Genetic contribution of retinoid-related genes to neural tube defects

Up-regulation of miR-10a-5p expression inhibits the proliferation and differentiation of neural stem cells by targeting Chl1

Neural tube defects (NTDs) are characterized by the failure of neural tube closure during embryogenesis and are considered the most common and severe central nervous system anomalies during early development. Recent microRNA (miRNA) expression profiling studies have revealed that the dysregulation of several miRNAs plays an important role in retinoic acid (RA)-induced NTDs. However, the molecular functions of these miRNAs in NTDs remain largely unidentified. Here, we show that miR-10a-5p is significantly upregulated in RA-induced NTDs and results in reduced cell growth due to cell cycle arrest and dysregulation of cell differentiation. Moreover, the cell adhesion molecule L1-like ( Chl1) is identified as a direct target of miR-10a-5p in neural stem cells (NSCs) in vitro, and its expression is reduced in RA-induced NTDs. siRNA-mediated knockdown of intracellular Chl1 affects cell proliferation and differentiation similar to those of miR-10a-5p overexpression, which further leads to the inhibition of the expressions of downstream ERK1/2 MAPK signaling pathway proteins. These cellular responses are abrogated by either increased expression of the direct target of miR-10a-5p ( Chl1) or an ERK agonist such as honokiol. Overall, our study demonstrates that miR-10a-5p plays a major role in the process of NSC growth and differentiation by directly targeting Chl1, which in turn induces the downregulation of the ERK1/2 cascade, suggesting that miR-10a-5p and Chl1 are critical for NTD formation in the development of embryos.

Alcohol induces neural tube defects by reducing retinoic acid signaling and promoting neural plate expansion

Introduction: Neural tube defects (NTDs) are among the most debilitating and common developmental defects in humans. The induction of NTDs has been attributed to abnormal folic acid (vitamin B9) metabolism, Wnt and BMP signaling, excess retinoic acid (RA), dietary components, environmental factors, and many others. In the present study we show that reduced RA signaling, including alcohol exposure, induces NTDs. Methods: Xenopus embryos were exposed to pharmacological RA biosynthesis inhibitors to study the induction of NTDs. Embryos were treated with DEAB, citral, or ethanol, all of which inhibit the biosynthesis of RA, or injected to overexpress Cyp26a1 to reduce RA. NTD induction was studied using neural plate and notochord markers together with morphological analysis. Expression of the neuroectodermal regulatory network and cell proliferation were analyzed to understand the morphological malformations of the neural plate. Results: Reducing RA signaling levels using retinaldehyde dehydrogenase inhibitors (ethanol, DEAB, and citral) or Cyp26a1-driven degradation efficiently induce NTDs. These NTDs can be rescued by providing precursors of RA. We mapped this RA requirement to early gastrula stages during the induction of neural plate precursors. This reduced RA signaling results in abnormal expression of neural network genes, including the neural plate stem cell maintenance genes, geminin, and foxd4l1.1. This abnormal expression of neural network genes results in increased proliferation of neural precursors giving rise to an expanded neural plate. Conclusion: We show that RA signaling is required for neural tube closure during embryogenesis. RA signaling plays a very early role in the regulation of proliferation and differentiation of the neural plate soon after the induction of neural progenitors during gastrulation. RA signaling disruption leads to the induction of NTDs through the mis regulation of the early neuroectodermal network, leading to increased proliferation resulting in the expansion of the neural plate. Ethanol exposure induces NTDs through this mechanism involving reduced RA levels.

