and now established therapy target. We discuss the ongoing development of FRα-targeting antibodies and antibody-drug conjugates (ADCs), the first of which has been approved for the treatment of ovarian cancer, providing the impetus for heightened research and therapy development. Novel insights into the tumor microenvironment, advances in antibody engineering to enhance immune-mediated effects, the emergence of ADCs, and several studies of anti-FRα agents combined with chemotherapy, targeted and immune therapy are offering new perspectives and treatment possibilities. Hence, we highlight key translational research and discuss several preclinical studies and clinical trials of interest, with an emphasis on agents and therapy combinations with potential to change future clinical practice.

Noncanonical function of folate through folate receptor 1 during neural tube formation

Folate supplementation reduces the occurrence of neural tube defects (NTDs), birth defects consisting in the failure of the neural tube to form and close. The mechanisms underlying NTDs and their prevention by folate remain unclear. Here we show that folate receptor 1 (FOLR1) is necessary for the formation of neural tube-like structures in human-cell derived neural organoids. FOLR1 knockdown in neural organoids and in Xenopus laevis embryos leads to NTDs that are rescued by pteroate, a folate precursor that is unable to participate in metabolism. We demonstrate that FOLR1 interacts with and opposes the function of CD2-associated protein, molecule essential for apical endocytosis and turnover of C-cadherin in neural plate cells. In addition, folates increase Ca2+ transient frequency, suggesting that folate and FOLR1 signal intracellularly to regulate neural plate folding. This study identifies a mechanism of action of folate distinct from its vitamin function during neural tube formation.

Folate Receptor Beta Signaling in the Regulation of Macrophage Antimicrobial Immune Response: A Scoping Review

Introduction

Folate, vitamin B9, is a water-soluble vitamin that is essential to cellular proliferation and division. In addition to the reduced folate carrier, eukaryotic cells take up folate through endocytosis mediated by one of two GPI-anchored folate receptors (FRs), FRα or FRβ. Two other isoforms of FR exist, FRγ and FRδ, neither of which support endocytic activities of FR signaling. FRβ is expressed primarily by monocytes and macrophages and highly expressed on activated macrophages. Macrophage expression of FRβ suggests a role for this receptor in modulating function of these immune sentinels, particularly as they engage in inflammatory processes. Despite several studies suggesting that folates can suppress inflammatory responses of macrophages to proinflammatory stimuli, there appears to be a lack of basic research examining the role of FRβ in modulating macrophage responses to microbial sensing. We therefore conducted a scoping review to assess evidence within the published literature addressing the question, "what is known about the extent to which FRβ regulates macrophage responses to sensing, and responding to, microorganisms?".

Methods

As a strategy for the study selection, we queried articles indexed in the research database PubMed and the search engine Google Scholar (up until August 12, 2023), including combinations of the research words: macrophage, folate receptor beta, FOLR2.

Results

We identified 2 relevant articles out of 153 that are worth discussing here, none of which directly addressed our research question.

Conclusion

There is an unmet need to better define the contribution of FRβ to regulating the macrophage response to microbes.

Maternal metabolism influences neural tube closure

Changes in maternal nutrient availability due to diet or disease significantly increase the risk of neural tube defects (NTDs). Because the incidence of metabolic disease continues to rise, it is urgent that we better understand how altered maternal nutrient levels can influence embryonic neural tube development. Furthermore, primary neurulation occurs before placental function during a period of histiotrophic nutrient exchange. In this review we detail how maternal metabolites are transported by the yolk sac to the developing embryo. We discuss recent advances in understanding how altered maternal levels of essential nutrients disrupt development of the neuroepithelium, and identify points of intersection between metabolic pathways that are crucial for NTD prevention.

Non-canonical function of folate/folate receptor 1 during neural tube formation

Folate supplementation reduces the occurrence of neural tube defects, one of the most common and serious birth defects, consisting in the failure of the neural tube to form and close early in pregnancy. The mechanisms underlying neural tube defects and folate action during neural tube formation remain unclear. Here we show that folate receptor 1 (FOLR1) is necessary for the formation of neural tube-like structures in human-cell derived neural organoids. Knockdown of FOLR1 in human neural organoids as well as in the Xenopus laevis in vivo model leads to neural tube defects that are rescued by pteroate, a folate precursor that binds to FOLR1 but is unable to participate in metabolic pathways. We demonstrate that FOLR1 interacts with and opposes the function of CD2-associated protein (CD2AP), a molecule that we find is essential for apical endocytosis and the spatiotemporal turnover of the cell adherens junction component C-cadherin in neural plate cells. The counteracting action of FOLR1 on these processes is mediated by regulating CD2AP protein level via a degradation-dependent mechanism. In addition, folate and pteroate increase Ca 2+ transient frequency in the neural plate in a FOLR1-dependent manner, suggesting that folate/FOLR1 signal intracellularly to regulate neural plate folding. This study identifies a mechanism of action of folate distinct from its vitamin function during neural tube formation.

Association of polymorphisms of FOLR1 gene and FOLR2 gene and maternal folic acid supplementation with risk of ventricular septal defect: a case-control study

OBJECTIVES

It was the first time to examine the role of maternal polymorphisms of FOLR1 gene and FOLR2 gene, as well as their interactions with maternal folic acid supplementation (FAS), in the risk of ventricular septal defect (VSD).

METHODS

A case-control study was conducted with 385 mothers of VSD infants and 652 controls. The exposures of interest were FAS and FOLR1 gene and FOLR2 gene polymorphisms. The logistic regression model was used for accessing the strength of association.

RESULTS

After controlling for the potential confounders, women who did not utilize folic acid had a substantially higher risk of VSD (aOR = 2.25; 95% CI: 1.48 to 3.43), compared to those who did. We also observed genetic polymorphisms of FOLR1 gene at rs2071010 (GA vs. GG: aOR = 0.63, 95%CI: 0.45 to 0.88) and rs11235462 (AA vs. TT: aOR = 0.53, 95%CI: 0.33 to 0.84), as well as FOLR2 gene at rs651646 (AA vs. TT: aOR = 0.46, 95%CI: 0.30 to 0.70), rs2298444 (CC vs. TT: aOR = 0.58, 95%CI: 0.36 to 0.91) and rs514933 (TC vs. TT: aOR = 0.57, 95%CI: 0.41 to 0.78) were associated with a lower risk of VSD. Furthermore, there was a statistically significant interaction between maternal FAS and genetic polymorphisms at rs514933 on the risk of VSD (FDR_P = 0.015).

CONCLUSIONS

The maternal genetic polymorphisms of the FOLR1 gene and FOLR2 gene, as well as FAS and their interactions, were shown to be significantly associated with the risk of VSD in offspring.

Review of Evidence for Environmental Causes of Uveal Coloboma

Uveal coloboma is a condition defined by missing ocular tissues and is a significant cause of childhood blindness. It occurs from a failure of the optic fissure to close during embryonic development,and may lead to missing parts of the iris, ciliary body, retina, choroid, and optic nerve. Because there is no treatment for coloboma, efforts have focused on prevention. While several genetic causes of coloboma have been identified, little definitive research exists regarding the environmental causes of this condition. We review the current literature on environmental factors associated with coloboma in an effort to guide future research and preventative counseling related to this condition.

