Grainyhead genes and mammalian neural tube closure

Inositols: From Established Knowledge to Novel Approaches

Myo-inositol (myo-Ins) and D-chiro-inositol (D-chiro-Ins) are natural compounds involved in many biological pathways. Since the discovery of their involvement in endocrine signal transduction, myo-Ins and D-chiro-Ins supplementation has contributed to clinical approaches in ameliorating many gynecological and endocrinological diseases. Currently both myo-Ins and D-chiro-Ins are well-tolerated, effective alternative candidates to the classical insulin sensitizers, and are useful treatments in preventing and treating metabolic and reproductive disorders such as polycystic ovary syndrome (PCOS), gestational diabetes mellitus (GDM), and male fertility disturbances, like sperm abnormalities. Moreover, besides metabolic activity, myo-Ins and D-chiro-Ins deeply influence steroidogenesis, regulating the pools of androgens and estrogens, likely in opposite ways. Given the complexity of inositol-related mechanisms of action, many of their beneficial effects are still under scrutiny. Therefore, continuing research aims to discover new emerging roles and mechanisms that can allow clinicians to tailor inositol therapy and to use it in other medical areas, hitherto unexplored. The present paper outlines the established evidence on inositols and updates on recent research, namely concerning D-chiro-Ins involvement into steroidogenesis. In particular, D-chiro-Ins mediates insulin-induced testosterone biosynthesis from ovarian thecal cells and directly affects synthesis of estrogens by modulating the expression of the aromatase enzyme. Ovaries, as well as other organs and tissues, are characterized by a specific ratio of myo-Ins to D-chiro-Ins, which ensures their healthy state and proper functionality. Altered inositol ratios may account for pathological conditions, causing an imbalance in sex hormones. Such situations usually occur in association with medical conditions, such as PCOS, or as a consequence of some pharmacological treatments. Based on the physiological role of inositols and the pathological implications of altered myo-Ins to D-chiro-Ins ratios, inositol therapy may be designed with two different aims: (1) restoring the inositol physiological ratio; (2) altering the ratio in a controlled way to achieve specific effects.

Loss of Grhl3 is correlated with altered cellular protrusions in the non-neural ectoderm during neural tube closure

BACKGROUND

The transcription factor Grainyhead-like 3 (GRHL3) has multiple roles in a variety of tissues during development including epithelial patterning and actin cytoskeletal regulation. During neural tube closure (NTC) in the mouse embryo, GRHL3 is expressed and functions in the non-neural ectoderm (NNE). Two important functions of GRHL3 are regulating the actin cytoskeleton during NTC and regulating the boundary between the NNE and neural ectoderm. However, an open question that remains is whether these functions explain the caudally restricted neural tube defect (NTD) of spina bifida observed in Grhl3 mutants.

RESULTS

Using scanning electron microscopy and immunofluorescence based imaging on Grhl3 mutants and wildtype controls, we show that GRHL3 is dispensable for NNE identity or epithelial maintenance in the caudal NNE but is needed for regulation of cellular protrusions during NTC. Grhl3 mutants have decreased lamellipodia relative to wildtype embryos during caudal NTC, first observed at the onset of delays when lamellipodia become prominent in wildtype embryos. At the axial level of NTD, half of the mutants show increased and disorganized filopodia and half lack cellular protrusions.

CONCLUSION

These data suggest that altered cellular protrusions during NTC contribute to the etiology of NTD in Grhl3 mutants.

Grainy head signaling regulates epithelium development and ecdysis in Blattella germanica

The transcription factor grainy head (Grh) functions in the protection of the epithelium against the external environment by generating strongly adhesive layers, and this function is conserved in vertebrates and invertebrates. In Drosophila, the top model for holometabolous insects, Grh is necessary during embryonic development, epidermal differentiation, central nervous system specification and epithelial repair. However, the function of this gene in hemimetabolous insect epithelia remains unknown. To examine the function of Grh signaling in regulating epithelium development in Hemimetabola, we focused on the Blattella germanica epidermal layer using a gene knockdown strategy. The spatiotemporal expression pattern of BgGrh was detected, and knockdown of BgGrh and BgCad96ca, which provide positive feedback to BgGrh, caused severe defects in new epithelium development and impeded the molting process required to discard the old integument. Knockdown of the expression of BgGrh and BgCad96ca caused increased expression of chitin synthase gene (BgCHS1) and chitinase gene (BgCht5), the upregulations of which should be mediated by the higher level of hormone receptor 3 (BgHr3) gene. In conclusion, epithelium development is regulated by Grh signaling, which might represent a potential target for the control of urban pest cockroaches.

An update on the use of inositols in preventing gestational diabetes mellitus (GDM) and neural tube defects (NTDs)

INTRODUCTION

Obstetric history and maternal body composition and lifestyle may be associated with serious complications both for the mother, such as gestational diabetes mellitus (GDM), and for the fetus, including congenital malformations such as neural tube defects (NTDs).

AREAS COVERED

In view of the recent knowledge, changes in nutritional and physical activity habits ameliorate glycemic control during pregnancy and in turn improve maternal and neonatal health outcomes. Recently, a series of small clinical and experimental studies indicated that supplemenation with inositols, a family of insulin sensitizers, was associated with beneficial impact for both GDM and NTDs.

EXPERT OPINION

Herein, we discuss the most significant scientific evidence supporting myo-inositol administration as a prophylaxis for the above-mentioned conditions.

Maternal Inositol Status and Neural Tube Defects: A Role for the Human Yolk Sac in Embryonic Inositol Delivery?

Supplementation with myo-inositol during the periconceptional period of pregnancy may ameliorate the recurrence risk of having a fetus affected by a neural tube defect (NTD; e.g., spina bifida). This could be of particular importance in providing a means for preventing NTDs that are unresponsive to folic acid. This review highlights the characteristics of inositol and describes the role of myo-inositol in the prevention of NTDs in rodent studies and the evidence for its efficacy in reducing NTD risk in human pregnancy. The possible reduction in NTD risk by maternal myo-inositol implies functional and developmentally important maternal-embryonic inositol interrelationships and also suggests that embryonic uptake of myo-inositol is crucial for embryonic development. The establishment of active myo-inositol cellular uptake mechanisms in the embryonic stages of human pregnancy, when the neural tube is closing, is likely to be an important determinant of normal development. We draw attention to the generation of materno-fetal inositol concentration gradients and relationships, and outline a transport pathway by which myo-inositol may be delivered to the early developing human embryo. These considerations provide novel insights into the mechanisms that may underpin inositol's ability to confer embryonic developmental benefit.

