Rescue of neural tube defects in Pax-3-deficient embryos by p53 loss of function: implications for Pax-3- dependent development and tumorigenesis

Pax3 lineage-specific deletion of Gpr161 is associated with spinal neural tube and craniofacial malformations during embryonic development

Sonic hedgehog (Shh) signaling is the morphogen signaling that regulates embryonic craniofacial and neural tube development. G protein-coupled receptor 161 (Gpr161) is a negative regulator of Shh signaling, and its inactivation in mice results in embryo lethality associated with craniofacial defects and neural tube defects. However, the structural defects of later embryonic stages and cell lineages underlying abnormalities have not been well characterized due to the limited lifespan of Gpr161 null mice. We found that embryos with Pax3 lineage-specific deletion of Gpr161 presented with tectal hypertrophy (anterior dorsal neuroepithelium), cranial vault and facial bone hypoplasia (cranial neural crest), vertebral abnormalities (somite) and the closed form of spina bifida (posterior dorsal neuroepithelium). In particular, the closed form of spina bifida was partly due to reduced Pax3 and Cdx4 gene expression in the posterior dorsal neural tubes of Gpr161 mutant embryos with decreased Wnt signaling, whereas Shh signaling was increased. We describe a previously unreported role for Gpr161 in the development of posterior neural tubes and confirm its role in cranial neural crest- and somite-derived skeletogenesis and midbrain morphogenesis in mice.

Pax3 lineage-specific deletion of Gpr161 is associated with spinal neural tube and craniofacial malformations during embryonic development

Shh signaling is the morphogen signaling that regulates embryonic craniofacial and neural tube development. G protein-coupled receptor 161 (Gpr161) is a negative regulator of Shh signaling, and its inactivation in mice results in embryo lethality with craniofacial and neural tube defects (NTDs). However, the structural defects of later embryonic stages in Gpr161 null mice and cell lineages underlying abnormalities were not well characterized due to their limited lifespan. We found the Pax3 lineage-specific deletion of Gpr161 in mice presented with tectal hypertrophy (anterior dorsal neuroepithelium), cranial vault and facial bone hypoplasia (cranial neural crest (CNC)), vertebral abnormalities (somite), and the closed form of spina bifida (posterior dorsal neuroepithelium). In particular, the closed form of spina bifida is partly due to the reduced Pax3 and Cdx4 gene expression of the posterior dorsal neural tubes of Gpr161 mutant embryos involving decreased Wnt signaling whereas Shh signaling was increased. This study provides the novel role of Gpr161 in the posterior neural tube development and confirms its role on CNC- and somite-derived skeletogenesis and midbrain morphogenesis in mice.

The impact of TP53 activation and apoptosis in primary hereditary microcephaly

Autosomal recessive primary microcephaly (MCPH) is a constellation of disorders that share significant brain size reduction and mild to moderate intellectual disability, which may be accompanied by a large variety of more invalidating clinical signs. Extensive neural progenitor cells (NPC) proliferation and differentiation are essential to determine brain final size. Accordingly, the 30 MCPH loci mapped so far (MCPH1-MCPH30) encode for proteins involved in microtubule and spindle organization, centriole biogenesis, nuclear envelope, DNA replication and repair, underscoring that a wide variety of cellular processes is required for sustaining NPC expansion during development. Current models propose that altered balance between symmetric and asymmetric division, as well as premature differentiation, are the main mechanisms leading to MCPH. Although studies of cellular alterations in microcephaly models have constantly shown the co-existence of high DNA damage and apoptosis levels, these mechanisms are less considered as primary factors. In this review we highlight how the molecular and cellular events produced by mutation of the majority of MCPH genes may converge on apoptotic death of NPCs and neurons, via TP53 activation. We propose that these mechanisms should be more carefully considered in the alterations of the sophisticated equilibrium between proliferation, differentiation and death produced by MCPH gene mutations. In consideration of the potential druggability of cell apoptotic pathways, a better understanding of their role in MCPH may significantly facilitate the development of translational approaches.

Synergistic Effects of Folic Acid and n-3 Polyunsaturated Fatty Acid in Preventing Neural Tube Defects in Diabetic Mice

The present study aimed to investigate whether a combination of folic acid (FA) and n-3 polyunsaturated fatty acids (PUFA) has a better preventive effect on maternal diabetes-induced neural tube defects (NTD) than FA alone. The experiment included five groups of pregnant mice: healthy control (HC), diabetes mellitus control (DMC), diabetes + n-3 PUFA (DMn-3), diabetes + FA (DMFA), and diabetes + FA + n-3 PUFA (DMFA + n-3). The incidence of NTD in DMFA + n-3 (1.04%) was significantly lower than that in DMFA (8.57%) and DMn-3 (7.82%). The incidence of NTD in DMFA and DMn-3 was significantly lower than that in DMC (19.41%). DMFA + n-3 had a lower apoptosis of neuroepithelial cells, a lower expression of P53 and Bax, and a higher expression of Pax3 and Bcl-2, compared with DMFA and DMn-3. Combination of FA and n-3 PUFA attenuated diabetes-induced hypermethylation of Pax3, overexpression and overactivity of Dnmt3b, abnormal expression of genes involved in one-carbon metabolism and elevation of homocysteine, and these improving effects were better than FA or n-3 PUFA alone. In conclusion, the combination of FA and n-3 PUFA has a synergistic effect on preventing maternal diabetes-induced NTD.

Tripeptide Leu-Pro-Phe from Corn Protein Hydrolysates Attenuates Hyperglycemia-Induced Neural Tube Defect in Chicken Embryos

Neural tube defect (NTD) is the most common and severe embryopathy causing embryonic malformation and even death associated with gestational diabetes mellitus (GDM). Leu-Pro-Phe (LPF) is an antioxidative tripeptide isolated from hydrolysates of corn protein. However, the biological activity of LPF in vivo and in vitro remains unclear. This study is aimed at investigating the protective effects of tripeptide LPF against NTD in the high glucose exposure condition and delineate the underlying biological mechanism. We found that LPF alleviated NTD in the high glucose-exposed chicken embryo model. In addition, DF-1 chicken embryo fibroblast was loaded with high glucose for induction of oxidative stress and abnormal O-GlcNAcylation in vitro. LPF significantly decreased accumulation of reactive oxygen species and content of malondialdehyde in DF-1 cells but increased the ratio of reduced glutathione and oxidized glutathione in chick embryo. Oxygen radical absorbance capacity results showed that LPF itself had good free radical scavenging capacity and could enhance antioxidant activity of the cell content. Mechanistic studies suggested that the resistance of LPF to oxidative damage may be related to promotion of NRF2 expression and nuclear translocation. LPF alleviated the overall O-GlcNAcylation level of cellular proteins under high glucose conditions and restored the level of Pax3 protein. Collectively, our findings indicate that LPF peptide could act as a nutritional supplement for the protection of development of embryonic neural tube affected by GDM.

PAX Genes in Cardiovascular Development

The mammalian heart is a four-chambered organ with systemic and pulmonary circulations to deliver oxygenated blood to the body, and a tightly regulated genetic network exists to shape normal development of the heart and its associated major arteries. A key process during cardiovascular morphogenesis is the septation of the outflow tract which initially forms as a single vessel before separating into the aorta and pulmonary trunk. The outflow tract connects to the aortic arch arteries which are derived from the pharyngeal arch arteries. Congenital heart defects are a major cause of death and morbidity and are frequently associated with a failure to deliver oxygenated blood to the body. The Pax transcription factor family is characterised through their highly conserved paired box and DNA binding domains and are crucial in organogenesis, regulating the development of a wide range of cells, organs and tissues including the cardiovascular system. Studies altering the expression of these genes in murine models, notably Pax3 and Pax9, have found a range of cardiovascular patterning abnormalities such as interruption of the aortic arch and common arterial trunk. This suggests that these Pax genes play a crucial role in the regulatory networks governing cardiovascular development.