Interaction between dolutegravir and folate transporters and receptor in human and rodent placenta

Background

Due to the critical role of folates in neurodevelopment, it is important to understand potential interactions between anti-HIV drugs used during pregnancy, and folate delivery pathways in the placenta. This study investigates the effect of dolutegravir (DTG) exposure on the functional expression of the reduced folate carrier (RFC), proton-coupled folate transporter (PCFT), and folate receptor-α (FRα) in the placenta.

Methods

Human placental cell lines, human placental explants, and a pregnant mouse model treated with clinically relevant concentrations of DTG were used. Gene and protein expression were assessed by qPCR, immunoblot and immunohistochemical assays. Folate transport function was measured by applying radioisotope-based transport assays.

Findings

In placental cells, clinically relevant DTG exposure for 3h or 6h was associated with a modest but significant reduction in the expression of RFC and PCFT both at the mRNA and protein levels, as well as decreased uptake of RFC and PCFT substrates [³H]-methotrexate and [³H]-folic acid, respectively. In pregnant mice, DTG administration was associated with an increase in both placental RFC and PCFT mRNA expression, accompanied by a decrease in placental FRα mRNA under folate-deficient dietary conditions.

Interpretation

These findings demonstrate a potential interaction between DTG and folate transport pathways in the placenta, particularly in vivo, under folate deficient conditions, potentially impacting folate delivery to the foetus in the context of DTG-based ART during pregnancy.

Funding

Funded by Ontario HIV Treatment Network, grant #506657; and Eunice Kennedy Shriver National Institute of Child Health & Human Development of the National Institutes of Health, award #R01HD104553.

The Incoherent Fluctuation of Folate Pools and Differential Regulation of Folate Enzymes Prioritize Nucleotide Supply in the Zebrafish Model Displaying Folate Deficiency-Induced Microphthalmia and Visual Defects

Objective

Congenital eye diseases are multi-factorial and usually cannot be cured. Therefore, proper preventive strategy and understanding the pathomechanism underlying these diseases become important. Deficiency in folate, a water-soluble vitamin B, has been associated with microphthalmia, a congenital eye disease characterized by abnormally small and malformed eyes. However, the causal-link and the underlying mechanism between folate and microphthalmia remain incompletely understood.

Methods

We examined the eye size, optomotor response, intracellular folate distribution, and the expression of folate-requiring enzymes in zebrafish larvae displaying folate deficiency (FD) and ocular defects.

Results

FD caused microphthalmia and impeded visual ability in zebrafish larvae, which were rescued by folate and dNTP supplementation. Cell cycle analysis revealed cell accumulation at S-phase and sub-G1 phase. Decreased cell proliferation and increased apoptosis were found in FD larvae during embryogenesis in a developmental timing-specific manner. Lowered methylenetetrahydrofolate reductase (mthfr) expression and up-regulated methylenetetrahydrofolate dehydrogenase (NADP⁺-dependent)-1-like (mthfd1L) expression were found in FD larvae. Knocking-down mthfd1L expression worsened FD-induced ocular anomalies; whereas increasing mthfd1L expression provided a protective effect. 5-CH₃-THF is the most sensitive folate pool, whose levels were the most significantly reduced in response to FD; whereas 10-CHO-THF levels were less affected. 5-CHO-THF is the most effective folate adduct for rescuing FD-induced microphthalmia and defective visual ability.

Conclusion

FD impeded nucleotides formation, impaired cell proliferation and differentiation, caused apoptosis and interfered active vitamin A production, contributing to ocular defects. The developmental timing-specific and incoherent fluctuation among folate adducts and increased expression of mthfd1L in response to FD reflect the context-dependent regulation of folate-mediated one-carbon metabolism, endowing the larvae to prioritize the essential biochemical pathways for supporting the continuous growth in response to folate depletion.

Gestational folic acid deficiency alters embryonic eye development: Possible role of basement membrane proteins in eye malformations

OBJECTIVES

Folic acid (FA) is crucial before and during early pregnancy. FA deficiency can occur because dietary FA intake is low in mothers at the time of conception. Likewise, various ocular pathologies are related to the alteration of extracellular matrices. The present study aimed to investigate the association between maternal FA deficiency and congenital eye defects. We also investigated whether maternal diet deficient in FA alters the expression of collagen IV and laminin-1 as a possible mechanism responsible for the appearance of ocular malformations. Both proteins are the main components of the basal lamina, and form an interlaced network that creates a relevant scaffold basement membrane. Basal laminae are involved in tissues maintenance and implicated in regulating many cellular processes.

METHODS

A total of 57 mouse embryos were classified into the following groups: Control group, (mothers were fed a standard rodent diet), and D2 and D8 groups (mothers were fed FA-deficient [FAD] diet for 2 or 8 wk, respectively). Female mice from group D2 were fed a FAD diet (0 mg/kg diet + 1% succinyl sulfathiazole used to block the synthesis of FA) for 2 wk from the day after mating until day 14.5 of gestation (E14.5). On the other hand, female mice from group D8 were fed a FAD diet for 8 wk (6 wk before conception and during the first 2 wk of pregnancy). For the data analysis, we first estimated the incidence of malformations in each group. Then, the statistical analysis was performed using IBM SPSS Statistics, version 25.0. Expression patterns of collagen IV and laminin-1 were examined with the immunohistochemical technique.

RESULTS

Our results showed that mice born to FA-deficient mothers had several congenital eye abnormalities. Embryos from dams fed a short-term FAD diet were found to have many significant abnormalities in both anterior and posterior segments, as well as choroidal vessel abnormalities. However, embryos from dams fed a long-term FAD diet had a significantly higher incidence of eye defects. Finally, maternal FA deficiency increased the expression of both collagen IV and laminin-1. Likewise, changes in the spatial localization and organization of collagen IV were observed.

CONCLUSIONS

A maternal FAD diet for a short-term period causes eye developmental defects and induces overexpression of both collagen IV and laminin-1. The malformations observed are probably related to alterations in the expression of basement membrane proteins.

Programming With Varying Dietary Fat Content Alters Cardiac Insulin Receptor, Glut4 and FoxO1 Immunoreactivity in Neonatal Rats, Whereas High Fat Programming Alters Cebpa Gene Expression in Neonatal Female Rats

Fetal programming refers to an intrauterine stimulus or insult that shapes growth, development and health outcomes. Dependent on the quality and quantity, dietary fats can be beneficial or detrimental for the growth of the fetus and can alter insulin signaling by regulating the expression of key factors. The effects of varying dietary fat content on the expression profiles of factors in the neonatal female and male rat heart were investigated and analyzed in control (10% fat), 20F (20% fat), 30F (30% fat) and 40F (40% fat which was a high fat diet used to induce high fat programming) neonatal rats. The whole neonatal heart was immunostained for insulin receptor, glucose transporter 4 (Glut4) and forkhead box protein 1 (FoxO1), followed by image analysis. The expression of 84 genes, commonly associated with the insulin signaling pathway, were then examined in 40F female and 40F male offspring. Maintenance on diets, varying in fat content during fetal life, altered the expression of cardiac factors, with changes induced from 20% fat in female neonates, but from 30% fat in male neonates. Further, CCAAT/enhancer-binding protein alpha (Cebpa) was upregulated in 40F female neonates. There was, however, differential expression of several insulin signaling genes in 40F (high fat programmed) offspring, with some tending to significance but most differences were in fold changes (≥1.5 fold). The increased immunoreactivity for insulin receptor, Glut4 and FoxO1 in 20F female and 30F male neonatal rats may reflect a compensatory response to programming to maintain cardiac physiology. Cebpa was upregulated in female offspring maintained on a high fat diet, with fold increases in other insulin signaling genes viz. Aebp1, Cfd (adipsin), Adra1d, Prkcg, Igfbp, Retn (resistin) and Ucp1. In female offspring maintained on a high fat diet, increased Cebpa gene expression (concomitant with fold increases in other insulin signaling genes) may reflect cardiac stress and an adaptative response to cardiac inflammation, stress and/or injury, after high fat programming. Diet and the sex are determinants of cardiac physiology and pathophysiology, reflecting divergent mechanisms that are sex-specific.