Interrogating the Grainyhead-like 2 (Grhl2) genomic locus identifies an enhancer element that regulates palatogenesis in mouse

The highly-conserved Grainyhead-like (Grhl) transcription factors are critical regulators of embryogenesis that regulate cellular survival, proliferation, migration and epithelial integrity, especially during the formation of the craniofacial skeleton. Family member Grhl2 is expressed throughout epithelial tissues during development, and loss of Grhl2 function leads to significant defects in neurulation, abdominal wall closure, formation of the face and fusion of the maxilla/palate. Whereas numerous downstream target genes of Grhl2 have been identified, very little is known about how this crucial developmental transcription factor itself is regulated. Here, using in silico and in utero expression analyses and functional deletion in mice, we have identified a novel 2.4 ​kb enhancer element (mm1286) that drives reporter gene expression in a pattern that strongly recapitulates endogenous Grhl2 in the craniofacial primordia, modulates Grhl2 expression in these tissues, and augments Grhl2-mediated closure of the secondary palate. Deletion of this genomic element, in the context of inactivation of one allele of Grhl2 (through generation of double heterozygous Grhl2^+/-;mm1286^+/- mice), results in a significant predisposition to palatal clefting at birth. Moreover, we found that a highly conserved 325 bp region of mm1286 is both necessary and sufficient for mediating the craniofacial-specific enhancer activity of this region, and that an extremely well-conserved 12-bp sequence within this element (CTGTCAAACAGGT) substantially determines full enhancer function. Together, these data provide valuable new insights into the upstream genomic regulatory landscape responsible for transcriptional control of Grhl2 during palatal closure.

Functional roles of Grainyhead-like transcription factors in renal development and disease

Proper renal function relies on the tightly regulated development of nephrons and collecting ducts. This process, known as tubulogenesis, involves dynamic cellular and molecular changes that instruct cells to form highly organized tubes of epithelial cells which compartmentalize the renal interstitium and tubular lumen via assembly of a selective barrier. The integrity and diversity of the various renal epithelia is achieved via formation of intercellular protein complexes along the apical-basal axis of the epithelial cells. In recent years, the evolutionarily conserved family of Grainyhead-like (GRHL) transcription factors which encompasses three mammalian family members (Grainyhead-like 1, 2, 3) has emerged as a group of critical regulators for organ development, epithelial differentiation, and barrier formation. Evidence from transgenic animal models supports the presence of Grainyhead-like-dependent transcriptional mechanisms that promote formation and maintenance of epithelial barriers in the kidney. In this review, we highlight different Grhl-dependent mechanisms that modulate epithelial differentiation in the kidney. Additionally, we discuss how disruptions in these mechanisms result in impaired renal function later in life.

Genetic variants in GRHL3 and risk for neural tube defects: A case-control and case-parent triad/control study

BACKGROUND

Neural tube defects (NTDs) are the most common severe birth defects with complex etiologies. Previous studies conducted on animals have suggested that the Grhl3 gene is essential for closure of the spinal neural tube, but little evidence from human studies on the variants of GRHL3 gene has been provided, especially the common genetic variants.

METHODS

To investigate the relationship between common genetic variants of GRHL3 and the risk for NTDs, we performed a case-control study and a case-parent triad/control study. Fast-target enrichment sequencing was performed to screen exon regions from 503 NTD cases, and three tag SNPs (single nucleotide polymorphisms, including rs12030057, rs2486668, and rs545809) were selected according to the sequencing results. Then, Sequenom MassARRAY genotyping was performed in 757 case parents and 519 controls to obtain genotype information of the target variant sites among all NTD triads and controls.

RESULTS

The genotype distributions of all SNPs were in accordance with Hardy-Weinberg Equilibrium (HWE) in the control population. In the case-control study, significant associations were found between C27G genetic variants on rs2486668 and risk for spina bifida and encephalocele, respectively, under different genetic models. Consistently, in the case-parent triad/control study, GG genotype on rs2486668 was associated with increased risk for spina bifida, with a RR of 2.15 (95% CI: 1.20-3.83). However, no parent-of-origin effect was found for any tag SNPs.

CONCLUSION

The GRHL3 C67G missense variant may increase the risk for spina bifida and encephalocele phenotypes.

Exploring GRHL3 polymorphisms and SNP-SNP interactions in the risk of non-syndromic oral clefts in the Brazilian population

OBJECTIVE

To investigate the association of single-nucleotide polymorphisms (SNP) in grainyhead-like 3 (GRHL3) and to verify its possible interactions with others genes responsible for craniofacial development in the risk of non-syndromic oral cleft (NSOC).

METHODS

Applying TaqMan allelic discrimination assays, we evaluated GRHL3 SNPs (rs10903078, rs41268753, and rs4648975) in an ancestry-structured case-control sample composed of 1,127 Brazilian participants [272 non-syndromic cleft palate only (NSCPO), 242 non-syndromic cleft lip only (NSCLO), 319 non-syndromic cleft lip and palate (NSCLP), and 294 healthy controls]. Additionally, SNP-SNP interactions of GRHL3 and previously reported variants in FAM49A, FOXE1, NTN1, and VAX1 were verified in non-syndromic cleft lip with or without cleft palate (NSCL ± P). To eliminate false-positive associations, Bonferroni correction or 1,000 permutation method was applied.

RESULTS

The multiple logistic regression analysis showed that the CC genotype of rs10903078 (p = .03) and the haplotype C-C formed by the SNPs rs10903078 and rs41268753 (p = .04) were associated with NSCLO, but the p-values did not withstand Bonferroni correction. However, SNP-SNP test revealed significant interactions between GRHL3 SNPs and FAM49A (rs7552), FOXE1 (rs3758249), VAX1 (rs7078160 and rs751231), and NTN1 (rs9891446).