Functional cargos of exosomes derived from Flk-1+ vascular progenitors enable neurulation and ameliorate embryonic anomalies in diabetic pregnancy

Various types of progenitors initiate individual organ formation and their crosstalk orchestrates morphogenesis for the entire embryo. Here we show that progenitor exosomal communication across embryonic organs occurs in normal development and is altered in embryos of diabetic pregnancy. Endoderm fibroblast growth factor 2 (FGF2) stimulates mesoderm Flk-1+ vascular progenitors to produce exosomes containing the anti-stress protein Survivin. These exosomes act on neural stem cells of the neuroepithelium to facilitate neurulation by inhibiting cellular stress and apoptosis. Maternal diabetes causes Flk-1+ progenitor dysfunction by suppressing FGF2 through DNA hypermethylation. Restoring endoderm FGF2 prevents diabetes-induced survivin reduction in Flk-1+ progenitor exosomes. Transgenic Survivin expression in Flk-1+ progenitors or in utero delivery of survivin-enriched exosomes restores cellular homeostasis and prevents diabetes-induced neural tube defects (NTDs), whereas inhibiting exosome production induces NTDs. Thus, functional inter-organ communication via Flk-1 exosomes is vital for neurulation and its disruption leads to embryonic anomalies.

Exploring epigenomic mechanisms of neural tube defects using multi-omics methods and data

Neural tube defects (NTDs) are a heterogeneous set of malformations attributed to disruption in normal neural tube closure during early embryogenesis. An in-depth understanding of NTD etiology and mechanisms remains elusive, however. Among the proposed mechanisms, epigenetic changes are thought to play an important role in the formation of NTDs. Epigenomics covers a wide spectrum of genomic DNA sequence modifications that can be investigated via high-throughput techniques. Recent advances in epigenomic technologies have enabled epigenetic studies of congenital malformations and facilitated the integration of big data into the understanding of NTDs. Herein, we review clinical epigenomic data that focuses on DNA methylation, histone modification, and miRNA alterations in human neural tissues, placental tissues, and leukocytes to explore potential mechanisms by which candidate genes affect human NTD pathogenesis. We discuss the links between epigenomics and gene regulatory mechanisms, and the effects of epigenetic alterations in human tissues on neural tube closure.

The role of Lrp6-mediated Wnt/β-catenin signaling in the development and intervention of spinal neural tube defects in mice

Neural tube defects (NTDs) are among the common and severe birth defects with poorly understood etiology. Mutations in the Wnt co-receptor LRP6 are associated with NTDs in humans. Either gain-of-function (GOF) or loss-of-function (LOF) mutations of Lrp6 can cause NTDs in mice. NTDs in Lrp6-GOF mutants may be attributed to altered β-catenin-independent noncanonical Wnt signaling. However, the mechanisms underlying NTDs in Lrp6-LOF mutants and the role of Lrp6-mediated canonical Wnt/β-catenin signaling in neural tube closure remain unresolved. We previously demonstrated that β-catenin signaling is required for posterior neuropore (PNP) closure. In the current study, conditional ablation of Lrp6 in dorsal PNP caused spinal NTDs with diminished activities of Wnt/β-catenin signaling and its downstream target gene Pax3, which is required for PNP closure. β-catenin-GOF rescued NTDs in Lrp6-LOF mutants. Moreover, maternal supplementation of a Wnt/β-catenin signaling agonist reduced the frequency and severity of spinal NTDs in Lrp6-LOF mutants by restoring Pax3 expression. Together, these results demonstrate the essential role of Lrp6-mediated Wnt/β-catenin signaling in PNP closure, which could also provide a therapeutic target for NTD intervention through manipulation of canonical Wnt/β-catenin signaling activities.

Summary: Conditional ablation of Lrp6 in dorsal neural folds causes spinal neural tube defects that can be rescued by genetic activation of β-catenin or maternal supplementation of Wnt signaling agonists.

Pax3 Hypomorphs Reveal Hidden Pax7 Functional Genetic Compensation in Utero

Pax3 and Pax7 transcription factors are paralogs within the Pax gene family that that are expressed in early embryos in partially overlapping expression domains and have distinct functions. Significantly, mammalian development is largely unaffected by Pax7 systemic deletion but systemic Pax3 deletion results in defects in neural tube closure, neural crest emigration, cardiac outflow tract septation, muscle hypoplasia and in utero lethality by E14. However, we previously demonstrated that Pax3 hypomorphs expressing only 20% functional Pax3 protein levels exhibit normal neural tube and heart development, but myogenesis is selectively impaired. To determine why only some Pax3-expressing cell lineages are affected and to further titrate Pax3 threshold levels required for neural tube and heart development, we generated hypomorphs containing both a hypomorphic and a null Pax3 allele. This resulted in mutants only expressing 10% functional Pax3 protein with exacerbated neural tube, neural crest and muscle defects, but still a normal heart. To examine why the cardiac neural crest appears resistant to very low Pax3 levels, we examined its paralog Pax7. Significantly, Pax7 expression is both ectopically expressed in Pax3-expressing dorsal neural tube cells and is also upregulated in the Pax3-expressing lineages. To test whether this compensatory Pax7 expression is functional, we deleted Pax7 both systemically and lineage-specifically in hypomorphs expressing only 10% Pax3. Removal of one Pax7 allele resulted in partial outflow tract defects, and complete loss of Pax7 resulted in full penetrance outflow tract defects and in utero lethality. Moreover, combinatorial loss of Pax3 and Pax7 resulted in severe craniofacial defects and a total block of neural crest cell emigration from the neural tube. Pax7^Cre lineage mapping revealed ectopic labeling of Pax3-derived neural crest tissues and within the outflow tract of the heart, experimentally confirming the observation of ectopic activation of Pax7 in 10% Pax3 hypomorphs. Finally, genetic cell ablation of Pax7^Cre-marked cells is sufficient to cause outflow tract defects in hypomorphs expressing only 10% Pax3, confirming that ectopic and induced Pax7 can play an overlapping functional genetic compensational role in both cardiac neural crest lineage and during craniofacial development, which is normally masked by the dominant role of Pax3.

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.

N-3 polyunsaturated fatty acids effectively protect against neural tube defects in diabetic mice induced by streptozotocin

Folate cannot prevent all neural tube defects (NTD), indicating that other pathogeneses still exist except for the folate deficiency. Maternal diabetes mellitus during pregnancy can increase the risk of offspring NTD. Our previous study showed that polyunsaturated fatty acids (PUFA) were lower in the placenta of human NTD cases than in healthy controls, and the supplementation of fish oil (rich in long-chain (LC) n-3 PUFA, mainly C20:5n-3 and C22:6n-3) had a better prevention effect against sodium valproate induced NTD than corn oil (rich in C18:2n-6) and flaxseed oil (rich in C18:3n-3). The aim of the present study was to investigate whether PUFA could prevent diabetes-induced NTD in mice. Streptozotocin (STZ)-induced diabetic pregnant mice were fed with a normal diet (DMC), a diet containing a low dose of fish oil (DMLn-3), a diet containing a high dose of fish oil (DMHn-3) or a diet rich in corn oil (DMn-6). Healthy pregnant mice were fed with a normal diet (HC). Compared with the DMC group, the rate of NTD was significantly lower in the DMHn-3 group (4.44% vs. 12.50%), but not in the DMLn-3 (11.11%) or DMn-6 group (12.03%). The NTD rate in the DMHn-3 group was comparable with that in the HC group (1.33%) (p = 0.246), and lower than that in the DMn-6 group (p = 0.052). The NTD rate in DMLn-3 and DMn-6 groups was significantly higher than that in the HC group. No significant difference was observed in NTD rate between DMLn-3 and DMHn-3 groups, and between DMLn-3 and DMn-6 groups. Compared with the HC group, the DMC group had a significantly lower C22:6n-3 in both serum and embryos. Fish oil supplementation ameliorated neuroepithelial cell apoptosis, and the apoptotic rate was comparable between DMHn-3 and HC groups. Although the apoptotic rate was significantly lower in the DMn-6 group than the DMC group, it was still much higher than that in the HC group. The proteins P53 and Bax in embryos were higher, while the proteins Bcl-2 and Pax3 were lower in the DMC group than in the HC group. The disturbance of Pax3, P53 and Bax induced by diabetes was abolished in DMLn-3, DMHn-3 and DMn-6 groups. Importantly, Bcl-2 in embryos was restored to the normal level only in the DMHn-3 group but not in the DMLn-3 or DMn-6 group. In conclusion, LC n-3 PUFA enriched fish oil has a protective effect against NTD in diabetes induced by STZ through improving neuroepithelial cell apoptosis, and the mechanism may be by increasing the anti-apoptosis protein Bcl-2 independently of Pax3 and P53.