Gestational folate deficiency alters embryonic gene expression and cell function

Folic acid is a nutrient essential for embryonic development. Folate deficiency can cause embryonic lethality or neural tube defects and orofacial anomalies. Folate receptor 1 (Folr1) is a folate binding protein that facilitates the cellular uptake of dietary folate. To better understand the biological processes affected by folate deficiency, gene expression profiles of gestational day 9.5 (gd9.5) Folr1^-/- embryos were compared to those of gd9.5 Folr1^+/+ embryos. The expression of 837 genes/ESTs was found to be differentially altered in Folr1^-/- embryos, relative to those observed in wild-type embryos. The 837 differentially expressed genes were subjected to Ingenuity Pathway Analysis. Among the major biological functions affected in Folr1^-/- mice were those related to 'digestive system development/function', 'cardiovascular system development/function', 'tissue development', 'cellular development', and 'cell growth and differentiation', while the major canonical pathways affected were those associated with blood coagulation, embryonic stem cell transcription and cardiomyocyte differentiation (via BMP receptors). Cellular proliferation, apoptosis and migration were all significantly affected in the Folr1^-/- embryos. Cranial neural crest cells (NCCs) and neural tube explants, grown under folate-deficient conditions, exhibited marked reduction in directed migration that can be attributed, in part, to an altered cytoskeleton caused by perturbations in F-actin formation and/or assembly. The present study revealed that several developmentally relevant biological processes were compromised in Folr1^-/- embryos.

Exploiting the folate receptor α in oncology

Folate receptor α (FRα) came into focus as an anticancer target many decades after the successful development of drugs targeting intracellular folate metabolism, such as methotrexate and pemetrexed. Binding to FRα is one of several methods by which folate is taken up by cells; however, this receptor is an attractive anticancer drug target owing to the overexpression of FRα in a range of solid tumours, including ovarian, lung and breast cancers. Furthermore, using FRα to better localize effective anticancer therapies to their target tumours using platforms such as antibody-drug conjugates, small-molecule drug conjugates, radioimmunoconjugates and, more recently, chimeric antigen receptor T cells could further improve the outcomes of patients with FRα-overexpressing cancers. FRα can also be harnessed for predictive biomarker research. Moreover, imaging FRα radiologically or in real time during surgery can lead to improved functional imaging and surgical outcomes, respectively. In this Review, we describe the current status of research into FRα in cancer, including data from several late-phase clinical trials involving FRα-targeted therapies, and the use of new technologies to develop FRα-targeted agents with improved therapeutic indices.

Cellular mechanisms underlying Pax3-related neural tube defects and their prevention by folic acid

Neural tube defects (NTDs), including spina bifida and anencephaly, are among the most common birth defects worldwide, but their underlying genetic and cellular causes are not well understood. Some NTDs are preventable by supplemental folic acid. However, despite widespread use of folic acid supplements and implementation of food fortification in many countries, the protective mechanism is unclear. Pax3 mutant (splotch; Sp2H ) mice provide a model in which NTDs are preventable by folic acid and exacerbated by maternal folate deficiency. Here, we found that cell proliferation was diminished in the dorsal neuroepithelium of mutant embryos, corresponding to the region of abolished Pax3 function. This was accompanied by premature neuronal differentiation in the prospective midbrain. Contrary to previous reports, we did not find evidence that increased apoptosis could underlie failed neural tube closure in Pax3 mutant embryos, nor that inhibition of apoptosis could prevent NTDs. These findings suggest that Pax3 functions to maintain the neuroepithelium in a proliferative, undifferentiated state, allowing neurulation to proceed. NTDs in Pax3 mutants were not associated with abnormal abundance of specific folates and were not prevented by formate, a one-carbon donor to folate metabolism. Supplemental folic acid restored proliferation in the cranial neuroepithelium. This effect was mediated by enhanced progression of the cell cycle from S to G2 phase, specifically in the Pax3 mutant dorsal neuroepithelium. We propose that the cell-cycle-promoting effect of folic acid compensates for the loss of Pax3 and thereby prevents cranial NTDs.

The antagonism of folate receptor by dolutegravir: developmental toxicity reduction by supplemental folic acid

OBJECTIVE

Maternal folate (vitamin B9) status is the largest known modifier of neural tube defect risk, so we evaluated folate-related mechanisms of action for dolutegravir (DTG) developmental toxicity.

DESIGN

Folate receptor 1 (FOLR1) was examined as a target for DTG developmental toxicity using protein and cellular interaction studies and an animal model.

METHODS

FOLR1 competitive binding studies were used to test DTG for FOLR1 antagonism. Human placenta cell line studies were used to test interactions with DTG, folate, and cations. Zebrafish were selected as an animal model to examine DTG-induced developmental toxicity and rescue strategies.

RESULTS

FOLR1 binding studies indicate DTG is a noncompetitive FOLR1 antagonist at therapeutic concentrations. In-vitro testing indicates calcium (2 mmol/l) increases FOLR1-folate interactions and alters DTG-FOLR1-folate interactions and cytotoxicity. DTG does not inhibit downstream folate metabolism by dihydrofolate reductase. Early embryonic exposure to DTG is developmentally toxic in zebrafish, and supplemental folic acid can mitigate DTG developmental toxicity.

CONCLUSION

Folates and FOLR1 are established modifiers of risk for neural tube defects, and binding data indicates DTG is a partial antagonist of FOLR1. Supplemental folate can ameliorate increased developmental toxicity due to DTG in zebrafish. The results from these studies are expected to inform and guide future animal models and clinical studies of DTG-based antiretroviral therapy in women of childbearing age.