CONCLUSIONS

Our results confirm the importance of GRHL3 and its interactions with previously NSOC-associated genes, including FAM49A, FOXE1, NTN1, and VAX1, in the pathogenesis of NSOC in the Brazilian population.

Spinal neural tube closure depends on regulation of surface ectoderm identity and biomechanics by Grhl2

Lack or excess expression of the surface ectoderm-expressed transcription factor Grainyhead-like2 (Grhl2), each prevent spinal neural tube closure. Here we investigate the causative mechanisms and find reciprocal dysregulation of epithelial genes, cell junction components and actomyosin properties in Grhl2 null and over-expressing embryos. Grhl2 null surface ectoderm shows a shift from epithelial to neuroepithelial identity (with ectopic expression of N-cadherin and Sox2), actomyosin disorganisation, cell shape changes and diminished resistance to neural fold recoil upon ablation of the closure point. In contrast, excessive abundance of Grhl2 generates a super-epithelial surface ectoderm, in which up-regulation of cell-cell junction proteins is associated with an actomyosin-dependent increase in local mechanical stress. This is compatible with apposition of the neural folds but not with progression of closure, unless myosin activity is inhibited. Overall, our findings suggest that Grhl2 plays a crucial role in regulating biomechanical properties of the surface ectoderm that are essential for spinal neurulation.

Loss or over-expression of Grainyhead-like transcription factors (Grhl) prevents closure of the neural tube but the mechanism underlying this is unclear. Here, the authors show that Grhl2 regulates murine posterior-neuropore closure via changes in the identity and biomechanics of the non-neural, surface ectoderm cells.

Cytoplasmic localization of GRHL3 upon epidermal differentiation triggers cell shape change for epithelial morphogenesis

Epithelial cell shape change is a pivotal driving force for morphogenesis of complex three-dimensional architecture. However, molecular mechanisms triggering shape changes of epithelial cells in the course of growth and differentiation have not been entirely elucidated. Grhl3 plays a crucial role as a downstream transcription factor of Wnt/β-catenin in epidermal differentiation. Here, we show Grhl3 induced large, mature epidermal cells, enriched with actomyosin networks, from embryoid bodies in vitro. Such epidermal cells were apparently formed by the simultaneous activation of canonical and non-canonical Wnt signaling pathways. A nuclear transcription factor, GRHL3 is localized in the cytoplasm and cell membrane during epidermal differentiation. Subsequently, such extranuclear GRHL3 is essential for the membrane-associated expression of VANGL2 and CELSR1. Cytoplasmic GRHL3, thereby, allows epidermal cells to acquire mechanical properties for changes in epithelial cell shape. Thus, we propose that cytoplasmic localization of GRHL3 upon epidermal differentiation directly triggers epithelial morphogenesis.

Hypomethylation of GRHL3 gene is associated with the occurrence of neural tube defects

AIM

To investigate the relationship between GRHL3 methylation and the etiology of neural tube defects (NTDs).

MATERIALS & METHODS

Analyze data from a genome-wide DNA methylation array. Targeted DNA methylation analysis was performed for 46 cases and 23 controls. At last, grhl3 overexpression and gene depletion experiments were conducted in zebrafish.

RESULTS

Five hypomethylated CpGs were discovered in the methylation arrays performed on NTD cases. In a validation study, 15 hypomethylated CpGs were found and the overall methylation levels decreased in brain/spinal cord tissue from NTD cases. The knockdown and overexpression of grhl3 in zebrafish damaged embryonic convergent extension processes.

CONCLUSION

Hypomethylation of GRHL3 in central nervous tissue is associated with NTDs, further supporting the importance of GRHL3 and methylation in proper neural tube closure.

Neural tube closure depends on expression of Grainyhead-like 3 in multiple tissues

Failure of neural tube closure leads to neural tube defects (NTDs), common congenital abnormalities in humans. Among the genes whose loss of function causes NTDs in mice, Grainyhead-like3 (Grhl3) is essential for spinal neural tube closure, with null mutants exhibiting fully penetrant spina bifida. During spinal neurulation Grhl3 is initially expressed in the surface (non-neural) ectoderm, subsequently in the neuroepithelial component of the neural folds and at the node-streak border, and finally in the hindgut endoderm. Here, we show that endoderm-specific knockout of Grhl3 causes late-arising spinal NTDs, preceded by increased ventral curvature of the caudal region which was shown previously to suppress closure of the spinal neural folds. This finding supports the hypothesis that diminished Grhl3 expression in the hindgut is the cause of spinal NTDs in the curly tail, carrying a hypomorphic Grhl3 allele. Complete loss of Grhl3 function produces a more severe phenotype in which closure fails earlier in neurulation, before the stage of onset of expression in the hindgut of wild-type embryos. This implicates additional tissues and NTD mechanisms in Grhl3 null embryos. Conditional knockout of Grhl3 in the neural plate and node-streak border has minimal effect on closure, suggesting that abnormal function of surface ectoderm, where Grhl3 transcripts are first detected, is primarily responsible for early failure of spinal neurulation in Grhl3 null embryos.

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Conditional knockout of Grhl3 in the hindgut causes spinal NTDs owing to incomplete closure of the posterior neuropore late in spinal neurulation.

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On the other hand, closure fails early in spinal neurulation in Grhl3 null embryos, prior to the normal stage of hindgut expression.

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Stage-dependent analysis of Grhl3 expression implicates the non-neural ectoderm in the early failure of closure.

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Grhl3 is also expressed in neural plate and neuromesodermal precursors, but knock-out in these tissues does not cause NTDs.