Diabetic Embryopathy Susceptibility in Mice Is Associated with Differential Dependence on Glucosamine and Modulation of High Glucose-Induced Oxidative Stress

The high K_M glucose transporter, GLUT2 (SLC2A2), is expressed by embryos and causes high rates of glucose transport during maternal hyperglycemic episodes in diabetic pregnancies and causes congenital malformations (diabetic embryopathy). GLUT2 is also a low K_M transporter of the amino sugar, glucosamine (GlcN), which enters the hexosamine biosynthetic pathway (HBP) and provides substrate for glycosylation reactions. Exogenous GlcN also increases activity of the pentose phosphate pathway (PPP), which increases production of NADPH reducing equivalents. GLUT2-transported GlcN is inhibited by high glucose concentrations. Not all mouse strains are susceptible to diabetic embryopathy. The aim of this study was to test the hypothesis that susceptibility to diabetic embryopathy is related to differential dependence on exogenous GlcN for glycosylation or stimulation of the PPP. We tested this using murine embryonic stem cell (ESC) lines that were derived from embryopathy-susceptible FVB/NJ (FVB), and embryopathy-resistant C57Bl/6J (B6), embryos in the presence of low or high glucose, and in the presence or absence of GlcN. There were no significant differences in Glut2 expression, or of glucose or GlcN transport, between FVB and B6 ESC. GlcN effects on growth and incorporation into glycoproteins indicated that FVB ESC are more dependent on exogenous GlcN than are B6 ESC. GlcN stimulated PPP activity in FVB but not in B6 ESC. High glucose induced oxidative stress in FVB ESC but not in B6 ESC. These results indicate that FVB embryos are more dependent on exogenous GlcN for glycosylation, but also for stimulation of the PPP and NADPH production, than are B6 embryos, thereby rendering FVB embryos more susceptible to high glucose to induce oxidative stress.

Diabetes, Oxidative Stress, and DNA Damage Modulate Cranial Neural Crest Cell Development and the Phenotype Variability of Craniofacial Disorders

Craniofacial malformations are among the most common birth defects in humans and they often have significant detrimental functional, aesthetic, and social consequences. To date, more than 700 distinct craniofacial disorders have been described. However, the genetic, environmental, and developmental origins of most of these conditions remain to be determined. This gap in our knowledge is hampered in part by the tremendous phenotypic diversity evident in craniofacial syndromes but is also due to our limited understanding of the signals and mechanisms governing normal craniofacial development and variation. The principles of Mendelian inheritance have uncovered the etiology of relatively few complex craniofacial traits and consequently, the variability of craniofacial syndromes and phenotypes both within families and between families is often attributed to variable gene expression and incomplete penetrance. However, it is becoming increasingly apparent that phenotypic variation is often the result of combinatorial genetic and non-genetic factors. Major non-genetic factors include environmental effectors such as pregestational maternal diabetes, which is well-known to increase the risk of craniofacial birth defects. The hyperglycemia characteristic of diabetes causes oxidative stress which in turn can result in genotoxic stress, DNA damage, metabolic alterations, and subsequently perturbed embryogenesis. In this review we explore the importance of gene-environment associations involving diabetes, oxidative stress, and DNA damage during cranial neural crest cell development, which may underpin the phenotypic variability observed in specific craniofacial syndromes.

Mutation in Eftud2 causes craniofacial defects in mice via mis-splicing of Mdm2 and increased P53

EFTUD2 is mutated in patients with mandibulofacial dysostosis with microcephaly (MFDM). We generated a mutant mouse line with conditional mutation in Eftud2 and used Wnt1-Cre2 to delete it in neural crest cells. Homozygous deletion of Eftud2 causes brain and craniofacial malformations, affecting the same precursors as in MFDM patients. RNAseq analysis of embryonic heads revealed a significant increase in exon skipping and increased levels of an alternatively spliced Mdm2 transcript lacking exon 3. Exon skipping in Mdm2 was also increased in O9-1 mouse neural crest cells after siRNA knock-down of Eftud2 and in MFDM patient cells. Moreover, we found increased nuclear P53, higher expression of P53-target genes and increased cell death. Finally, overactivation of the P53 pathway in Eftud2 knockdown cells was attenuated by overexpression of non-spliced Mdm2, and craniofacial development was improved when Eftud2-mutant embryos were treated with Pifithrin-α, an inhibitor of P53. Thus, our work indicates that the P53-pathway can be targeted to prevent craniofacial abnormalities and shows a previously unknown role for alternative splicing of Mdm2 in the etiology of MFDM.

Progenitor death drives retinal dysplasia and neuronal degeneration in a mouse model of ATRIP-Seckel syndrome

Seckel syndrome is a type of microcephalic primordial dwarfism (MPD) that is characterized by growth retardation and neurodevelopmental defects, including reports of retinopathy. Mutations in key mediators of the replication stress response, the mutually dependent partners ATR and ATRIP, are among the known causes of Seckel syndrome. However, it remains unclear how their deficiency disrupts the development and function of the central nervous system (CNS). Here, we investigated the cellular and molecular consequences of ATRIP deficiency in different cell populations of the developing murine neural retina. We discovered that conditional inactivation of Atrip in photoreceptor neurons did not affect their survival or function. In contrast, Atrip deficiency in retinal progenitor cells (RPCs) led to severe lamination defects followed by secondary photoreceptor degeneration and loss of vision. Furthermore, we showed that RPCs lacking functional ATRIP exhibited higher levels of replicative stress and accumulated endogenous DNA damage that was accompanied by stabilization of TRP53. Notably, inactivation of Trp53 prevented apoptosis of Atrip-deficient progenitor cells and was sufficient to rescue retinal dysplasia, neurodegeneration and loss of vision. Together, these results reveal an essential role of ATRIP-mediated replication stress response in CNS development and suggest that the TRP53-mediated apoptosis of progenitor cells might contribute to retinal malformations in Seckel syndrome and other MPD disorders.This article has an associated First Person interview with the first author of the paper.

Mechanisms of Congenital Malformations in Pregnancies with Pre-existing Diabetes

PURPOSE OF REVIEW

Fetuses of diabetic mothers are at increased risk for congenital malformations. Research in recent decades using animal and embryonic stem cell models has revealed many embryonic developmental processes that are disturbed by maternal diabetes. The aim of this review is to give clinicians a better understanding of the reasons for rigorous glycemic control in early pregnancy, and to provide background to guide future research.

RECENT FINDINGS

Mouse models of diabetic pregnancy have revealed mechanisms for altered expression of tissue-specific genes that lead to malformations that are more common in diabetic pregnancies, such as neural tube defects (NTDs) and congenital heart defects (CHDs), and how altered gene expression causes apoptosis that leads to malformations. Embryos express the glucose transporter, GLUT2, which confers susceptibility to malformation, due to high rates of glucose uptake during maternal hyperglycemia and subsequent oxidative stress; however, the teleological function of GLUT2 for mammalian embryos may be to transport the amino sugar glucosamine (GlcN) from maternal circulation to be used as substrate for glycosylation reactions and to promote embryo cell growth. Malformations in diabetic pregnancy may be not only due to excess glucose uptake but also due to insufficient GlcN uptake. Avoiding maternal hyperglycemia during early pregnancy should prevent excess glucose uptake via GLUT2 into embryo cells, and also permit sufficient GLUT2-mediated GlcN uptake.