Exploring the association between congenital vertebral malformations and neural tube defects

Congenital vertebral malformations (CVMs) and neural tube defects (NTDs) are common birth defects affecting the spine and nervous system, respectively, due to defects in somitogenesis and neurulation. Somitogenesis and neurulation rely on factors secreted from neighbouring tissues and the integrity of the axial structure. Crucial signalling pathways like Wnt, Notch and planar cell polarity regulate somitogenesis and neurulation with significant crosstalk. While previous studies suggest an association between CVMs and NTDs, the exact mechanism underlying this relationship remains unclear. In this review, we explore embryonic development, signalling pathways and clinical phenotypes involved in the association between CVMs and NTDs. Moreover, we provide a summary of syndromes that exhibit occurrences of both CVMs and NTDs. We aim to provide insights into the potential mechanisms underlying the association between CVMs and NTDs, thereby facilitating clinical diagnosis and management of these anomalies.

A quest for genetic causes underlying signaling pathways associated with neural tube defects

Neural tube defects (NTDs) are serious congenital deformities of the nervous system that occur owing to the failure of normal neural tube closures. Genetic and non-genetic factors contribute to the etiology of neural tube defects in humans, indicating the role of gene-gene and gene-environment interaction in the occurrence and recurrence risk of neural tube defects. Several lines of genetic studies on humans and animals demonstrated the role of aberrant genes in the developmental risk of neural tube defects and also provided an understanding of the cellular and morphological programs that occur during embryonic development. Other studies observed the effects of folate and supplementation of folic acid on neural tube defects. Hence, here we review what is known to date regarding altered genes associated with specific signaling pathways resulting in NTDs, as well as highlight the role of various genetic, and non-genetic factors and their interactions that contribute to NTDs. Additionally, we also shine a light on the role of folate and cell adhesion molecules (CAMs) in neural tube defects.

Zebrafish as a Model to Study Retinoic Acid Signaling in Development and Disease

Retinoic acid (RA) is a metabolite of vitamin A (retinol) that plays various roles in development to influence differentiation, patterning, and organogenesis. RA also serves as a crucial homeostatic regulator in adult tissues. The role of RA and its associated pathways are well conserved from zebrafish to humans in both development and disease. This makes the zebrafish a natural model for further interrogation into the functions of RA and RA-associated maladies for the sake of basic research, as well as human health. In this review, we explore both foundational and recent studies using zebrafish as a translational model for investigating RA from the molecular to the organismal scale.

Neural Tube Defects and Folate Deficiency: Is DNA Repair Defective?

Neural tube defects (NTDs) are complex congenital malformations resulting from failure of neural tube closure during embryogenesis, which is affected by the interaction of genetic and environmental factors. It is well known that folate deficiency increases the incidence of NTDs; however, the underlying mechanism remains unclear. Folate deficiency not only causes DNA hypomethylation, but also blocks the synthesis of 2'-deoxythymidine-5'-monophosphate (dTMP) and increases uracil misincorporation, resulting in genomic instabilities such as base mismatch, DNA breakage, and even chromosome aberration. DNA repair pathways are essential for ensuring normal DNA synthesis, genomic stability and integrity during embryonic neural development. Genomic instability or lack of DNA repair has been implicated in risk of development of NTDs. Here, we reviewed the relationship between folate deficiency, DNA repair pathways and NTDs so as to reveal the role and significance of DNA repair system in the pathogenesis of NTDs and better understand the pathogenesis of NTDs.

ALDH1A2-related disorder: A new genetic syndrome due to alteration of the retinoic acid pathway

Aldehyde Dehydrogenase 1, Family Member A2 (ALDH1A2) is essential for the synthesis of retinoic acid from vitamin A. Studies in model organisms demonstrate a critical role for ALDH1A2 in embryonic development, yet few pathogenic variants are linked to congenital anomalies in humans. We present three siblings with multiple congenital anomaly syndrome linked to biallelic sequence variants in ALDH1A2. The major congenital malformations affecting these children include tetralogy of Fallot, absent thymus, diaphragmatic eventration, and talipes equinovarus. Upper airway anomalies, hypocalcemia, and dysmorphic features are newly reported in this manuscript. In vitro functional validation of variants indicated that substitutions reduced the expression of the enzyme. Our clinical and functional data adds to a recent report of biallelic ALDH1A2 pathogenic variants in two families with a similar constellation of congenital malformations. These findings provide further evidence for an autosomal recessive ALDH1A2-deficient recognizable malformation syndrome involving the diaphragm, cardiac and musculoskeletal systems.