Developmental Toxicity of Mycotoxin Fumonisin B₁ in Animal Embryogenesis: An Overview

A teratogenic agent or teratogen can disturb the development of an embryo or a fetus. Fumonisin B₁ (FB₁), produced by Fusarium verticillioides and F. proliferatum, is among the most commonly seen mycotoxins and contaminants from stale maize and other farm products. It may cause physical or functional defects in embryos or fetuses, if the pregnant animal is exposed to mycotoxin FB₁. Due to its high similarity in chemical structure with lipid sphinganine (Sa) and sphingosine (So), the primary component of sphingolipids, FB₁ plays a role in competitively inhibiting Sa and So, which are key enzymes in de novo ceramide synthase in the sphingolipid biosynthetic pathway. Therefore, it causes growth retardation and developmental abnormalities to the embryos of hamsters, rats, mice, and chickens. Moreover, maternal FB₁ toxicity can be passed onto the embryo or fetus, leading to mortality. FB₁ also disrupts folate metabolism via the high-affinity folate transporter that can then result in folate insufficiency. The deficiencies are closely linked to incidences of neural tube defects (NTDs) in mice or humans. The purpose of this review is to understand the toxicity and mechanisms of mycotoxin FB₁ on the development of embryos or fetuses.

Folic acid modifies the shape of epithelial cells during morphogenesis via a Folr1 and MLCK dependent mechanism

Folic acid supplementation can prevent neural tube defects, but the specific molecular mechanisms by which it does have not been elucidated. During neural plate morphogenesis, epithelial cell apical constriction cooperates with other events to drive tissue-bending, and when defective, can result in neural tube defects. A Rho-kinase deficient binding mutant of the apical constriction regulating protein, Shroom3 (Shroom3^R1838C), is one of only a handful of mouse mutant lines with neural tube defects that can be rescued by folic acid supplementation. This provided a unique opportunity to probe the functional rescue of a protein linked to neural tube development by folic acid. Utilizing an epithelial cell culture model of apical constriction, it was observed that treatment with exogenous folic acid, as well as co-expression of the folic acid receptor Folr1, can rescue the function of the Rho-kinase binding deficient mutant of Shroom3 in vitro It was also determined that the rescuing ability of folic acid is RhoA and Rho-kinase independent but myosin light chain kinase (MLCK) and Src-kinase dependent. Inhibition of Rho-kinase-dependent apical constriction in chick embryo neural epithelium was also observed to be rescued by exogenous folic acid and that treatment with folic acid is accompanied by elevated activated myosin light chain and MLCK. Furthermore, doubly heterozygous mouse embryos lacking one copy each of Shroom3 and Folr1 exhibit a low rate of neural tube defects and also have lower levels of activated myosin light chain and MLCK. These studies suggest a novel mechanism by which folic acid modifies epithelial cell shape during morphogenesis, shedding light onto how folic acid may prevent neural tube defects.

Biometric Alterations of Mouse Embryonic Eye Structures Due to Short-Term Folic Acid Deficiency

PURPOSE

Folic acid (FA) is an essential nutrient for normal embryonic development. FA deficiency (FAD) in maternal diet increases the risk of several defects among the progeny, especially, neural tube defects. The eye begins its development from the neural tube; however, the relationship between FAD and ocular development in the offspring has been little explored and it isn't known how the FAD affects the formation of the eye. Our objective was to analyze the effect of maternal FAD on mouse embryos ocular biometry.

METHODS

Female mice C57/BL/6J were distributed into three different groups, according to the assigned diet: control group fed a standard FA diet (2 mg FA/kg), FAD group for short term fed (0 mg FA/kg + 1% succinylsulfathiazole) from the day after mating until day 14.5 of gestation, and FAD group for long term fed the same FA-deficient diet for 6 weeks prior mating and continued with this diet during gestation. A total of 57 embryos (19 embryos of each dietary group) at 14.5 gestational days were evaluated. As indicators of changes in ocular biometry, we analyze two parameters: area and circularity of the lens and whole eye, and the area of the retina. The program used in the treatment and selection of the areas of interest was ImageJ. The statistical analysis was performed by IBM SPSS Statistics 19.

RESULTS

Regarding the measures of the area, FA-deficient lenses and eyes were smaller than that of controls. We have also observed increase in the size of the neural retina, spatially, in embryos from females fed FAD diet during long term. On the other hand, as regard to circularity measures, we have seen that eyes and lenses were more circular than control.

CONCLUSION

Maternal FAD diet for a very short term generates morphological changes in ocular structures to the offspring.

Insights into the Etiology of Mammalian Neural Tube Closure Defects from Developmental, Genetic and Evolutionary Studies

The human neural tube defects (NTD), anencephaly, spina bifida and craniorachischisis, originate from a failure of the embryonic neural tube to close. Human NTD are relatively common and both complex and heterogeneous in genetic origin, but the genetic variants and developmental mechanisms are largely unknown. Here we review the numerous studies, mainly in mice, of normal neural tube closure, the mechanisms of failure caused by specific gene mutations, and the evolution of the vertebrate cranial neural tube and its genetic processes, seeking insights into the etiology of human NTD. We find evidence of many regions along the anterior–posterior axis each differing in some aspect of neural tube closure—morphology, cell behavior, specific genes required—and conclude that the etiology of NTD is likely to be partly specific to the anterior–posterior location of the defect and also genetically heterogeneous. We revisit the hypotheses explaining the excess of females among cranial NTD cases in mice and humans and new developments in understanding the role of the folate pathway in NTD. Finally, we demonstrate that evidence from mouse mutants strongly supports the search for digenic or oligogenic etiology in human NTD of all types.

Folate deficiency prevents neural crest fate by disturbing the epigenetic Sox2 repression on the dorsal neural tube

Folate deficiency has been known to contribute to neural tube and neural crest defects, but why these tissues are particularly affected, and which are the molecular mechanisms involved in those abnormalities are important human health questions that remain unanswered. Here we study the function of two of the main folate transporters, FolR1 and Rfc1, which are robustly expressed in these tissues. Folate is the precursor of S-adenosylmethionine, which is the main donor for DNA, protein and RNA methylation. Our results show that knockdown of FolR1 and/or Rfc1 reduced the abundance of histone H3 lysine and DNA methylation, two epigenetic modifications that play an important role during neural and neural crest development. Additionally, by knocking down folate transporter or pharmacologically inhibiting folate transport and metabolism, we observed ectopic Sox2 expression at the expense of neural crest markers in the dorsal neural tube. This is correlated with neural crest associated defects, with particular impact on orofacial formation. By using bisulfite sequencing, we show that this phenotype is consequence of reduced DNA methylation on the Sox2 locus at the dorsal neural tube, which can be rescued by the addition of folinic acid. Taken together, our in vivo results reveal the importance of folate as a source of the methyl groups necessary for the establishment of the correct epigenetic marks during neural and neural crest fate-restriction.

Neural tube closure and embryonic metabolism

Neural tube closure (NTC) is an embryonic process during formation of the mammalian central nervous system. Disruption of the dynamic, sequential events of NTC can cause neural tube defects (NTD) leading to spina bifida and anencephaly in the newborn. NTC is affected by inherent factors such as genetic mutation or if the mother is exposed to certain environmental factors such as intake of harmful chemicals, maternal infection, irradiation, malnutrition, and inadequate or excessive intake of specific nutrients. Although effects of these stress factors on NTC have been intensively studied, the metabolic state of a normally developing embryo remains unclear. State-of-the art mass spectrometry techniques have enabled detailed study of embryonic metabolite profiles and their distribution within tissues. This approach has demonstrated that glucose metabolism is altered during NTC stages involving chorioallantoic branching. An understanding of embryonic metabolic rewiring would help reveal the etiology of NTD caused by environmental factors.