A Glance of p53 Functions in Brain Development, Neural Stem Cells, and Brain Cancer

p53 is one of the most intensively studied tumor suppressors. It transcriptionally regulates a broad range of genes to modulate a series of cellular events, including DNA damage repair, cell cycle arrest, senescence, apoptosis, ferroptosis, autophagy, and metabolic remodeling, which are fundamental for both development and cancer. This review discusses the role of p53 in brain development, neural stem cell regulation and the mechanisms of inactivating p53 in gliomas. p53 null or p53 mutant mice show female biased exencephaly, potentially due to X chromosome inactivation failure and/or hormone-related gene expression. Oxidative cellular status, increased PI3K/Akt signaling, elevated ID1, and metabolism are all implicated in p53-loss induced neurogenesis. However, p53 has also been shown to promote neuronal differentiation. In addition, p53 mutations are frequently identified in brain tumors, especially glioblastomas. Mechanisms underlying p53 inactivation in brain tumor cells include disruption of p53 protein stability, gene expression and transactivation potential as well as p53 gene loss or mutation. Loss of p53 function and gain-of-function of mutant p53 are both implicated in brain development and tumor genesis. Further understanding of the role of p53 in the brain may provide therapeutic insights for brain developmental syndromes and cancer.

Spliceosomopathies and neurocristopathies: Two sides of the same coin?

Mutations in core components of the spliceosome are responsible for a group of syndromes collectively known as spliceosomopathies. Patients exhibit microcephaly, micrognathia, malar hypoplasia, external ear anomalies, eye anomalies, psychomotor delay, intellectual disability, limb, and heart defects. Craniofacial malformations in these patients are predominantly found in neural crest cells-derived structures of the face and head. Mutations in eight genes SNRPB, RNU4ATAC, SF3B4, PUF60, EFTUD2, TXNL4, EIF4A3, and CWC27 are associated with craniofacial spliceosomopathies. In this review, we provide a brief description of the normal development of the head and the face and an overview of mutations identified in genes associated with craniofacial spliceosomopathies. We also describe a model to explain how and when these mutations are most likely to impact neural crest cells. We speculate that mutations in a subset of core splicing factors lead to disrupted splicing in neural crest cells because these cells have increased sensitivity to inefficient splicing. Hence, disruption in splicing likely activates a cellular stress response that includes increased skipping of regulatory exons in genes such as MDM2 and MDM4, key regulators of P53. This would result in P53-associated death of neural crest cells and consequently craniofacial malformations associated with spliceosomopathies.

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.

TP53 in bone and soft tissue sarcomas

Genomic and functional study of existing and emerging sarcoma targets, such as fusion proteins, chromosomal aberrations, reduced tumor suppressor activity, and oncogenic drivers, is broadening our understanding of sarcomagenesis. Among these mechanisms, the tumor suppressor p53 (TP53) plays significant roles in the suppression of bone and soft tissue sarcoma progression. Although mutations in TP53 were thought to be relatively low in sarcomas, modern techniques including whole-genome sequencing have recently illuminated unappreciated alterations in TP53 in osteosarcoma. In addition, oncogenic gain-of-function activities of missense mutant p53 (mutp53) have been reported in sarcomas. Moreover, new targeting strategies for TP53 have been discovered: restoration of wild-type p53 (wtp53) activity through inhibition of TP53 negative regulators, reactivation of the wtp53 activity from mutp53, depletion of mutp53, and targeting of vulnerabilities in cells with TP53 deletions or mutations. These discoveries enable development of novel therapeutic strategies for therapy-resistant sarcomas. We have outlined nine bone and soft tissue sarcomas for which TP53 plays a crucial tumor suppressive role. These include osteosarcoma, Ewing sarcoma, chondrosarcoma, rhabdomyosarcoma (RMS), leiomyosarcoma (LMS), synovial sarcoma, liposarcoma (LPS), angiosarcoma, and undifferentiated pleomorphic sarcoma (UPS).

Lineage Tracing Reveals a Subset of Reserve Muscle Stem Cells Capable of Clonal Expansion under Stress

Stem cell heterogeneity is recognized as functionally relevant for tissue homeostasis and repair. The identity, context dependence, and regulation of skeletal muscle satellite cell (SC) subsets remains poorly understood. We identify a minor subset of Pax7⁺ SCs that is indelibly marked by an inducible Mx1-Cre transgene in vivo, is enriched for Pax3 expression, and has reduced ROS (reactive oxygen species) levels. Mx1⁺ SCs possess potent stem cell activity upon transplantation but minimally contribute to endogenous muscle repair, due to their relative low abundance. In contrast, a dramatic clonal expansion of Mx1⁺ SCs allows extensive contribution to muscle repair and niche repopulation upon selective pressure of radiation stress, consistent with reserve stem cell (RSC) properties. Loss of Pax3 in RSCs increased ROS content and diminished survival and stress tolerance. These observations demonstrate that the Pax7⁺ SC pool contains a discrete population of radiotolerant RSCs that undergo clonal expansion under severe stress.

The role of p53 in developmental syndromes

While it is well appreciated that loss of the p53 tumor suppressor protein promotes cancer, growing evidence indicates that increased p53 activity underlies the developmental defects in a wide range of genetic syndromes. The inherited or de novo mutations that cause these syndromes affect diverse cellular processes, such as ribosome biogenesis, DNA repair, and centriole duplication, and analysis of human patient samples and mouse models demonstrates that disrupting these cellular processes can activate the p53 pathway. Importantly, many of the developmental defects in mouse models of these syndromes can be rescued by loss of p53, indicating that inappropriate p53 activation directly contributes to their pathogenesis. A role for p53 in driving developmental defects is further supported by the observation that mouse strains with broad p53 hyperactivation, due to mutations affecting p53 pathway components, display a host of tissue-specific developmental defects, including hematopoietic, neuronal, craniofacial, cardiovascular, and pigmentation defects. Furthermore, germline activating mutations in TP53 were recently identified in two human patients exhibiting bone marrow failure and other developmental defects. Studies in mice suggest that p53 drives developmental defects by inducing apoptosis, restraining proliferation, or modulating other developmental programs in a cell type-dependent manner. Here, we review the growing body of evidence from mouse models that implicates p53 as a driver of tissue-specific developmental defects in diverse genetic syndromes.

Deoxyuracil in DNA and disease: Genomic signal or managed situation?

Genomic instability is implicated in the etiology of several deleterious health outcomes including megaloblastic anemia, neural tube defects, and neurodegeneration. Uracil misincorporation and its repair are known to cause genomic instability by inducing DNA strand breaks leading to apoptosis, but there is emerging evidence that uracil incorporation may also result in broader modifications of gene expression, including: changes in transcriptional stalling, strand break-mediated transcriptional upregulation, and direct promoter inhibition. The factors that influence uracil levels in DNA are cytosine deamination, de novo thymidylate (dTMP) biosynthesis, salvage dTMP biosynthesis, dUTPase, and DNA repair. There is evidence that the nuclear localization of the enzymes in these pathways in mammalian cells may modify and/or control the levels of uracil accumulation into nuclear DNA. Uracil sequencing technologies demonstrate that uracil in DNA is not distributed stochastically across the genome, but instead shows patterns of enrichment. Nuclear localization of the enzymes that modify uracil in DNA may serve to change these patterns of enrichment in a tissue-specific manner, and thereby signal the genome in response to metabolic and/or nutritional state of the cell.