Abnormal methylation caused by folic acid deficiency in neural tube defects

Neural tube closure disorders, including anencephaly, spina bifida, and encephalocele, cause neural tube defects (NTDs). This congenital disability remained not only a major contributor to the prevalence of stillbirths and neonatal deaths but also a significant cause of lifelong physical disability in surviving infants. NTDs are complex diseases caused by multiple etiologies, levels, and mechanisms. Currently, the pathogenesis of NTDs is considered to be associated with both genetic and environmental factors. Here, we aimed to review the research progress on the etiology and mechanism of NTDs induced by methylation modification caused by folic acid deficiency. Folic acid supplementation in the diet is reported to be beneficial in preventing NTDs. Methylation modification is one of the most important epigenetic modifications crucial for brain neurodevelopment. Disturbances in folic acid metabolism and decreased S-adenosylmethionine levels lead to reduced methyl donors and methylation modification disorders. In this review, we summarized the relationship between NTDs, folic acid metabolism, and related methylation of DNA, imprinted genes, cytoskeletal protein, histone, RNA, and non-coding RNA, so as to clarify the role of folic acid and methylation in NTDs and to better understand the various pathogenesis mechanisms of NTDs and the effective prevention.

Cannabidiol impairs neural tube closure in mouse whole embryo culture

BACKGROUND

Cannabidiol (CBD) is a nonpsychoactive constituent of cannabis widely available as a dietary supplement. Previous reports that it impairs the retinoid, sonic hedgehog, and folate metabolism pathways raise concern that it may impair closure of the embryonic neural tube (NT), producing NT defects including spina bifida and exencephaly.

METHODS

We undertook teratogenicity testing of CBD in mouse whole embryo culture.

RESULTS

At concentrations that do not diminish embryo viability, growth, or axial rotation, CBD dose-dependently impairs cranial NT closure, increasing the proportion of embryos that develop exencephaly. It concomitantly diminishes closure of the spinal NT, the posterior neuropore (PNP), producing longer neuropores at the end of culture which is a hallmark of spina bifida risk. Exposure to CBD does not disrupt the formation of long F-actin cables in surface ectoderm cells flanking the PNP or folding of the neuroepithelium at predictable hinge points. At the cellular level, CBD exposure does not alter proliferation or apoptosis of the spinal neuroepithelium.

DISCUSSION

Thus, CBD acts selectively as a neuroteratogen predisposing to spina bifida and exencephaly in mouse whole embryo culture at exposure levels not associated with overt toxicity. Large-scale testing of CBD's effects on NT closure, particularly in at-risk groups, is warranted to inform its marketing to women of childbearing age.

Pathogenesis of neural tube defects: The regulation and disruption of cellular processes underlying neural tube closure

Neural tube closure (NTC) is crucial for proper development of the brain and spinal cord and requires precise morphogenesis from a sheet of cells to an intact three-dimensional structure. NTC is dependent on successful regulation of hundreds of genes, a myriad of signaling pathways, concentration gradients, and is influenced by epigenetic and environmental cues. Failure of NTC is termed a neural tube defect (NTD) and is a leading class of congenital defects in the United States and worldwide. Though NTDs are all defined as incomplete closure of the neural tube, the pathogenesis of an NTD determines the type, severity, positioning, and accompanying phenotypes. In this review, we survey pathogenesis of NTDs relating to disruption of cellular processes arising from genetic mutations, altered epigenetic regulation, and environmental influences by micronutrients and maternal condition. This article is categorized under: Congenital Diseases > Genetics/Genomics/Epigenetics Neurological Diseases > Genetics/Genomics/Epigenetics Neurological Diseases > Stem Cells and Development.