Transcriptional mechanisms coordinating tight junction assembly during epithelial differentiation

Epithelial tissues form a selective barrier via direct cell-cell interactions to separate and establish concentration gradients between the different compartments of the body. Proper function and formation of this barrier rely on the establishment of distinct intercellular junction complexes. These complexes include tight junctions, adherens junctions, desmosomes, and gap junctions. The tight junction is by far the most diverse junctional complex in the epithelial barrier. Its composition varies greatly across different epithelial tissues to confer various barrier properties. Thus, epithelial cells rely on tightly regulated transcriptional mechanisms to ensure proper formation of the epithelial barrier and to achieve tight junction diversity. Here, we review different transcriptional mechanisms utilized during embryogenesis and disease development to promote tight junction assembly and maintenance of intercellular barrier integrity. We focus particularly on the Grainyhead-like transcription factors and ligand-activated nuclear hormone receptors, two central families of proteins in epithelialization.

Rare deleterious variants in GRHL3 are associated with human spina bifida

Neural tube defects, including spina bifida, are among the most common birth defects caused by failure of neural tube closure during development. They have a complex etiology involving largely undetermined environmental and genetic factors. Previous studies in mouse models have implicated the transcription factor Grhl3 as an important factor in the pathogenesis of spina bifida. In the present study, we conducted a resequencing analysis of GRHL3 in a cohort of 233 familial and sporadic cases of spina bifida. We identified two novel truncating variants: one homozygous frameshift variant, p.Asp16Aspfs*10, in two affected siblings and one heterozygous intronic splicing variant, p.Ala318Glyfs*26. We also identified five missense variants, one of which was demonstrated to reduce the activation of gene targets in a luciferase reporter assay. With the previously identified p.Arg391Cys variant, eight variants were found in GRHL3. Comparison of the variant rate between our cohort and the ExAC database identified a significant enrichment of deleterious variants in GRHL3 in the whole gene and the transactivation region in spina bifida patients. These data provide strong evidence for a role of GRHL3 as a predisposing factor to spina bifida and will help dissect the complex etiology and pathogenic mechanisms of these malformations.

Stable Binding of the Conserved Transcription Factor Grainy Head to its Target Genes Throughout Drosophila melanogaster Development

It has been suggested that transcription factor binding is temporally dynamic, and that changes in binding determine transcriptional output. Nonetheless, this model is based on relatively few examples in which transcription factor binding has been assayed at multiple developmental stages. The essential transcription factor Grainy head (Grh) is conserved from fungi to humans, and controls epithelial development and barrier formation in numerous tissues. Drosophila melanogaster, which possess a single grainy head (grh) gene, provide an excellent system to study this conserved factor. To determine whether temporally distinct binding events allow Grh to control cell fate specification in different tissue types, we used a combination of ChIP-seq and RNA-seq to elucidate the gene regulatory network controlled by Grh during four stages of embryonic development (spanning stages 5-17) and in larval tissue. Contrary to expectations, we discovered that Grh remains bound to at least 1146 genomic loci over days of development. In contrast to this stable DNA occupancy, the subset of genes whose expression is regulated by Grh varies. Grh transitions from functioning primarily as a transcriptional repressor early in development to functioning predominantly as an activator later. Our data reveal that Grh binds to target genes well before the Grh-dependent transcriptional program commences, suggesting it sets the stage for subsequent recruitment of additional factors that execute stage-specific Grh functions.

Mice lacking the conserved transcription factor Grainyhead-like 3 (Grhl3) display increased apposition of the frontal and parietal bones during embryonic development

BACKGROUND

Increased apposition of the frontal and parietal bones of the skull during embryogenesis may be a risk factor for the subsequent development of premature skull fusion, or craniosynostosis. Human craniosynostosis is a prevalent, and often serious embryological and neonatal pathology. Other than known mutations in a small number of contributing genes, the aetiology of craniosynostosis is largely unknown. Therefore, the identification of novel genes which contribute to normal skull patterning, morphology and premature suture apposition is imperative, in order to fully understand the genetic regulation of cranial development.

RESULTS

Using advanced imaging techniques and quantitative measurement, we show that genetic deletion of the highly-conserved transcription factor Grainyhead-like 3 (Grhl3) in mice (Grhl3 ^-/- ) leads to decreased skull size, aberrant skull morphology and premature apposition of the coronal sutures during embryogenesis. Furthermore, Grhl3 ^-/- mice also present with premature collagen deposition and osteoblast alignment at the sutures, and the physical interaction between the developing skull, and outermost covering of the brain (the dura mater), as well as the overlying dermis and subcutaneous tissue, appears compromised in embryos lacking Grhl3. Although Grhl3 ^-/- mice die at birth, we investigated skull morphology and size in adult animals lacking one Grhl3 allele (heterozygous; Grhl3 ^+/- ), which are viable and fertile. We found that these adult mice also present with a smaller cranial cavity, suggestive of post-natal haploinsufficiency in the context of cranial development.

CONCLUSIONS

Our findings show that our Grhl3 mice present with increased apposition of the frontal and parietal bones, suggesting that Grhl3 may be involved in the developmental pathogenesis of craniosynostosis.

Grainyhead-like 2 inhibits the coactivator p300, suppressing tubulogenesis and the epithelial-mesenchymal transition

GRHL2 suppresses EMT to give a default epithelial phenotype. GRHL2 inhibits this process through the histone acetyltransferase coactivator p300, repressing the partial EMT and preventing induction of MMPs. The results demonstrate novel roles for p300 and GRHL2 in promoting or suppressing EMT in morphogenesis and tumor progression.

Developmental morphogenesis and tumor progression require a transient or stable breakdown of epithelial junctional complexes to permit programmed migration, invasion, and anoikis resistance, characteristics endowed by the epithelial–mesenchymal transition (EMT). The epithelial master-regulatory transcription factor Grainyhead-like 2 (GRHL2) suppresses and reverses EMT, causing a mesenchymal–epithelial transition to the default epithelial phenotype. Here we investigated the role of GRHL2 in tubulogenesis of Madin–Darby canine kidney cells, a process requiring transient, partial EMT. GRHL2 was required for cystogenesis, but it suppressed tubulogenesis in response to hepatocyte growth factor. Surprisingly, GRHL2 suppressed this process by inhibiting the histone acetyltransferase coactivator p300, preventing the induction of matrix metalloproteases and other p300-dependent genes required for tubulogenesis. A 13–amino acid region of GRHL2 was necessary for inhibition of p300, suppression of tubulogenesis, and interference with EMT. The results demonstrate that p300 is required for partial or complete EMT occurring in tubulogenesis or tumor progression and that GRHL2 suppresses EMT in both contexts through inhibition of p300.