High-throughput gene expression analysis identifies p53-dependent and -independent pathways contributing to the adrenocortical dysplasia (acd) phenotype

In mammalian cells TPP1, encoded by the Acd gene, is a key component of the shelterin complex, which is required for telomere length maintenance and telomere protection. In mice, a hypomorphic mutation in Acd causes the adrenocortical dysplasia (acd) phenotype, which includes limb and body axis anomalies, and perinatal lethality. p53 deficiency partially rescues limb and body axis anomalies in acd mutant embryos, but not perinatal lethality, implicating p53-independent mechanisms in the acd phenotype. Loss of function of most shelterin proteins results in early embryonic lethality. Thus, study of the hypomorphic acd allele provides a unique opportunity to understand telomere dysfunction at an organismal level. The aim of this study was to identify transcriptome alterations in acd mutant and acd, p53 double mutant embryos to understand the p53-dependent and -independent factors that contribute to the mutant phenotypes in the context of the whole organism. Genes involved in developmental processes, cell cycle, metabolic pathways, tight junctions, axon guidance and signaling pathways were regulated by p53-driven mechanisms in acd mutant embryos, while genes functioning in immune response, and RNA processing were altered independently of p53 in acd, p53 double mutant embryos. To our best of knowledge, this is the first study revealing detailed transcriptomic alterations, reflecting novel p53-dependent and -independent pathways contributing to the acd phenotype. Our data confirm the importance of cell cycle and DNA repair pathways, and suggest novel links between telomere dysfunction and immune system regulation and the splicing machinery. Given the broad applicability of telomere maintenance in growth, development, and genome stability, our data will also provide a rich resource for others studying telomere maintenance and DNA damage responses in mammalian model systems.

PAX3: A Molecule with Oncogenic or Tumor Suppressor Function Is Involved in Cancer

Metastasis is the most deadly aspect of cancer and results from acquired gene regulation abnormalities in tumor cells. Transcriptional regulation is an essential component of controlling of gene function and its failure could contribute to tumor progression and metastasis. During cancer progression, deregulation of oncogenic or tumor suppressive transcription factors, as well as master cell fate regulators, collectively influences multiple steps of the metastasis cascade, including local invasion and dissemination of the tumor to distant organs. Transcription factor PAX3/Pax3, which contributes to diverse cell lineages during embryonic development, plays a major role in tumorigenesis. Mutations in this gene can cause neurodevelopmental disease and the existing literature supports that there is a potential link between aberrant expression of PAX3 genes in adult tissues and a wide variety of cancers. PAX3 function is tissue-specific and could contribute to tumorigenesis either directly as oncogene or as a tumor suppressor by losing its function. In this review, we discuss comprehensively the differential role played by PAX3 in various tissues and how its aberrant expression is implicated in disease development. This review particularly highlights the oncogenic and tumor suppressor role played by PAX3 in different cancers and underlines the importance of precisely identifying tissue-specific role of PAX3 in order to determine its exact role in development of cancer.

The expression and function of PAX3 in development and disease

The PAX3 gene encodes a member of the PAX family of transcription factors that is characterized by a highly conserved paired box motif. The PAX3 protein is a transcription factor consisting of an N-terminal DNA binding domain (containing a paired box and homeodomain) and a C-terminal transcriptional activation domain. This protein is expressed during development of skeletal muscle, central nervous system and neural crest derivatives, and regulates expression of target genes that impact on proliferation, survival, differentiation and motility in these lineages. Germline mutations of the murine Pax3 and human PAX3 genes cause deficiencies in these developmental lineages and result in the Splotch phenotype and Waardenburg syndrome, respectively. Somatic genetic rearrangements that juxtapose the PAX3 DNA binding domain to the transcriptional activation domain of other transcription factors deregulate PAX3 function and contribute to the pathogenesis of the soft tissue cancers alveolar rhabdomyosarcoma and biphenotypic sinonasal sarcoma. The wild-type PAX3 protein is also expressed in other cancers related to developmental lineages that normally express this protein and exerts phenotypic effects related to its normal developmental role.

Oxidative Stress and Apoptosis in Benzo[a]pyrene-Induced Neural Tube Defects

Neural tube defects (NTDs) are among the most common and severe congenital malformations and result from incomplete closure of the neural tube during early development. Maternal exposure to polycyclic aromatic hydrocarbons (PAHs) has been suggested to be a risk factor for NTDs and previous studies imply that the mechanism underlying the association between PAH exposure and NTDs may involve oxidative stress and apoptosis. The objectives of this study were to investigate whether there is a direct effect of maternal benzo[α] pyrene (BaP) exposure on the closure of the neural tube in mice, and to examine the underlying mechanisms by combining animal experiments and human subject studies. We found that intraperitoneal injection of BaP from embryonic day 7 at a dose of 250 mg kg^-1 induced NTDs (13.3% frequency) in ICR mice. BaP exposure significantly increased expression of genes associated with oxidative stress, Cyp1a1, Sod1 and Sod2, while repressing Gpx1. Elevated apoptosis and higher protein expression of cleaved caspase-3 in the neuroepithelium of treated embryos were observed. Pre-treatment with vitamin E, added to food, significantly protected against BaP-induced NTDs (1.4% frequency) (P < 0.05). Vitamin E also partly normalized oxidative stress related gene expression and excess apoptosis in BaP-treated embryos. Examination of human neural tissues revealed that increased levels of protein carbonyl and apoptosis were related with maternal exposure to PAHs and the risk of NTDs. Collectively, these results suggest that BaP exposure could induce NTDs and that this may involve increased oxidative stress and apoptosis, while vitamin E may have a protective effect.

Autophagy-dependent crosstalk between GILT and PAX-3 influences radiation sensitivity of human melanoma cells

Melanoma represents an ever-increasing problem in the western world as incidence rates continue to climb. Though manageable during early stages, late stage metastatic disease is highly resistant to current intervention. We have previously shown that gamma-interferon-inducible lysosomal thiol-reductase (GILT) enhances HLA class II antigen processing and immune detection of human melanoma cells. Here we report that GILT expression inhibits a potential target, paired box-3 (PAX-3) protein, in late stage human metastatic melanoma. We also show that GILT transfection or induction by IFN-γ, decreases PAX-3 protein expression while upregulating the expression of Daxx, which is also a repressor of PAX-3. Confocal microscopic analysis demonstrated that GILT co-localizes with PAX-3 protein, but not with Daxx within melanoma cells. Immunoprecipitation and immunoblotting studies suggest that GILT expression negatively regulates PAX-3 through the autophagy pathway, potentially resulting in increased susceptibility to conventional treatment in the form of chemotherapy or radiotherapy. While high-dose radiation is a common treatment for melanoma patients, our data suggest that GILT expression significantly increased the susceptibility of melanoma cells to low-dose radiation therapy via upregulation of tumor suppressor protein p53. Overall, these data suggest that GILT has multiple roles in inducing human melanoma cells as better targets for radiation and immunotherapy.

Apoptosis, Expression of PAX3 and P53, and Caspase Signal in Fetuses with Neural Tube Defects

BACKGROUND

Neural tube defects (NTDs) are among the most common and severe congenital malformations of the central nervous system. Animal studies have shown that apoptosis is involved in the development of NTDs. However, little evidence is available from human studies. We aim to examine the level of apoptosis and expression of apoptosis-regulating proteins of human terminated fetuses.

METHODS

A total of 37 NTD cases and 21 controls from pregnancy terminations were recruited. Tissues of the central nervous system were obtained through autopsy. Apoptosis of neuroepithelial cells was examined by terminal deoxynucleotidyl transferase-mediated deoxyuridinetriphosphate nick end-labeling (TUNEL) assay. Expression of PAX3, p53, and caspase 3/8/9 in central nervous tissue was measured using Western blotting.