Time Course Transcriptome Analysis of Spina Bifida Progression in Fetal Rats

A better understanding of the transcriptomic modifications that occur in spina bifida may lead to identify mechanisms involved in the progression of spina bifida in utero and the development of new therapeutic strategies that aid in spinal cord regeneration after surgical interventions. In this study, RNA-sequencing was used to identify differentially expressed genes in fetal spinal cords from rats with retinoic acid-induced spina bifida at E15, E17, and E20. Gene ontology, KEGG, and protein–protein interaction analysis were conducted to predict pathways involved in the evolution of the disease. Approximately 3000, 1000 and 300 genes were differentially expressed compared to the control groups at E15, E17 and E20, respectively. Overall, the results suggest common alterations in certain pathways between gestational time points, such as upregulation in p53 and sonic hedgehog signaling at E15 and E17 and downregulation in the myelin sheath at E17 and E20. However, there were other modifications specific to gestational time points, including skeletal muscle development at E15, downregulated glucose metabolism at E17, and upregulated inflammation at E20. In conclusion, this work provides evidence that gestational age during spina bifida repair may be a significant variable to consider during the development of new regenerative therapeutics approaches.

Micronutrient imbalance and common phenotypes in neural tube defects

Neural tube defects (NTDs) are among the most common birth defects, with a prevalence of close to 19 per 10,000 births worldwide. The etiology of NTDs is complex involving the interplay of genetic and environmental factors. Since nutrient deficiency is a risk factor and dietary changes are the major preventative measure to reduce the risk of NTDs, a more detailed understanding of how common micronutrient imbalances contribute to NTDs is crucial. While folic acid has been the most discussed environmental factor due to the success that population-wide fortification has had on prevention of NTDs, folic acid supplementation does not prevent all NTDs. The imbalance of several other micronutrients has been implicated as risks for NTDs by epidemiological studies and in vivo studies in animal models. In this review, we highlight recent literature deciphering the multifactorial mechanisms underlying NTDs with an emphasis on mouse and human data. Specifically, we focus on advances in our understanding of how too much or too little retinoic acid, zinc, and iron alter gene expression and cellular processes contributing to the pathobiology of NTDs. Synthesis of the discussed literature reveals common cellular phenotypes found in embryos with NTDs resulting from several micronutrient imbalances. The goal is to combine knowledge of these common cellular phenotypes with mechanisms underlying micronutrient imbalances to provide insights into possible new targets for preventative measures against NTDs.

Functional Validation of CLDN Variants Identified in a Neural Tube Defect Cohort Demonstrates Their Contribution to Neural Tube Defects

Neural tube defects (NTDs) are severe malformations of the central nervous system that affect 1–2 individuals per 2,000 births. Their etiology is complex and involves both genetic and environmental factors. Our recent discovery that simultaneous removal of Cldn3, -4, and -8 from tight junctions results in cranial and spinal NTDs in both chick and mouse embryos suggests that claudins play a conserved role in neural tube closure in vertebrates. To determine if claudins were associated with NTDs in humans, we used a Fluidigm next generation sequencing approach to identify genetic variants in CLDN loci in 152 patients with spinal NTDs. We identified eleven rare and four novel missense mutations in ten CLDN genes. In vivo validation of variant pathogenicity using a chick embryo model system revealed that overexpression of four variants caused a significant increase in NTDs: CLDN3 A128T, CLDN8 P216L, CLDN19 I22T, and E209G. Our data implicate rare missense variants in CLDN genes as risk factors for spinal NTDs and suggest a new family of proteins involved in the pathogenesis of these malformations.

Association between rare variants in specific functional pathways and human neural tube defects multiple subphenotypes

BACKGROUND

Neural tube defects (NTDs) are failure of neural tube closure, which includes multiple central nervous system phenotypes. More than 300 mouse mutant strains exhibits NTDs phenotypes and give us some clues to establish association between biological functions and subphenotypes. However, the knowledge about association in human remains still very poor.