Grainyhead-like 2 downstream targets act to suppress epithelial-to-mesenchymal transition during neural tube closure

The transcription factor grainyhead-like 2 (GRHL2) is expressed in non-neural ectoderm (NNE) and Grhl2 loss results in fully penetrant cranial neural tube defects (NTDs) in mice. GRHL2 activates expression of several epithelial genes; however, additional molecular targets and functional processes regulated by GRHL2 in the NNE remain to be determined, as well as the underlying cause of the NTDs in Grhl2 mutants. Here, we find that Grhl2 loss results in abnormal mesenchymal phenotypes in the NNE, including aberrant vimentin expression and increased cellular dynamics that affects the NNE and neural crest cells. The resulting loss of NNE integrity contributes to an inability of the cranial neural folds to move toward the midline and results in NTD. Further, we identified Esrp1, Sostdc1, Fermt1, Tmprss2 and Lamc2 as novel NNE-expressed genes that are downregulated in Grhl2 mutants. Our in vitro assays show that they act as suppressors of the epithelial-to-mesenchymal transition (EMT). Thus, GRHL2 promotes the epithelial nature of the NNE during the dynamic events of neural tube formation by both activating key epithelial genes and actively suppressing EMT through novel downstream EMT suppressors.

Fate Specification of Neural Plate Border by Canonical Wnt Signaling and Grhl3 is Crucial for Neural Tube Closure

During primary neurulation, the separation of a single-layered ectodermal sheet into the surface ectoderm (SE) and neural tube specifies SE and neural ectoderm (NE) cell fates. The mechanisms underlying fate specification in conjunction with neural tube closure are poorly understood. Here, by comparing expression profiles between SE and NE lineages, we observed that uncommitted progenitor cells, expressing stem cell markers, are present in the neural plate border/neural fold prior to neural tube closure. Our results also demonstrated that canonical Wnt and its antagonists, DKK1/KREMEN1, progressively specify these progenitors into SE or NE fates in accord with the progress of neural tube closure. Additionally, SE specification of the neural plate border via canonical Wnt signaling is directed by the grainyhead-like 3 (Grhl3) transcription factor. Thus, we propose that the fate specification of uncommitted progenitors in the neural plate border by canonical Wnt signaling and its downstream effector Grhl3 is crucial for neural tube closure. This study implicates that failure in critical genetic factors controlling fate specification of progenitor cells in the neural plate border/neural fold coordinated with neural tube closure may be potential causes of human neural tube defects.

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Neural plate border/neural fold possesses stem cell-like characters during primary neurulation.

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Canonical Wnt and its antagonists progressively specify progenitors into surface or neural fates upon neural tube closure.

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Fate specification into surface ectoderm in the neural fold is directed by the Grhl3 transcription factor.

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Fate specification of uncommitted progenitors in the neural plate border is intimately coupled to neural tube closure.

Loss-of-function de novo mutations play an important role in severe human neural tube defects

BACKGROUND

Neural tube defects (NTDs) are very common and severe birth defects that are caused by failure of neural tube closure and that have a complex aetiology. Anencephaly and spina bifida are severe NTDs that affect reproductive fitness and suggest a role for de novo mutations (DNMs) in their aetiology.

METHODS

We used whole-exome sequencing in 43 sporadic cases affected with myelomeningocele or anencephaly and their unaffected parents to identify DNMs in their exomes.

RESULTS

We identified 42 coding DNMs in 25 cases, of which 6 were loss of function (LoF) showing a higher rate of LoF DNM in our cohort compared with control cohorts. Notably, we identified two protein-truncating DNMs in two independent cases in SHROOM3, previously associated with NTDs only in animal models. We have demonstrated a significant enrichment of LoF DNMs in this gene in NTDs compared with the gene specific DNM rate and to the DNM rate estimated from control cohorts. We also identified one nonsense DNM in PAX3 and two potentially causative missense DNMs in GRHL3 and PTPRS.

CONCLUSIONS

Our study demonstrates an important role of LoF DNMs in the development of NTDs and strongly implicates SHROOM3 in its aetiology.

GRHL1 acts as tumor suppressor in neuroblastoma and is negatively regulated by MYCN and HDAC3

Neuroblastoma is an embryonic solid tumor of neural crest origin and accounts for 11% of all cancer-related deaths in children. Novel therapeutic strategies are therefore urgently required. MYCN oncogene amplification, which occurs in 20% of neuroblastomas, is a hallmark of high risk. Here, we aimed to exploit molecular mechanisms that can be pharmacologically addressed with epigenetically modifying drugs, such as histone deacetylase (HDAC) inhibitors. Grainyhead-like 1 (GRHL1), a gene critical for Drosophila neural development, belonged to the genes most strongly responding to HDAC inhibitor treatment of neuroblastoma cells in a genome-wide screen. An increase in the histone H4 pan-acetylation associated with its promoter preceded transcriptional activation. Physically adjacent, HDAC3 and MYCN colocalized to the GRHL1 promoter and repressed its transcription. High-level GRHL1 expression in primary neuroblastomas correlated on transcriptional and translational levels with favorable patient survival and established clinical and molecular markers for favorable tumor biology, including lack of MYCN amplification. Enforced GRHL1 expression in MYCN-amplified neuroblastoma cells with low endogenous GRHL1 levels abrogated anchorage-independent colony formation, inhibited proliferation, and retarded xenograft growth in mice. GRHL1 knockdown in MYCN single-copy cells with high endogenous GRHL1 levels promoted colony formation. GRHL1 regulated 170 genes genome-wide, and most were involved in pathways regulated during neuroblastomagenesis, including nervous system development, proliferation, cell-cell adhesion, cell spreading, and cellular differentiation. In summary, the data presented here indicate a significant role of HDAC3 in the MYCN-mediated repression of GRHL1 and suggest drugs that block HDAC3 activity and suppress MYCN expression as promising candidates for novel treatment strategies of high-risk neuroblastoma.