RESULTS

More TUNEL-positive apoptosis cells were observed in the central nervous tissue of NTD cases than those of controls (p < 0.05). In spinal cord tissue, lower PAX3 expression, higher p53 expression, and increased levels of cleaved caspase 3(17kD) and cleaved caspase 8 (18kD) were found in anencephaly cases but not in spina bifida cases when compared with controls. In brain tissue, levels of PAX3 were significantly reduced in both encephalocele and spina bifida subtypes; the expression levels of cleaved caspase 3(17 kD) of encephalocele cases and cleaved caspase 8(47/45 kD) in spina bifida cases were higher than in controls; no difference was found in the expression of p53 or caspase 9 between NTDs and controls.

CONCLUSION

These findings provide some evidence that excessive apoptosis in fetal central nervous tissues may be associated with the development of human NTDs. Birth Defects Research 109:1596-1604, 2017. © 2017 Wiley Periodicals, Inc.

An Examination of the Role of Transcriptional and Posttranscriptional Regulation in Rhabdomyosarcoma

Rhabdomyosarcoma (RMS) is an aggressive family of soft tissue tumors that most commonly manifests in children. RMS variants express several skeletal muscle markers, suggesting myogenic stem or progenitor cell origin of RMS. In this review, the roles of both recently identified and well-established microRNAs in RMS are discussed and summarized in a succinct, tabulated format. Additionally, the subtypes of RMS are reviewed along with the involvement of basic helix-loop-helix (bHLH) proteins, Pax proteins, and microRNAs in normal and pathologic myogenesis. Finally, the current and potential future treatment options for RMS are outlined.

Tcof1-Related Molecular Networks in Treacher Collins Syndrome

Treacher Collins syndrome (TCS) is a rare, autosomal-dominant disorder characterized by craniofacial deformities, and is primarily caused by mutations in the Tcof1 gene. This article was aimed to perform a comprehensive literature review and systematic bioinformatic analysis of Tcof1-related molecular networks in TCS. First, the up- and down-regulated genes in Tcof1 heterozygous haploinsufficient mutant mice embryos and Tcof1 knockdown and Tcof1 over-expressed neuroblastoma N1E-115 cells were obtained from the Gene Expression Omnibus database. The GeneDecks database was used to calculate the 500 genes most closely related to Tcof1. Then, the relationships between 4 gene sets (a predicted set and sets comparing the wildtype with the 3 Gene Expression Omnibus datasets) were analyzed using the DAVID, GeneMANIA and STRING databases. The analysis results showed that the Tcof1-related genes were enriched in various biological processes, including cell proliferation, apoptosis, cell cycle, differentiation, and migration. They were also enriched in several signaling pathways, such as the ribosome, p53, cell cycle, and WNT signaling pathways. Additionally, these genes clearly had direct or indirect interactions with Tcof1 and between each other. Literature review and bioinformatic analysis finds imply that special attention should be given to these pathways, as they may offer target points for TCS therapies.

A Novel Mouse Model of Diffuse Intrinsic Pontine Glioma Initiated in Pax3-Expressing Cells

Diffuse intrinsic pontine glioma (DIPG) is a rare and incurable brain tumor that arises predominately in children and involves the pons, a structure that along with the midbrain and medulla makes up the brainstem. We have previously developed genetically engineered mouse models of brainstem glioma using the RCAS/Tv-a system by targeting PDGF-B overexpression, p53 loss, and H3.3K27M mutation to Nestin-expressing brainstem progenitor cells of the neonatal mouse. Here we describe a novel mouse model targeting these same genetic alterations to Pax3-expressing cells, which in the neonatal mouse pons consist of a Pax3 +/Nestin +/Sox2 + population lining the fourth ventricle and a Pax3 +/NeuN + parenchymal population. Injection of RCAS-PDGF-B into the brainstem of Pax3-Tv-a mice at postnatal day 3 results in 40% of mice developing asymptomatic low-grade glioma. A mixture of low- and high-grade glioma results from injection of Pax3-Tv-a;p53^fl/fl mice with RCAS-PDGF-B and RCAS-Cre, with or without RCAS-H3.3K27M. These tumors are Ki67 +, Nestin +, Olig2 +, and largely GFAP − and can arise anywhere within the brainstem, including the classic DIPG location of the ventral pons. Expression of the H3.3K27M mutation reduces overall H3K27me3 as compared with tumors without the mutation, similar to what has been previously shown in human and mouse tumors. Thus, we have generated a novel genetically engineered mouse model of DIPG, which faithfully recapitulates the human disease and represents a novel platform with which to study the biology and treatment of this deadly disease.

Tissue-specific roles for sonic hedgehog signaling in establishing thymus and parathyroid organ fate

The thymus and parathyroids develop from third pharyngeal pouch (3rd pp) endoderm. Our previous studies show that Shh null mice have smaller, aparathyroid primordia in which thymus fate specification extends into the pharynx. SHH signaling is active in both dorsal pouch endoderm and neighboring neural crest (NC) mesenchyme. It is unclear which target tissue of SHH signaling is required for the patterning defects in Shh mutants. Here, we used a genetic approach to ectopically activate or delete the SHH signal transducer Smo in either pp endoderm or NC mesenchyme. Although no manipulation recapitulated the Shh null phenotype, manipulation of SHH signaling in either the endoderm or NC mesenchyme had direct and indirect effects on both cell types during fate specification and organogenesis. SHH pathway activation throughout pouch endoderm activated ectopic Tbx1 expression and partially suppressed the thymus-specific transcription factor Foxn1, identifying Tbx1 as a key target of SHH signaling in the 3rd pp. However, ectopic SHH signaling was insufficient to expand the GCM2-positive parathyroid domain, indicating that multiple inputs, some of which might be independent of SHH signaling, are required for parathyroid fate specification. These data support a model in which SHH signaling plays both positive and negative roles in patterning and organogenesis of the thymus and parathyroids.

The status of diabetic embryopathy

Diabetic embryopathy is a theoretical enigma and a clinical challenge. Both type 1 and type 2 diabetic pregnancy carry a significant risk for fetal maldevelopment, and the precise reasons for the diabetes-induced teratogenicity are not clearly identified. The experimental work in this field has revealed a partial, however complex, answer to the teratological question, and we will review some of the latest suggestions.

Melatonin prevents neural tube defects in the offspring of diabetic pregnancy

Melatonin, an endogenous neurohormone secreted by the pineal gland, has a variety of physiological functions and neuroprotective effects. However, its protective role on the neural tube defects (NTDs) was not very clear. The aim of this study was to investigate the effects of melatonin on the incidence of NTDs (including anencephaly, encephalocele, and spina bifida) of offspring from diabetic pregnant mice as well as its underlying mechanisms. Pregnant mice were given 10 mg/kg melatonin by daily i.p. injection from embryonic day (E) 0.5 until being killed on E11.5. Here, we showed that melatonin decreased the NTDs (especially exencephaly) rate of embryos exposed to maternal diabetes. Melatonin stimulated proliferation of neural stem cells (NSCs) under hyperglycemic condition through the extracellular regulated protein kinases (ERK) pathway. Furthermore, as a direct free radical scavenger, melatonin decreased apoptosis of NSCs exposed to hyperglycemia. In the light of these findings, it suggests that melatonin supplementation may play an important role in the prevention of neural malformations in diabetic pregnancy.

Functional Insights into Chromatin Remodelling from Studies on CHARGE Syndrome

CHARGE syndrome is a rare genetic syndrome characterised by a unique combination of multiple organ anomalies. Dominant loss-of-function mutations in the gene encoding chromodomain helicase DNA binding protein 7 (CHD7), which is an ATP-dependent chromatin remodeller, have been identified as the cause of CHARGE syndrome. Here, we review recent work aimed at understanding the mechanism of CHD7 function in normal and pathological states, highlighting results from biochemical and in vivo studies. The emerging picture from this work suggests that the mechanisms by which CHD7 fine-tunes gene expression are context specific, consistent with the pleiotropic nature of CHARGE syndrome.