Folate receptors and neural tube closure

Neural tube defects (NTD) are among the most common human congenital malformations, affecting 0.5-8.0/1000 of live births. Human clinical trials have shown that periconceptional folate supplementation significantly decreases the occurrence of NTD in offspring. However, the mechanism by which folate acts on NTD remains largely unknown. Folate receptor (Folr) is one of the three membrane proteins that mediate cellular uptake of folates. Recent studies suggest that mouse Folr1 (formerly referred to as Fbp1) is essential for neural tube closure. Therefore, we examined spatial and temporal expression patterns of Folr1 in developing mouse embryos, showing a close association between Folr1 and anterior neural tube closure. Transient transgenic analysis was performed using lacZ as a reporter; we identified a 1.1-kb enhancer that directs lacZ expression in the neural tube and optic vesicle in a manner that is similar to endogenous Folr1. The 1.1-kb enhancer sequences were highly conserved between humans and mice, suggesting that human FOLR1 is associated with anterior neural tube closure in humans. Several experimental studies in mice and human epidemiological and genetics studies have suggested that folate receptor abnormalities are involved in a portion of human NTDs, although the solo defect of FOLR1 did not cause NTD.

Folate receptor 1 is necessary for neural plate cell apical constriction during Xenopus neural tube formation

Folate supplementation prevents up to 70% of neural tube defects (NTDs), which result from a failure of neural tube closure during embryogenesis. The elucidation of the mechanisms underlying folate action has been challenging. This study introduces Xenopus laevis as a model to determine the cellular and molecular mechanisms involved in folate action during neural tube formation. We show that knockdown of folate receptor 1 (Folr1; also known as FRα) impairs neural tube formation and leads to NTDs. Folr1 knockdown in neural plate cells only is necessary and sufficient to induce NTDs. Folr1-deficient neural plate cells fail to constrict, resulting in widening of the neural plate midline and defective neural tube closure. Pharmacological inhibition of folate action by methotrexate during neurulation induces NTDs by inhibiting folate interaction with its uptake systems. Our findings support a model in which the folate receptor interacts with cell adhesion molecules, thus regulating the apical cell membrane remodeling and cytoskeletal dynamics necessary for neural plate folding. Further studies in this organism could unveil novel cellular and molecular events mediated by folate and lead to new ways of preventing NTDs.

Hypermethylation of AKT2 gene is associated with neural-tube defects in fetus

INTRODUCTION

Neural-tube defects (NTDs) are common birth defects of complex etiology. Although many studies have confirmed a genetic component, the exact mechanism between DNA methylation and NTDs remains unclear.

METHODS

In this work, we investigated the alteration of methylation from placental tissues obtained from 152 normal infants or with NTDs in 130 children with neural-tube defects. Genome-wide changes in DNA methylation were measured using the NimbleGen microarray. The expression levels of 12 genes were also determined, and two genes (AKT2 and CDC25C) showed low expression in NTDs by quantitative real-time PCR analysis. Then, the methyhlated region of AKT2 promoter sequences were confirmed by massARRAY.

RESULTS

A total of 150 differentially methylated regions (81 low methylated regions and 69 high methylated regions) were selected by microarray. The expression levels of AKT2 and CDC25C showed lower expression in NTDs. And the percentage of methyhlated region of AKT2 promoter were increased in NTDs.

CONCLUSIONS

DNA mythelation was one of the possible epigenetic variations correlated with the occurrence of NTDs, and AKT2 may be a candidate gene for NTDs.

Evolutionary development of embryonic cerebrospinal fluid composition and regulation: an open research field with implications for brain development and function

Within the consolidated field of evolutionary development, there is emerging research on evolutionary aspects of central nervous system development and its implications for adult brain structure and function, including behaviour. The central nervous system is one of the most intriguing systems in complex metazoans, as it controls all body and mind functions. Its failure is responsible for a number of severe and largely incurable diseases, including neurological and neurodegenerative ones. Moreover, the evolution of the nervous system is thought to be a critical step in the adaptive radiation of vertebrates. Brain formation is initiated early during development. Most embryological, genetic and evolutionary studies have focused on brain neurogenesis and regionalisation, including the formation and function of organising centres, and the comparison of homolog gene expression and function among model organisms from different taxa. The architecture of the vertebrate brain primordium also reveals the existence of connected internal cavities, the cephalic vesicles, which in fetuses and adults become the ventricular system of the brain. During embryonic and fetal development, brain cavities and ventricles are filled with a complex, protein-rich fluid called cerebrospinal fluid (CSF). However, CSF has not been widely analysed from either an embryological or evolutionary perspective. Recently, it has been demonstrated in higher vertebrates that embryonic cerebrospinal fluid has key functions in delivering diffusible signals and nutrients to the developing brain, thus contributing to the proliferation, differentiation and survival of neural progenitor cells, and to the expansion and patterning of the brain. Moreover, it has been shown that the composition and homeostasis of CSF are tightly controlled in a time-dependent manner from the closure of the anterior neuropore, just before the initiation of primary neurogenesis, up to the formation of functional choroid plexuses. In this review, we draw together existing literature about the formation, function and homeostatic regulation of embryonic cerebrospinal fluid, from the closure of the anterior neuropore to the formation of functional fetal choroid plexuses, from an evolutionary perspective. The relevance of these processes to the normal functions and diseases of adult brain will also be discussed.

Inhibition of fumonisin B1 cytotoxicity by nanosilicate platelets during mouse embryo development

Nanosilicate platelets (NSP), the form of natural silicate clay that was exfoliated from montmorillonite (MMT), is widely used as a feed additive for its high non-specific binding capacity with mycotoxins such as fumonisin B₁ (FB₁), and has been evaluated its safety for biomedical use including cytotoxicity, genotoxicity, and lethal dosage (LD). In the study, we further examined its toxicity on the development of CD1 mouse embryos and its capacity to prevent teratogenesis-induced by FB₁. In vitro cultures, NSP did not disturb the development and the quality of intact pre-implantation mouse embryos. Further, newborn mice from females consumed with NSP showed no abnormalities. NSP had an unexpected high adsorption capacity in vitro. In contrast to female mice consumed with FB₁ only, a very low residual level of FB₁ in the circulation, reduced incidence of neutral tube defects and significantly increased fetal weight were observed in the females consumed with FB₁ and NSP, suggesting a high alleviation effect of NSP on FB₁in vivo. Furthermore, FB₁ treatment disturbed the gene expression of sphingolipid metabolism enzymes (longevity assurance homolog 5, LASS 5; sphingosine kinase 1, Sphk1; sphingosine kinase 2, Sphk2; sphingosine 1- phosphate lyase, Sgpl1; sphingosine 1-phosphate phosphatase, Sgpp1) in the maternal liver, uterus, fetus, and placenta, but NSP administration reversed the perturbations. Based on these findings, we conclude that NSP is a feasible and effective agent for supplementary use in reducing the toxicity of FB₁ to animals.

Maternal folic acid-deficient diet causes congenital malformations in the mouse eye

BACKGROUND

The eye is a very complex structure derived from the neural tube, surface ectoderm, and migratory mesenchyme from a neural crest origin. Because structures that evolve from the neural tube may be affected by a folate/folic acid (FA) deficiency, the aim of this work was to investigate whether a maternal folic acid-deficient diet may cause developmental alterations in the mouse eye.