Lamin b1 polymorphism influences morphology of the nuclear envelope, cell cycle progression, and risk of neural tube defects in mice

Neural tube defects (NTDs), including spina bifida and anencephaly, are common birth defects whose complex multigenic causation has hampered efforts to delineate their molecular basis. The effect of putative modifier genes in determining NTD susceptibility may be investigated in mouse models, particularly those that display partial penetrance such as curly tail, a strain in which NTDs result from a hypomorphic allele of the grainyhead-like-3 gene. Through proteomic analysis, we found that the curly tail genetic background harbours a polymorphic variant of lamin B1, lacking one of a series of nine glutamic acid residues. Lamins are intermediate filament proteins of the nuclear lamina with multiple functions that influence nuclear structure, cell cycle properties, and transcriptional regulation. Fluorescence loss in photobleaching showed that the variant lamin B1 exhibited reduced stability in the nuclear lamina. Genetic analysis demonstrated that the variant also affects neural tube closure: the frequency of spina bifida and anencephaly was reduced three-fold when wild-type lamin B1 was bred into the curly tail strain background. Cultured fibroblasts expressing variant lamin B1 show significantly increased nuclear dysmorphology and diminished proliferative capacity, as well as premature senescence, associated with reduced expression of cyclins and Smc2, and increased expression of p16. The cellular basis of spinal NTDs in curly tail embryos involves a proliferation defect localised to the hindgut epithelium, and S-phase progression was diminished in the hindgut of embryos expressing variant lamin B1. These observations indicate a mechanistic link between altered lamin B1 function, exacerbation of the Grhl3-mediated cell proliferation defect, and enhanced susceptibility to NTDs. We conclude that lamin B1 is a modifier gene of major effect for NTDs resulting from loss of Grhl3 function, a role that is likely mediated via the key function of lamin B1 in maintaining integrity of the nuclear envelope and ensuring normal cell cycle progression.

Mouse as a model for multifactorial inheritance of neural tube defects

Neural tube defects (NTDs) such as spina bifida and anencephaly are some of the most common structural birth defects found in humans. These defects occur due to failures of neurulation, a process where the flat neural plate rolls into a tube. In spite of their prevalence, the causes of NTDs are poorly understood. The multifactorial threshold model best describes the pattern of inheritance of NTDs where multiple undefined gene variants interact with environmental factors to cause an NTD. To date, mouse models have implicated a multitude of genes as required for neurulation, providing a mechanistic understanding of the cellular and molecular pathways that control neurulation. However, the majority of these mouse models exhibit NTDs with a Mendelian pattern of inheritance. Still, many examples of multifactorial inheritance have been demonstrated in mouse models of NTDs. These include null and hypomorphic alleles of neurulation genes that interact in a complex fashion with other genetic mutations or environmental factors to cause NTDs. These models have implicated several genes and pathways for testing as candidates for the genetic basis of NTDs in humans, resulting in identification of putative pathogenic mutations in some patients. Mouse models also provide an experimental paradigm to gain a mechanistic understanding of the environmental factors that influence NTD occurrence, such as folic acid and maternal diabetes, and have led to the discovery of additional preventative nutritional supplements such as inositol. This review provides examples of how multifactorial inheritance of NTDs can be modeled in the mouse.

The transcription factors Grainyhead-like 2 and NK2-homeobox 1 form a regulatory loop that coordinates lung epithelial cell morphogenesis and differentiation

The Grainyhead family of transcription factors controls morphogenesis and differentiation of epithelial cell layers in multicellular organisms by regulating cell junction- and proliferation-related genes. Grainyhead-like 2 (Grhl2) is expressed in developing mouse lung epithelium and is required for normal lung organogenesis. The specific epithelial cells expressing Grhl2 and the genes regulated by Grhl2 in normal lungs are mostly unknown. In these studies we identified the NK2-homeobox 1 transcription factor (Nkx2-1) as a direct transcriptional target of Grhl2. By binding and transcriptional assays and by confocal microscopy we showed that these two transcription factors form a positive feedback loop in vivo and in cell lines and are co-expressed in lung bronchiolar and alveolar type II cells. The morphological changes observed in flattening lung alveolar type II cells in culture are associated with down-regulation of Grhl2 and Nkx2-1. Reduction of Grhl2 in lung epithelial cell lines results in lower expression levels of Nkx2-1 and of known Grhl2 target genes. By microarray analysis we identified that in addition to Cadherin1 and Claudin4, Grhl2 regulates other cell interaction genes such as semaphorins and their receptors, which also play a functional role in developing lung epithelium. Impaired collective cell migration observed in Grhl2 knockdown cell monolayers is associated with reduced expression of these genes and may contribute to the altered epithelial phenotype reported in Grhl2 mutant mice. Thus, Grhl2 functions at the nexus of a novel regulatory network, connecting lung epithelial cell identity, migration, and cell-cell interactions.

The functions of grainy head-like proteins in animals and fungi and the evolution of apical extracellular barriers

The Grainy head (GRH) family of transcription factors are crucial for the development and repair of epidermal barriers in all animals in which they have been studied. This is a high-level functional conservation, as the known structural and enzymatic genes regulated by GRH proteins differ between species depending on the type of epidermal barrier being formed. Interestingly, members of the CP2 superfamily of transcription factors, which encompasses the GRH and LSF families in animals, are also found in fungi--organisms that lack epidermal tissues. To shed light on CP2 protein function in fungi, we characterized a Neurospora crassa mutant lacking the CP2 member we refer to as grainy head-like (grhl). We show that Neurospora GRHL has a DNA-binding specificity similar to that of animal GRH proteins and dissimilar to that of animal LSF proteins. Neurospora grhl mutants are defective in conidial-spore dispersal due to an inability to remodel the cell wall, and we show that grhl mutants and the long-known conidial separation-2 (csp-2) mutants are allelic. We then characterized the transcriptomes of both Neurospora grhl mutants and Drosophila grh mutant embryos to look for similarities in the affected genes. Neurospora grhl appears to play a role in the development and remodeling of the cell wall, as well as in the activation of genes involved in defense and virulence. Drosophila GRH is required to activate the expression of many genes involved in cuticular/epidermal-barrier formation. We also present evidence that GRH plays a role in adult antimicrobial defense. These results, along with previous studies of animal GRH proteins, suggest the fascinating possibility that the apical extracellular barriers of some animals and fungi might share an evolutionary connection, and that the formation of physical barriers in the last common ancestor was under the control of a transcriptional code that included GRH-like proteins.