METHODS

High throughput targeted genome DNA sequencing were performed on 280 neural tube closure-related genes in 355 NTDs cases and 225 ethnicity matched controls, RESULTS: We explored that potential damaging rare variants in genes functioning in chromatin modification, apoptosis, retinoid metabolism and lipid metabolism are associated with human NTDs. Importantly, our data indicate that except for planar cell polarity pathway, craniorachischisis is also genetically related with chromatin modification and retinoid metabolism. Furthermore, single phenotype in cranial or spinal regions displays significant association with specific biological function, such as anencephaly is associated with potentially damaging rare variants in genes functioning in chromatin modification, encephalocele is associated with apoptosis, retinoid metabolism and one carbon metabolism, spina bifida aperta and spina bifida cystica are associated with apoptosis; lumbar sacral spina bifida aperta and spina bifida occulta are associated with lipid metabolism. By contrast, complex phenotypes in both cranial and spinal regions display association with various biological functions given the different phenotypes.

CONCLUSIONS

Our study links genetic variant to subphenotypes of human NTDs and provides a preliminary but direct clue to investigate pathogenic mechanism for human NTDs.

Regulating Retinoic Acid Availability during Development and Regeneration: The Role of the CYP26 Enzymes

This review focuses on the role of the Cytochrome p450 subfamily 26 (CYP26) retinoic acid (RA) degrading enzymes during development and regeneration. Cyp26 enzymes, along with retinoic acid synthesising enzymes, are absolutely required for RA homeostasis in these processes by regulating availability of RA for receptor binding and signalling. Cyp26 enzymes are necessary to generate RA gradients and to protect specific tissues from RA signalling. Disruption of RA homeostasis leads to a wide variety of embryonic defects affecting many tissues. Here, the function of CYP26 enzymes is discussed in the context of the RA signalling pathway, enzymatic structure and biochemistry, human genetic disease, and function in development and regeneration as elucidated from animal model studies.

Role of carotenoids and retinoids during heart development

The nutritional requirements of the developing embryo are complex. In the case of dietary vitamin A (retinol, retinyl esters and provitamin A carotenoids), maternal derived nutrients serve as precursors to signaling molecules such as retinoic acid, which is required for embryonic patterning and organogenesis. Despite variations in the composition and levels of maternal vitamin A, embryonic tissues need to generate a precise amount of retinoic acid to avoid congenital malformations. Here, we summarize recent findings regarding the role and metabolism of vitamin A during heart development and we survey the association of genes known to affect retinoid metabolism or signaling with various inherited disorders. A better understanding of the roles of vitamin A in the heart and of the factors that affect retinoid metabolism and signaling can help design strategies to meet nutritional needs and to prevent birth defects and disorders associated with altered retinoid metabolism. This article is part of a Special Issue entitled Carotenoids recent advances in cell and molecular biology edited by Johannes von Lintig and Loredana Quadro.

Low folate concentration impacts mismatch repair deficiency in neural tube defects

Aim: To know the cause of sequence variants in neural tube defect (NTD). Materials & methods: We sequenced genes implicated in neural tube closure (NTC) in a Chinese cohort and elucidated the molecular mechanism-driving mutations. Results: In NTD cases, an increase in specific variants was identified, potentially deleterious rare variants harbored in H3K36me3 occupancy regions that recruits mismatch repair (MMR) machinery. Lower folate concentrations in local brain tissues were also observed. In neuroectoderm cells, folic acid insufficiency attenuated association of Msh6 to H3K36me3, and reduced bindings to NTC genes. Rare variants in human NTDs were featured by MMR deficiency and more severe microsatellite instability. Conclusion: Our work suggests a mechanistic link between folate insufficiency and MMR deficiency that correlates with an increase of rare variants in NTC genes.