Hyperglycemia impairs left-right axis formation and thereby disturbs heart morphogenesis in mouse embryos

Congenital heart defects with heterotaxia are associated with pregestational diabetes mellitus. To provide insight into the mechanisms underlying such diabetes-related heart defects, we examined the effects of high-glucose concentrations on formation of the left-right axis in mouse embryos. Expression of Pitx2, which plays a key role in left-right asymmetric morphogenesis and cardiac development, was lost in the left lateral plate mesoderm of embryos of diabetic dams. Embryos exposed to high-glucose concentrations in culture also failed to express Nodal and Pitx2 in the left lateral plate mesoderm. The distribution of phosphorylated Smad2 revealed that Nodal activity in the node was attenuated, accounting for the failure of left-right axis formation. Consistent with this notion, Notch signal-dependent expression of Nodal-related genes in the node was also down-regulated in association with a reduced level of Notch signaling, suggesting that high-glucose concentrations impede Notch signaling and thereby hinder establishment of the left-right axis required for heart morphogenesis.

Guilty as CHARGED: p53's expanding role in disease

Unrestrained p53 activity during development, as occurs upon loss of the p53 negative regulators Mdm2 or Mdmx, causes early embryonic lethality. Surprisingly, co-expression of wild-type p53 and a transcriptionally-dead variant of p53, with mutations in both transactivation domains (p53(L25Q,W26S,F53Q,F54S)), also causes lethality, but later in gestation and in association with a host of very specific phenotypes reminiscent of a syndrome known as CHARGE. Molecular analyses revealed that wild-type p53 is inappropriately activated in p53(5,26,53,54/)(+) embryos, triggering cell-cycle arrest or apoptosis during development to cause CHARGE phenotypes. In addition, CHARGE syndrome is typically caused by mutations in the CHD7 chromatin remodeler, and we have shown that activated p53 contributes to phenotypes caused by CHD7-deficiency. Together, these studies provide new insight into CHARGE syndrome and expand our understanding of the role of p53 in diseases other than cancer.

Oxidative stress, unfolded protein response, and apoptosis in developmental toxicity

Physiological development requires precise spatiotemporal regulation of cellular and molecular processes. Disruption of these key events can generate developmental toxicity in the form of teratogenesis or mortality. The mechanism behind many developmental toxicants remains unknown. While recent work has focused on the unfolded protein response (UPR), oxidative stress, and apoptosis in the pathogenesis of disease, few studies have addressed their relationship in developmental toxicity. Redox regulation, UPR, and apoptosis are essential for physiological development and can be disturbed by a variety of endogenous and exogenous toxicants to generate lethality and diverse malformations. This review examines the current knowledge of the role of oxidative stress, UPR, and apoptosis in physiological development as well as in developmental toxicity, focusing on studies and advances in vertebrates model systems.

Impact of retinoic acid exposure on midfacial shape variation and manifestation of holoprosencephaly in Twsg1 mutant mice

Holoprosencephaly (HPE) is a developmental anomaly characterized by inadequate or absent midline division of the embryonic forebrain and midline facial defects. It is believed that interactions between genes and the environment play a role in the widely variable penetrance and expressivity of HPE, although direct investigation of such effects has been limited. The goal of this study was to examine whether mice carrying a mutation in a gene encoding the bone morphogenetic protein (BMP) antagonist twisted gastrulation (Twsg1), which is associated with a low penetrance of HPE, are sensitized to retinoic acid (RA) teratogenesis. Pregnant Twsg1(+/-) dams were treated by gavage with a low dose of all-trans RA (3.75 mg/kg of body weight). Embryos were analyzed between embryonic day (E)9.5 and E11.5 by microscopy and geometric morphometric analysis by micro-computed tomography. P19 embryonal carcinoma cells were used to examine potential mechanisms mediating the combined effects of increased BMP and retinoid signaling. Although only 7% of wild-type embryos exposed to RA showed overt HPE or neural tube defects (NTDs), 100% of Twsg1(-/-) mutants exposed to RA manifested severe HPE compared to 17% without RA. Remarkably, up to 30% of Twsg1(+/-) mutants also showed HPE (23%) or NTDs (7%). The majority of shape variation among Twsg1(+/-) mutants was associated with narrowing of the midface. In P19 cells, RA induced the expression of Bmp2, acted in concert with BMP2 to increase p53 expression, caspase activation and oxidative stress. This study provides direct evidence for modifying effects of the environment in a genetic mouse model carrying a predisposing mutation for HPE in the Twsg1 gene. Further study of the mechanisms underlying these gene-environment interactions in vivo will contribute to better understanding of the pathogenesis of birth defects and present an opportunity to explore potential preventive interventions.

Pax3 expression enhances PDGF-B-induced brainstem gliomagenesis and characterizes a subset of brainstem glioma

High-grade Brainstem Glioma (BSG), also known as Diffuse Intrinsic Pontine Glioma (DIPG), is an incurable pediatric brain cancer. Increasing evidence supports the existence of regional differences in gliomagenesis such that BSG is considered a distinct disease from glioma of the cerebral cortex (CG). In an effort to elucidate unique characteristics of BSG, we conducted expression analysis of mouse PDGF-B-driven BSG and CG initiated in Nestin progenitor cells and identified a short list of expression changes specific to the brainstem gliomagenesis process, including abnormal upregulation of paired box 3 (Pax3). In the neonatal mouse brain, Pax3 expression marks a subset of brainstem progenitor cells, while it is absent from the cerebral cortex, mirroring its regional expression in glioma. Ectopic expression of Pax3 in normal brainstem progenitors in vitro shows that Pax3 inhibits apoptosis. Pax3-induced inhibition of apoptosis is p53-dependent, however, and in the absence of p53, Pax3 promotes proliferation of brainstem progenitors. In vivo, Pax3 enhances PDGF-B-driven gliomagenesis by shortening tumor latency and increasing tumor penetrance and grade, in a region-specific manner, while loss of Pax3 function extends survival of PDGF-B-driven;p53-deficient BSG-bearing mice by 33%. Importantly, Pax3 is regionally expressed in human glioma as well, with high PAX3 mRNA characterizing 40% of human BSG, revealing a subset of tumors that significantly associates with PDGFRA alterations, amplifications of cell cycle regulatory genes, and is exclusive of ACVR1 mutations. Collectively, these data suggest that regional Pax3 expression not only marks a novel subset of BSG but also contributes to PDGF-B-induced brainstem gliomagenesis.

Increased DNA methyltransferase 3b (Dnmt3b)-mediated CpG island methylation stimulated by oxidative stress inhibits expression of a gene required for neural tube and neural crest development in diabetic pregnancy

Previous studies have shown that diabetic embryopathy results from impaired expression of genes that are required for formation of embryonic structures. We have focused on Pax3, a gene that is expressed in embryonic neuroepithelium and is required for neural tube closure. Pax3 expression is inhibited in embryos of diabetic mice due to hyperglycemia-induced oxidative stress. DNA methylation silences developmentally expressed genes before differentiation. We hypothesized that hypomethylation of Pax3 upon neuroepithelial differentiation may be inhibited by hyperglycemia-induced oxidative stress. We tested this using embryos of pregnant hyperglycemic mice and mouse embryonic stem cells (ESC). Methylation of a Pax3 CpG island decreased upon neurulation of embryos and formation of neuronal precursors from ESC. In ESC, this was inhibited by oxidative stress. Use of short hairpin RNA in ESC demonstrated that DNA methyltransferase 3b (Dnmt3b) was responsible for methylation and silencing of Pax3 before differentiation and by oxidative stress. Although expression of Dnmt3b was not affected by oxidative stress, DNA methyltransferase activity was increased. These results indicate that hyperglycemia-induced oxidative stress stimulates Dnmt3b activity, thereby inhibiting chromatin modifications necessary for induction of Pax3 expression during neurulation and thus providing a molecular mechanism for defects caused by Pax3 insufficiency in diabetic pregnancy.