Grainy head and its target genes in epithelial morphogenesis and wound healing

The Grainy head (Grh) family of transcription factors is characterized by a unique DNA-binding domain that binds to a conserved consensus sequence. Nematodes and flies have a single grh gene, whereas mice and humans have evolved three genes encoding Grainy head-like (Grhl) factors. We review the biological function of Grh in different animals and the mechanisms modulating its activity. grh and grhl genes play a remarkably conserved role in epithelial organ development and extracellular barrier repair after tissue damage. Recent studies in flies and vertebrates suggest that Grh factors may be primary determinants of cell adhesion and epithelial tissue formation. Grh proteins can dimerize and act as activators or repressors in different developmental contexts. In flies, tissue-specific, alternative splicing generates different Grh isoforms with different DNA-binding specificities and functions. Grh activity is also modulated by receptor tyrosine kinases: it is phosphorylated by extracellular signal regulated kinase, and this phosphorylation is selectively required for epidermal barrier repair. Two mechanisms have been proposed to explain the repressive function of Grh on target gene transcription. First, Grh can target the Polycomb silencing complex to specific response elements. Second, it can directly compete for DNA binding with transcriptional activators. Understanding the molecular mechanisms of gene regulation by Grh factors is likely to elucidate phylogenetically conserved mechanisms of epithelial cell morphogenesis and regeneration upon tissue damage.

Over-expression of Grhl2 causes spina bifida in the Axial defects mutant mouse

Cranial neural tube defects (NTDs) occur in mice carrying mutant alleles of many different genes, whereas isolated spinal NTDs (spina bifida) occur in fewer models, despite being common human birth defects. Spina bifida occurs at high frequency in the Axial defects (Axd) mouse mutant but the causative gene is not known. In the current study, the Axd mutation was mapped by linkage analysis. Within the critical genomic region, sequencing did not reveal a coding mutation whereas expression analysis demonstrated significant up-regulation of grainyhead-like 2 (Grhl2) in Axd mutant embryos. Expression of other candidate genes did not differ between genotypes. In order to test the hypothesis that over-expression of Grhl2 causes Axd NTDs, we performed a genetic cross to reduce Grhl2 function in Axd heterozygotes. Grhl2 loss of function mutant mice were generated and displayed both cranial and spinal NTDs. Compound heterozygotes carrying both loss (Grhl2 null) and putative gain of function (Axd) alleles exhibited normalization of spinal neural tube closure compared with Axd/+ littermates, which exhibit delayed closure. Grhl2 is expressed in the surface ectoderm and hindgut endoderm in the spinal region, overlapping with grainyhead-like 3 (Grhl3). Axd mutants display delayed eyelid closure, as reported in Grhl3 null embryos. Moreover, Axd mutant embryos exhibited increased ventral curvature of the spinal region and reduced proliferation in the hindgut, reminiscent of curly tail embryos, which carry a hypomorphic allele of Grhl3. Overall, our data suggest that defects in Axd mutant embryos result from over-expression of Grhl2.

Fbxl10/Kdm2b deficiency accelerates neural progenitor cell death and leads to exencephaly

Histone methylation is the important transcription regulatory system that affects mammalian development and cell differentiation. Alterations in epigenetic gene regulation are associated with disease. Fbxl10 (F-box and leucine-rich repeat protein 10) is a JmjC domain-containing histone demethylase. Although Fbxl10 has been implicated in cell cycle regulation, cell death, senescence, and tumorigenesis, these functions are controversial and its physiological function is unclear. To determine the in vivo function of Fbxl10, in this study, we generated a homozygous mutation in the mouse Fbxl10 gene. About half of Fbxl10-deficient mice exhibit failure of neural tube closure, resulting in exencephaly and die shortly after birth. Fbxl10 deficiency also causes retinal coloboma and a curled tail with low penetrances. Fbxl10 mRNA is specifically expressed in the cranial neural folds at E8.5 embryos, and apoptosis increased in the neuroepithelium and mesenchyme of Fbxl10-deficient E9.5 embryos, consistent with neural tube defects found in Fbxl10-deficient mice. Depletion of Fbxl10 induced the increased expression of p19ARF, an inducer of apoptosis, in E8.5 embryos and mouse embryonic fibroblast cells. In addition, the number of mitotic neural progenitor cells is significantly increased in the mutant E14.5 brain. Our findings suggest that the Fbxl10 gene makes important contributions to embryonic neural development by regulating cell proliferation and cell death in mice.

The transcription factor grainyhead-like 2 regulates the molecular composition of the epithelial apical junctional complex

Differentiation of epithelial cells and morphogenesis of epithelial tubes or layers is closely linked with the establishment and remodeling of the apical junctional complex, which includes adherens junctions and tight junctions. Little is known about the transcriptional control of apical junctional complex components. Here, we show that the transcription factor grainyhead-like 2 (Grhl2), an epithelium-specific mammalian homolog of Drosophila Grainyhead, is essential for adequate expression of the adherens junction gene E-cadherin and the tight junction gene claudin 4 (Cldn4) in several types of epithelia, including gut endoderm, surface ectoderm and otic epithelium. We have generated Grhl2 mutant mice to demonstrate defective molecular composition of the apical junctional complex in these compartments that coincides with the occurrence of anterior and posterior neural tube defects. Mechanistically, we show that Grhl2 specifically associates with cis-regulatory elements localized at the Cldn4 core promoter and within intron 2 of the E-cadherin gene. Cldn4 promoter activity in epithelial cells is crucially dependent on the availability of Grhl2 and on the integrity of the Grhl2-associated cis-regulatory element. At the E-cadherin locus, the intronic Grhl2-associated cis-regulatory region contacts the promoter via chromatin looping, while loss of Grhl2 leads to a specific decrease of activating histone marks at the E-cadherin promoter. Together, our data provide evidence that Grhl2 acts as a target gene-associated transcriptional activator of apical junctional complex components and, thereby, crucially participates in epithelial differentiation.