METHODS

Female C57BL/6J mice (8 weeks old) were assigned into two different folic acid groups for periods ranging between 2 and 16 weeks. Animals were killed at gestation day 17. Hepatic folate was analyzed, and the eyes from 287 fetuses were macroscopically studied, sectioned and immunolabeled with anti-transforming growth factor (TGF)-β2 and anti-TGF-βRII.

RESULTS

Mice exposed to a FA-deficient diet exhibited numerous eye macroscopic anomalies, such as anophthalmia and microphthalmia. Microscopically, the eye was the most affected organ (43.7% of the fetuses). The highest incidence of malformations occurred from the 8th week onward. A statistically significant linear association between the number of maternal weeks on the FA-deficient diet and embryonic microscopic eye malformations was observed. The optic cup derivatives and structures forming the eye anterior segment showed severe abnormalities. In addition, TGF-β2 and TGF-βRII expression in the eye was also altered.

CONCLUSION

This study suggests that an adequate folic acid/folate status plays a key role in the formation of ocular tissues and structures, whereas a vitamin deficiency is negatively associated with a normal eye development even after a short-term exposure.

Neural tube defects, folate, and immune modulation

Periconceptional supplementation with folic acid has led to a significant worldwide reduction in the incidence of neural tube defects (NTDs). However, despite increasing awareness of the benefits of folic acid supplementation and the implementation of food fortification programs in many countries, NTDs continue to be a leading cause of perinatal morbidity and mortality worldwide. Furthermore, there exists a significant subgroup of women who appear to be resistant to the protective effects of folic acid supplementation. The following review addresses emerging clinical and experimental evidence for a role of the immune system in the etiopathogenesis of NTDs, with the aim of developing novel preventative strategies to further reduce the incidence of NTD-affected pregnancies. In particular, recent studies demonstrating novel roles and interactions between innate immune factors such as the complement cascade, neurulation, and folate metabolism are explored.

Association of folic acid receptor α in maternal serum with neural tube defects

OBJECTIVE

To evaluate whether serum folic receptor α levels are changed in women whose previous pregnancies were complicated with neural tube defects (NTDs).

METHODS

This was a case-control study that included 41 women as the control group who had previously had at least one healthy pregnancy and 37 women as the study group who had a previous pregnancy complicated with NTDs. Blood samples were obtained from all of the participants six weeks after the termination of pregnancy or delivery of a baby. Serum folate receptor α concentrations were analyzed using a commercially available enzyme-linked immunosorbent assay (ELISA) kit.

RESULTS

The mean concentrations of serum folate receptor α were significantly lower in the NTD cases compared to those in the control group (p = 0.02). There was no significant difference in mean serum folate titers between the NTD cases and the control group (p = 0.07).

CONCLUSION

Low serum folic acid receptor α levels in the current study did not appear to be a regulatory marker of maternal folate homeostasis per se but rather a factor that contributed to the development of NTDs.

Epigenetic alterations in folate transport genes in placental tissue from fetuses with neural tube defects and in leukocytes from subjects with hyperhomocysteinemia

The objectives of this study were to identify tissue-specific differentially methylated regions (T-DMR’s) in the folate transport genes in placental tissue compared with leukocytes, and from placental tissues obtained from normal infants or with neural tube defects (NTDs). Using pyrosequencing, we developed methylation assays for the CpG islands (CGIs) and the CGI shore regions of the folate receptor α (FOLR1), proton-coupled folate transporter (PCFT) and reduced folate carrier 1 (RFC1) genes. The T-DMRs differed in location for each gene and the difference in methylation ranged between 2 and 54%. A higher T-DMR methylated fraction was associated with a lower mRNA level of the FOLR1 and RFC1 genes. Methylation fractions differed according to RFC1 80G > A genotype in the NTD cases and in leukocytes from subjects with high total plasma homocysteine (tHcy). There were no differences in methylated fraction of folate transporter genes between NTD cases and controls. We suggest that T-DMRs participate in the regulation of expression of the FOLR1 and RFC1 genes, that the RFC1 80G > A polymorphism exerts a gene-nutrition interaction on DNA methylation in the RFC1 gene, and that this interaction appears to be most prominent in NTD-affected births and in subjects with high tHcy concentrations.

How to form and close the brain: insight into the mechanism of cranial neural tube closure in mammals

The development of the embryonic brain critically depends on successfully completing cranial neural tube closure (NTC). Failure to properly close the neural tube results in significant and potentially lethal neural tube defects (NTDs). We believe these malformations are caused by disruptions in normal developmental programs such as those involved in neural plate morphogenesis and patterning, tissue fusion, and coordinated cell behaviors. Cranial NTDs include anencephaly and craniorachischisis, both lethal human birth defects. Newly emerging methods for molecular and cellular analysis offer a deeper understanding of not only the developmental NTC program itself but also mechanical and kinetic aspects of closure that may contribute to cranial NTDs. Clarifying the underlying mechanisms involved in NTC and how they relate to the onset of specific NTDs in various experimental models may help us develop novel intervention strategies to prevent NTDs.

Genetic and epigenomic footprints of folate

Dietary micronutrient composition has long been recognized as a determining factor for human health. Historically, biochemical research has successfully unraveled how vitamins serve as essential cofactors for enzymatic reactions in the biochemical machinery of the cell. Folate, also known as vitamin B9, follows this paradigm as well. Folate deficiency is linked to adverse health conditions, and dietary supplementation with folate has proven highly beneficial in the prevention of neural tube defects. With its function in single-carbon metabolism, folate levels affect nucleotide synthesis, with implications for cell proliferation, DNA repair, and genomic stability. Furthermore, by providing the single-carbon moiety in the synthesis pathway for S-adenosylmethionine, the main methyl donor in the cell, folate also impacts methylation reactions. It is this capacity that extends the reach of folate functions into the realm of epigenetics and gene regulation. Methylation reactions play a major role for several modalities of the epigenome. The specific methylation status of histones, noncoding RNAs, transcription factors, or DNA represents a significant determinant for the transcriptional output of a cell. Proper folate status is therefore necessary for a broad range of biological functions that go beyond the biochemistry of folate. In this review, we examine evolutionary, genetic, and epigenomic footprints of folate and the implications for human health.

Xenopus reduced folate carrier regulates neural crest development epigenetically

Folic acid deficiency during pregnancy causes birth neurocristopathic malformations resulting from aberrant development of neural crest cells. The Reduced folate carrier (RFC) is a membrane-bound receptor for facilitating transfer of reduced folate into the cells. RFC knockout mice are embryonic lethal and develop multiple malformations, including neurocristopathies. Here we show that XRFC is specifically expressed in neural crest tissues in Xenopus embryos and knockdown of XRFC by specific morpholino results in severe neurocristopathies. Inhibition of RFC blocked the expression of a series of neural crest marker genes while overexpression of RFC or injection of 5-methyltetrahydrofolate expanded the neural crest territories. In animal cap assays, knockdown of RFC dramatically reduced the mono- and trimethyl-Histone3-K4 levels and co-injection of the lysine methyltransferase hMLL1 largely rescued the XRFC morpholino phenotype. Our data revealed that the RFC mediated folate metabolic pathway likely potentiates neural crest gene expression through epigenetic modifications.