Zinc deficiency causes neural tube defects through attenuation of p53 ubiquitylation

Micronutrition is essential for neural tube closure, and zinc deficiency is associated with human neural tube defects. Here, we modeled zinc deficiency in mouse embryos, and used live imaging and molecular studies to determine how zinc deficiency affects neural tube closure. Embryos cultured with the zinc chelator TPEN failed to close the neural tube and showed excess apoptosis. TPEN-induced p53 protein stabilization in vivo and in neuroepithelial cell cultures and apoptosis was dependent on p53. Mechanistically, zinc deficiency resulted in disrupted interaction between p53 and the zinc-dependent E3 ubiquitin ligase Mdm2, and greatly reduced p53 ubiquitylation. Overexpression of human CHIP, a zinc-independent E3 ubiquitin ligase that targets p53, relieved TPEN-induced p53 stabilization and reduced apoptosis. Expression of p53 pro-apoptotic target genes was upregulated by zinc deficiency. Correspondingly, embryos cultured with p53 transcriptional activity inhibitor pifithrin-α could overcome TPEN-induced apoptosis and failure of neural tube closure. Our studies indicate that zinc deficiency disrupts neural tube closure through decreased p53 ubiquitylation, increased p53 stabilization and excess apoptosis.

Inhibition of NRF2 signaling and increased reactive oxygen species during embryogenesis in a rat model of retinoic acid-induced neural tube defects

Exposure to retinoic acid (RA) during pregnancy increases the risk of serious neural tube defects (NTDs) in the developing fetus. The precise molecular mechanism for this process is unclear; however, RA is associated with oxidative stress mediated by reactive oxygen species. Nuclear factor erythroid 2-related factor 2 (NRF2) is a master regulator of oxidative stress that directs the expression of antioxidant genes and detoxifying proteins to maintain redox homeostasis. We established a rat model of NTDs in which pregnant dams were administered all-trans (at)RA on gestational day 10, and oxidative stress levels and the spatiotemporal expression of NRF2 and its downstream targets were examined in the resulting embryos and in maternal blood. In the NTD group, total antioxidative capacity decreased and 8-hydroxy-2'-deoxyguanosine increased in maternal serum and fetal spinal cord tissues. Plasma GSH content, the GSH/GSSG ratio, and glutathione peroxidase activity in fetal spinal cords were lower in the NTD group relative to controls. We detected NRF2 protein reduction and concomitant upregulation of Kelch-like ECH-associated protein 1 (KEAP1) - a cytoplasmic inhibitor of NRF2 - in the NTD group. The mRNA and protein levels of downstream targets of NRF2 were downregulated in the spinal cords of NTD embryos. These data demonstrate substantial oxidative stress and NRF2 signaling pathway disruption in a model of NTDs induced by atRA. The inhibitory effects of atRA on NRF2 signaling may lower cellular defenses against RA-induced oxidative stress and could play important roles in NTD occurrence during embryonic development.

Formate rescues neural tube defects caused by mutations in Slc25a32

Periconceptional folic acid (FA) supplementation significantly reduces the prevalence of neural tube defects (NTDs). Unfortunately, some NTDs are FA resistant, and as such, NTDs remain a global public health concern. Previous studies have identified SLC25A32 as a mitochondrial folate transporter (MFT), which is capable of transferring tetrahydrofolate (THF) from cellular cytoplasm to the mitochondria in vitro. Herein, we show that gene trap inactivation of Slc25a32 (Mft) in mice induces NTDs that are folate (5-methyltetrahydrofolate, 5-mTHF) resistant yet are preventable by formate supplementation. Slc25a32gt/gt embryos die in utero with 100% penetrant cranial NTDs. 5-mTHF supplementation failed to promote normal neural tube closure (NTC) in mutant embryos, while formate supplementation enabled the majority (78%) of knockout embryos to complete NTC. A parallel genetic study in human subjects with NTDs identified biallelic loss of function SLC25A32 variants in a cranial NTD case. These data demonstrate that the loss of functional Slc25a32 results in cranial NTDs in mice and has also been observed in a human NTD patient.