The genetic background of the curly tail strain confers susceptibility to folate-deficiency-induced exencephaly

BACKGROUND: Suboptimal maternal folate status is considered a risk factor for neural tube defects (NTDs). However, the relationship between dietary folate status and risk of NTDs appears complex, as experimentally induced folate deficiency is insufficient to cause NTDs in nonmutant mice. In contrast, folate deficiency can exacerbate the effect of an NTD-causing mutation, as in splotch mice. The purpose of the present study was to determine whether folate deficiency can induce NTDs in mice with a permissive genetic background which do not normally exhibit defects. METHODS: Folate deficiency was induced in curly tail and genetically matched wild-type mice, and we analyzed the effect on maternal folate status, embryonic growth and development, and frequency of NTDs. RESULTS: Folate-deficient diets resulted in reduced maternal blood folate, elevated homocysteine, and a diminished embryonic folate content. Folate deficiency had a deleterious effect on reproductive success, resulting in smaller litter sizes and an increased rate of resorption. Notably, folate deficiency caused a similar-sized, statistically significant increase in the frequency of cranial NTDs among both curly tail (Grhl3 mutant) embryos and background-matched embryos that are wild type for Grhl3. The latter do not exhibit NTDs under normal dietary conditions. Maternal supplementation with myo-inositol reduced the incidence of NTDs in the folate-deficient wild-type strain. CONCLUSIONS: Dietary folate deficiency can induce cranial NTDs in nonmutant mice with a permissive genetic background, a situation that likely parallels gene-nutrient interactions in human NTDs. Our findings suggest that inositol supplementation may ameliorate NTDs resulting from insufficient dietary folate. Birth Defects Research (Part A), 2010. © 2009 Wiley-Liss, Inc.

Developmental control of inositol phosphate biosynthesis is altered in the brain of both curly and phenotypically normal straight tail mutant mice

BACKGROUND

Altered levels of inositol phosphate in the central nervous system (CNS) are hypothesized to produce distorted brain signaling and lead to numerous neurologic maladies. Little is known of mechanisms controlling the complex metabolic flux of inositol phosphate. Less is known of controls that regulate inositol-phosphate biosynthesis in the mammalian brain. The expression of 1L-myo-inositol-1 phosphate synthase (MIP), the only enzyme known to synthesize inositol phosphate, was studied in the brain of normal (CBA) and curly tail (CT) mutant mice. The CT strain exhibits a neural tube defect, spina bifida, responsive to inositol supplementation, but not to folic acid treatment.

METHODS

Utilizing enzyme assays to determine the specific activity of MIP, Western blotting to detect expression, gas chromatography/mass spectrometry to measure inositol concentration, and statistical analyses to evaluate quantitative data, MIP expression was analyzed in newborn, young, and adult brains of CBA and CT (curly tail [ct-CT] and straight tail [st-CT]) mutant mice.

RESULTS

Data analyses suggest there is a significant difference in MIP activity in the brain of CBA mice as compared to that of CT mutant mice and that temporal and spatial control of MIP expression and inositol concentrations are altered in the brain of both the ct-CT and phenotypically normal st-CT mutant. Moreover, two differentially expressed forms of MIP were identified in the adult mouse brain.

CONCLUSIONS

These findings implicate a role for MIP in the maturation of the CNS and evoke a hypothesis regarding the regulation of inositol phosphate biosynthesis in brain development.

Genetics of human neural tube defects

Neural tube defects (NTDs) are common, severe congenital malformations whose causation involves multiple genes and environmental factors. Although more than 200 genes are known to cause NTDs in mice, there has been rather limited progress in delineating the molecular basis underlying most human NTDs. Numerous genetic studies have been carried out to investigate candidate genes in cohorts of patients, with particular reference to those that participate in folate one-carbon metabolism. Although the homocysteine remethylation gene MTHFR has emerged as a risk factor in some human populations, few other consistent findings have resulted from this approach. Similarly, attention focused on the human homologues of mouse NTD genes has contributed only limited positive findings to date, although an emerging association between genes of the non-canonical Wnt (planar cell polarity) pathway and NTDs provides candidates for future studies. Priorities for the next phase of this research include: (i) larger studies that are sufficiently powered to detect significant associations with relatively minor risk factors; (ii) analysis of multiple candidate genes in groups of well-genotyped individuals to detect possible gene-gene interactions; (iii) use of high throughput genomic technology to evaluate the role of copy number variants and to detect 'private' and regulatory mutations, neither of which have been studied to date; (iv) detailed analysis of patient samples stratified by phenotype to enable, for example, hypothesis-driven testing of candidates genes in groups of NTDs with specific defects of folate metabolism, or in groups of fetuses with well-defined phenotypes such as craniorachischisis.

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

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

Development of the vertebrate central nervous system: formation of the neural tube

The developmental process of neurulation involves a series of coordinated morphological events, which result in conversion of the flat neural plate into the neural tube, the primordium of the entire central nervous system (CNS). Failure of neurulation results in neural tube defects (NTDs), severe abnormalities of the CNS, which are among the commonest of congenital malformations in humans. In order to gain insight into the embryological basis of NTDs, such as spina bifida and anencephaly, it is necessary to understand the morphogenetic processes and molecular mechanisms underlying neural tube closure. The mouse is the most extensively studied mammalian experimental model for studies of neurulation, while considerable insight into underlying developmental mechanisms has also arisen from studies in other model systems, particularly birds and amphibians. We describe the process of neural tube formation, discuss the cellular mechanisms involved and highlight recent findings that provide links between molecular signaling pathways and morphogenetic tissue movements.