Incrimination of heterogeneous nuclear ribonucleoprotein E1 (hnRNP-E1) as a candidate sensor of physiological folate deficiency

The mechanism underlying the sensing of varying degrees of physiological folate deficiency, prior to adaptive optimization of cellular folate uptake through the translational up-regulation of folate receptors (FR) is unclear. Because homocysteine, which accumulates intracellularly during folate deficiency, stimulated interactions between heterogeneous nuclear ribonucleoprotein E1 (hnRNP-E1) and an 18-base FR-α mRNA cis-element that led to increased FR biosynthesis and net up-regulation of FR at cell surfaces, hnRNP-E1 was a plausible candidate sensor of folate deficiency. Accordingly, using purified components, we evaluated the physiological basis whereby L-homocysteine triggered these RNA-protein interactions to stimulate FR biosynthesis. L-homocysteine induced a concentration-dependent increase in RNA-protein binding affinity throughout the range of physiological folate deficiency, which correlated with a proportionate increase in translation of FR in vitro and in cultured human cells. Targeted reduction of newly synthesized hnRNP-E1 proteins by siRNA to hnRNP-E1 mRNA reduced both constitutive and L-homocysteine-induced rates of FR biosynthesis. Furthermore, L-homocysteine covalently bound hnRNP-E1 via multiple protein-cysteine-S-S-homocysteine mixed disulfide bonds within K-homology domains known to interact with mRNA. These data suggest that a concentration-dependent, sequential disruption of critical cysteine-S-S-cysteine bonds by covalently bound L-homocysteine progressively unmasks an underlying RNA-binding pocket in hnRNP-E1 to optimize interaction with FR-α mRNA cis-element preparatory to FR up-regulation. Collectively, such data incriminate hnRNP-E1 as a physiologically relevant, sensitive, cellular sensor of folate deficiency. Because diverse mammalian and viral mRNAs also interact with this RNA-binding domain with functional consequences to their protein expression, homocysteinylated hnRNP-E1 also appears well positioned to orchestrate a novel, nutrition-sensitive (homocysteine-responsive), posttranscriptional RNA operon in folate-deficient cells.

SMOC1 is essential for ocular and limb development in humans and mice

Microphthalmia with limb anomalies (MLA) is a rare autosomal-recessive disorder, presenting with anophthalmia or microphthalmia and hand and/or foot malformation. We mapped the MLA locus to 14q24 and successfully identified three homozygous (one nonsense and two splice site) mutations in the SPARC (secreted protein acidic and rich in cysteine)-related modular calcium binding 1 (SMOC1) in three families. Smoc1 is expressed in the developing optic stalk, ventral optic cup, and limbs of mouse embryos. Smoc1 null mice recapitulated MLA phenotypes, including aplasia or hypoplasia of optic nerves, hypoplastic fibula and bowed tibia, and syndactyly in limbs. A thinned and irregular ganglion cell layer and atrophy of the anteroventral part of the retina were also observed. Soft tissue syndactyly, resulting from inhibited apoptosis, was related to disturbed expression of genes involved in BMP signaling in the interdigital mesenchyme. Our findings indicate that SMOC1/Smoc1 is essential for ocular and limb development in both humans and mice.

Alx3-deficient mice exhibit folic acid-resistant craniofacial midline and neural tube closure defects

Neural tube closure defects are among the most frequent congenital malformations in humans. Supplemental maternal intake of folic acid before and during pregnancy reduces their incidence significantly, but the mechanism underlying this preventive effect is unknown. As a number of genes that cause neural tube closure defects encode transcriptional regulators in mice, one possibility is that folic acid could induce the expression of transcription factors to compensate for the primary genetic defect. We report that folic acid is required in mouse embryos for the specific expression of the homeodomain gene Alx3 in the head mesenchyme, an important tissue for cranial neural tube closure. Alx3-deficient mice exhibit increased failure of cranial neural tube closure and increased cell death in the craniofacial region, two effects that are also observed in wild type embryos developing in the absence of folic acid. Folic acid cannot prevent these defects in Alx3-deficient embryos, indicating that one mechanism of folic acid action is through induced expression of Alx3. Thus, Alx3 emerges as a candidate gene for human neural tube defects and reveals the existence of induced transcription factor gene expression as a previously unknown mechanism by which folic acid prevents neural tube closure defects.

Can we prevent diabetic birth defects with micronutrients?

Congenital malformations are more common in infants of diabetic women than in children of non-diabetic women. The mechanisms behind diabetes-induced congenital anomalies are not known. Disturbed micronutrient metabolism, in concert with oxidative stress, has been suggested as a cause of diabetes-induced malformations by several studies. In experimental work, administration of inositol, arachidonic acid and several antioxidative compounds, as well as folic acid, to the embryo, has proven to attenuate the teratogenic effects of a diabetic environment. Future therapeutic efforts may include supplementation with antioxidants or micronutrients, such as folic acid, to the pregnant diabetic woman, although exact compounds and doses need to be determined.

Insights into prevention of human neural tube defects by folic acid arising from consideration of mouse mutants

Almost 30 years after the initial study by Richard W. Smithells and coworkers, it is still unknown how maternal periconceptional folic acid supplementation prevents human neural tube defects (NTDs). In this article, questions about human NTD prevention are considered in relation to three groups of mouse models: NTD mutants that respond to folate, NTD mutants and strains that do not respond to folate, and mutants involving folate-pathway genes. Of the 200 mouse NTD mutants, only a few have been tested with folate; half respond and half do not. Among responsive mutants, folic acid supplementation reduces exencephaly and/or spina bifida aperta frequency in the Sp(2H), Sp, Cd, Cited2, Cart1, and Gcn5 mutants. Prevention ranges from 35 to 85%. The responsive Sp(2H) (Pax3) mutant has abnormal folate metabolism, but the responsive Cited2 mutant does not. Neither folic nor folinic acid reduces NTD frequency in Axd, Grhl3, Fkbp8, Map3k4, or Nog mutants or in the curly tail or SELH/Bc strains. Spina bifida frequency is reduced in Axd by methionine and in curly tail by inositol. Exencephaly frequency is reduced in SELH/Bc by an alternative commercial ration. Mutations in folate-pathway genes do not cause NTDs, except for 30% exencephaly in folate-treated Folr1. Among folate-pathway mutants, neural tube closure is normal in Cbs, Folr2, Mthfd1, Mthfd2, Mthfr, and Shmt1 mutants. Embryos die by midgestation in Folr1, Mtr, Mtrr, and RFC1 mutants. The mouse models point to genetic heterogeneity in the ability to respond to folic acid and also to heterogeneity in genetic cause of NTDs that can be prevented by folic acid.

Regulation of folate receptor 1 gene expression in the visceral endoderm

BACKGROUND

Nutrient supply to the developing mammalian embryo is a fundamental requirement. Before completion of the chorioallantoic placenta, the visceral endoderm plays a crucial role in nurturing the embryo. We have found that visceral endoderm cells express folate receptor 1, a high-affinity receptor for the essential micronutrient folic acid, suggesting that the visceral endoderm has an important function for folate transport to the embryo. The mechanisms that direct expression of FOLR1 in the visceral endoderm are unknown.