A multi-resource data integration approach: identification of candidate genes regulating cell proliferation during neocortical development

Neurons of the mammalian neocortex are produced by proliferating cells located in the ventricular zone (VZ) lining the lateral ventricles. This is a complex and sequential process, requiring precise control of cell cycle progression, fate commitment and differentiation. We have analyzed publicly available databases from mouse and human to identify candidate genes that are potentially involved in regulating early neocortical development and neurogenesis. We used a mouse in situ hybridization dataset (The Allen Institute for Brain Science) to identify 13 genes (Cdon, Celsr1, Dbi, E2f5, Eomes, Hmgn2, Neurog2, Notch1, Pcnt, Sox3, Ssrp1, Tead2, Tgif2) with high correlation of expression in the proliferating cells of the VZ of the neocortex at early stages of development (E15.5). We generated a similar human brain network using microarray and RNA-seq data (BrainSpan Atlas) and identified 407 genes with high expression in the developing human VZ and subventricular zone (SVZ) at 8–9 post-conception weeks. Seven of the human genes were also present in the mouse VZ network. The human and mouse networks were extended using available genetic and proteomic datasets through GeneMANIA. A gene ontology search of the mouse and human networks indicated that many of the genes are involved in the cell cycle, DNA replication, mitosis and transcriptional regulation. The reported involvement of Cdon, Celsr1, Dbi, Eomes, Neurog2, Notch1, Pcnt, Sox3, Tead2, and Tgif2 in neural development or diseases resulting from the disruption of neurogenesis validates these candidate genes. Taken together, our knowledge-based discovery method has validated the involvement of many genes already known to be involved in neocortical development and extended the potential number of genes by 100's, many of which are involved in functions related to cell proliferation but others of which are potential candidates for involvement in the regulation of neocortical development.

Use of a murine embryonic stem cell line that is sensitive to high glucose environment to model neural tube development in diabetic pregnancy

BACKGROUND

Neural tube defects (NTDs) are significantly increased by maternal diabetes. Embryonic stem cells (ESC) that can differentiate into neuroepithelium and can sense supraphysiological glucose concentrations would be very valuable to simulate the effects of maternal diabetes on molecular and cellular processes during neural tube formation.

METHODS

LG-ESC, a recently established ESC line that expresses the glucose transporter, Scl2a2, and is sensitive to elevated glucose concentrations, were grown for up to 8 days in a three-dimensional culture to form neural cysts. We tested whether high glucose media inhibits expression of Pax3, a gene that is required for neural tube closure and whose expression is inhibited in embryos of diabetic mice, and inhibits formation of neural cysts.

RESULTS

Pax3 expression was detected after 4 days of culture and increased with time. Pax3 expression was inhibited by high glucose media, but not if cells had been cultured in low glucose media for the first 4 days of culture. Pax7, which is also expressed in dorsal neural tube, was not detected. Pax6, which is expressed in the ventral neural tube, was detected only after 8 days of culture, but was not inhibited by high glucose. High glucose media did not inhibit formation of neural cysts.

CONCLUSION

LG-ESC can be used as a model of embryonic exposure to a diabetic environment during neural tube development. While high glucose exposure inhibits expression of a gene required for neural tube closure, it may not inhibit all of the processes involved in formation of a neural tube-like structure.

β-catenin regulates Pax3 and Cdx2 for caudal neural tube closure and elongation

Non-canonical Wnt/planar cell polarity (PCP) signaling plays a primary role in the convergent extension that drives neural tube closure and body axis elongation. PCP signaling gene mutations cause severe neural tube defects (NTDs). However, the role of canonical Wnt/β-catenin signaling in neural tube closure and NTDs remains poorly understood. This study shows that conditional gene targeting of β-catenin in the dorsal neural folds of mouse embryos represses the expression of the homeobox-containing genes Pax3 and Cdx2 at the dorsal posterior neuropore (PNP), and subsequently diminishes the expression of the Wnt/β-catenin signaling target genes T, Tbx6 and Fgf8 at the tail bud, leading to spina bifida aperta, caudal axis bending and tail truncation. We demonstrate that Pax3 and Cdx2 are novel downstream targets of Wnt/β-catenin signaling. Transgenic activation of Pax3 cDNA can rescue the closure defect in the β-catenin mutants, suggesting that Pax3 is a key downstream effector of β-catenin signaling in the PNP closure process. Cdx2 is known to be crucial in posterior axis elongation and in neural tube closure. We found that Cdx2 expression is also repressed in the dorsal PNPs of Pax3-null embryos. However, the ectopically activated Pax3 in the β-catenin mutants cannot restore Cdx2 mRNA in the dorsal PNP, suggesting that the presence of both β-catenin and Pax3 is required for regional Cdx2 expression. Thus, β-catenin signaling is required for caudal neural tube closure and elongation, acting through the transcriptional regulation of key target genes in the PNP.

Familial dysautonomia model reveals Ikbkap deletion causes apoptosis of Pax3+ progenitors and peripheral neurons

Familial dysautonomia (FD) is a devastating developmental and progressive peripheral neuropathy caused by a mutation in the gene inhibitor of kappa B kinase complex-associated protein (IKBKAP). To identify the cellular and molecular mechanisms that cause FD, we generated mice in which Ikbkap expression is ablated in the peripheral nervous system and identify the steps in peripheral nervous system development that are Ikbkap-dependent. We show that Ikbkap is not required for trunk neural crest migration or pathfinding, nor for the formation of dorsal root or sympathetic ganglia, or the adrenal medulla. Instead, Ikbkap is essential for the second wave of neurogenesis during which the majority of tropomyosin-related kinase A (TrkA(+)) nociceptors and thermoreceptors arise. In its absence, approximately half the normal complement of TrkA(+) neurons are lost, which we show is partly due to p53-mediated premature differentiation and death of mitotically-active progenitors that express the paired-box gene Pax3 and give rise to the majority of TrkA(+) neurons. By the end of sensory development, the number of TrkC neurons is significantly increased, which may result from an increase in Runx3(+) cells. Furthermore, our data demonstrate that TrkA(+) (but not TrkC(+)) sensory and sympathetic neurons undergo exacerbated Caspase 3-mediated programmed cell death in the absence of Ikbkap and that this death is not due to a reduction in nerve growth factor synthesis. In summary, these data suggest that FD does not result from a failure in trunk neural crest migration, but rather from a critical function for Ikbkap in TrkA progenitors and TrkA(+) neurons.

Diabetic embryopathy: a developmental perspective from fertilization to adulthood

Maternal diabetes mellitus is one of the strongest human teratogens. Despite recent advances in the fields of clinical embryology, experimental teratology and preventive medicine, diabetes-related perturbations of the maternofetal unit maintain a considerable impact on the Healthcare System. Classic consequences of prenatal exposure to hyperglycemia encompass (early) spontaneous abortions, perinatal death and malformations. The spectrum of related malformations comprises some recurrent blastogenic monotopic patterns, i.e. holoprosencephaly, caudal dysgenesis and oculoauriculovertebral spectrum, as well as pleiotropic syndromes, i.e. femoral hypoplasia-unusual face syndrome. Despite this, most malformed fetuses display multiple blastogenic defects of the VACTERL type, whose (apparently) casual combination preclude recognizing recurrent patterns, but accurately testifies to their developmental stage at onset. With the application of developmental biology in modern medicine, the effects of diabetes on the unborn patient are expanded to include the predisposition to develop insulin resistance in adulthood. The mechanisms underlying the transgenerational correlation between maternal diabetes and proneness to adult disorders in the offspring remain unclear, and the epigenetic plasticity may represent the missing link. In this scenario, a development-driven summary of the multifaced consequences of maternal diabetes on fertility and child health may add a practical resource to the repertoire of available information on early stages of embryogenesis.