METHODS

Sequences were tested for transcriptional activation capabilities in the visceral endoderm utilizing reporter gene assays in a cell model for extraembryonic endoderm in vitro, and in transgenic mice in vivo.

RESULTS

With F9 embryo carcinoma cells as a model for extraembryonic endoderm, we demonstrate that the P4 promoter of the human FOLR1 gene is active during differentiation of the cells towards visceral endoderm. However, transgenic mouse experiments show that promoter sequences alone are insufficient to elicit reporter gene transcription in vivo. Using sequence conservation as guide to choose genomic sequences from the human FOLR1 gene locus, we demonstrate that the sequence termed F1CE2 exhibits specific enhancer activity in F9 cells in vitro, in the visceral endoderm, and later the yolk sac in transgenic mouse embryos in vivo. We further show that the transcription factor HNF4-alpha can activate this enhancer sequence.

CONCLUSIONS

We have identified a transcriptional enhancer sequence from the FOLR1 locus with specific activity in vitro and in vivo, and suggest that FOLR1 is a target for regulation by HNF4-alpha.

Valproic acid-induced deregulation in vitro of genes associated in vivo with neural tube defects

The utility of an in vitro system to search for molecular targets and markers of developmental toxicity was explored, using microarrays to detect genes susceptible to deregulation by the teratogen valproic acid (VPA) in the pluripotent mouse embryonal carcinoma cell line P19. Total RNA extracted from P19 cells cultured in the absence or presence of 1, 2.5, or 10mM VPA for 1.5, 6, or 24 h was subjected to replicated microarray analysis, using CodeLink UniSet I Mouse 20K Expression Bioarrays. A moderated F-test revealed a significant VPA response for 2972 (p < 10(-3)) array probes (19.4% of the filtered gene list), 421 of which were significant across all time points. In a core subset of VPA target genes whose expression was downregulated (68 genes) or upregulated (125 genes) with high probability (p < 10(-7)) after both 1.5 and 6 h of VPA exposure, there was a significant enrichment of the biological process Gene Ontology term transcriptional regulation among downregulated genes, and apoptosis among upregulated, and two of the downregulated genes (Folr1 and Gtf2i) have a knockout phenotype comprising exencephaly, the major malformation induced by VPA in mice. The VPA-induced gene expression response in P19 cells indicated that approximately 30% of the approximately 200 genes known from genetic mouse models to be associated with neural tube defects may be potential VPA targets, suggestive of a combined deregulation of multiple genes as a possible mechanism of VPA teratogenicity. Gene expression responses related to other known effects of VPA (histone deacetylase inhibition, G(1)-phase cell cycle arrest, induction of apoptosis) were also identified. This study indicates that toxicogenomic responses to a teratogenic compound in vitro may correlate with known in vitro and in vivo effects, and that short-time (< or =6 h) exposures in such an in vitro system could provide a useful component in mechanistic studies and screening tests in developmental toxicology.

Maternal fumonisin exposure as a risk factor for neural tube defects

Fumonisins are mycotoxins produced by the fungus F. verticillioides, a common contaminant of maize (corn) worldwide. Maternal consumption of fumonisin B(1)-contaminated maize during early pregnancy has recently been associated with increased risk for neural tube defects (NTDs) in human populations that rely heavily on maize as a dietary staple. Experimental administration of purified fumonisin to mice early in gestation also results in an increased incidence of NTDs in exposed offspring. Fumonisin inhibits the enzyme ceramide synthase in de novo sphingolipid biosynthesis, resulting in an elevation of free sphingoid bases and depletion of downstream glycosphingolipids. Increased sphingoid base metabolites (i.e., sphinganine-1-phosphate) may perturb signaling cascades involved in embryonic morphogenesis by functioning as ligands for sphingosine-1-P (S1P) receptors, a family of G-protein-coupled receptors that regulate key biological processes such as cell survival/proliferation, differentiation and migration. Fumonisin-induced depletion of glycosphingolipids impairs expression and function of the GPI-anchored folate receptor (Folr1), which may also contribute to adverse pregnancy outcomes. NTDs appear to be multifactorial in origin, involving complex gene-nutrient-environment interactions. Vitamin supplements containing folic acid have been shown to reduce the occurrence of NTDs, and may help protect the developing fetus from environmental teratogens. Fumonisins appear to be an environmental risk factor for birth defects, although other aspects of maternal nutrition and genetics play interactive roles in determining pregnancy outcome. Minimizing exposures to mycotoxins through enhanced agricultural practices, identifying biomarkers of exposure, characterizing mechanisms of toxicity, and improving maternal nutrition are all important strategies for reducing the NTD burden in susceptible human populations.

Gene-nutrient interactions: importance of folic acid and vitamin B12 during early embryogenesis

The role that nutritional factors play in mammalian development has received renewed attention over the past two decades as the scientific literature has exploded with reports that folic acid supplementation in the periconceptional period can protect embryos from a number of highly significant malformations. As is often the case, the relationship between B vitamin supplementation and improved pregnancy outcomes is more complicated than initially perceived, as the interaction between nutritional factors and selected genes must be considered. In this review, we attempt to summarize the complex clinical and experimental literature on nutritional factors, their biological transport mechanisms, and interactions with genetic polymorphisms that impact early embryogenesis. While not exhaustive, our goal was to provide an overview of important gene-nutrient interactions, focusing on folic acid and vitamin B12, to serve as a framework for understanding the multiple roles they play in early embryogenesis.

Embryonic development in the reduced folate carrier knockout mouse is modulated by maternal folate supplementation

BACKGROUND

The reduced folate carrier (RFC1) is a ubiquitously expressed integral membrane protein that mediates delivery of 5-methyltetrahydrofolate into mammalian cells. In this study, embryonic/fetal development is characterized in an RFC1 knockout mouse model in which pregnant dams receive different levels of folate supplementation.

METHODS

RFC1(+/-) males were mated to RFC1(+/-) females, and pregnant dams were treated with vehicle (control) or folic acid (25 or 50 mg/kg) by daily subcutaneous injection (0.1 mL/10 g bwt), beginning on E0.5 and continuing throughout gestation until the time of sacrifice.

RESULTS

Without maternal folate supplementation, RFC1 nullizygous embryos die shortly postimplantation. Supplementation of pregnant dams with 25 mg/kg/day folic acid prolongs survival of mutant embryos until E9.5-E10.5, but they are developmentally delayed relative to wild-type littermates, display a marked absence of erythropoiesis, severe neural tube and limb bud defects, and failure of chorioallantoic fusion. Fgfr2 protein levels are significantly reduced or absent in the extraembryonic membranes of RFC1 nullizygous embryos. Maternal folate supplementation with 50 mg/kg/day results in survival of 22% of RFC1 mutants to E18.5, but they develop with multiple malformations of the eyelids, lungs, heart, and skin.

CONCLUSIONS

High doses of daily maternal folate supplementation during embryonic/fetal development are necessary for early postimplantation embryonic viability of RFC1 nullizygous embryos, and play a critical role in chorioallantoic fusion, erythropoiesis, and proper development of the neural tube, limbs, lungs, heart, and skin.