The BMP antagonist Noggin promotes cranial and spinal neurulation by distinct mechanisms

Grainyhead-like 2 interacts with noggin to regulate tissue fusion in mouse

Defective tissue fusion during mammalian embryogenesis results in congenital anomalies, such as exencephaly, spina bifida and cleft lip and/or palate. The highly conserved transcription factor grainyhead-like 2 (Grhl2) is a crucial regulator of tissue fusion, with mouse models lacking GRHL2 function presenting with a fully penetrant open cranial neural tube, facial and abdominal clefting (abdominoschisis), and an open posterior neuropore. Here, we show that GRHL2 interacts with the soluble morphogen protein and bone morphogenetic protein (BMP) inhibitor noggin (NOG) to impact tissue fusion during development. The maxillary prominence epithelium in embryos lacking Grhl2 shows substantial morphological abnormalities and significant upregulation of NOG expression, together with aberrantly distributed pSMAD5-positive cells within the neural crest cell-derived maxillary prominence mesenchyme, indicative of disrupted BMP signalling. Reducing this elevated NOG expression (by generating Grhl2-/-;Nog+/- embryos) results in delayed embryonic lethality, partial tissue fusion rescue, and restoration of tissue form within the craniofacial epithelia. These data suggest that aberrant epithelial maintenance, partially regulated by noggin-mediated regulation of BMP-SMAD pathways, may underpin tissue fusion defects in Grhl2-/- mice.

Defects of the spliceosomal gene SNRPB affect osteo- and chondro-differentiation

Although gene splicing occurs throughout the body, the phenotype of spliceosomal defects is largely limited to specific tissues. Cerebro-costo-mandibular syndrome (CCMS) is one such spliceosomal disease, which presents as congenital skeletal dysmorphism and is caused by mutations of SNRPB gene encoding Small Nuclear Ribonucleoprotein Polypeptides B/B' (SmB/B'). This study employed in vitro cell cultures to monitor osteo- and chondro-differentiation and examined the role of SmB/B' in the differentiation process. We found that low levels of SmB/B' by knockdown or mutations of SNRPB led to suppressed osteodifferentiation in Saos-2 osteoprogenitor-like cells, which was accompanied by affected splicing of Dlx5. On the other hand, low SmB/B' led to promoted chondrogenesis in HEPM mesenchymal stem cells. Consistent with other reports, osteogenesis was promoted by the Wnt/β-catenin pathway activator and suppressed by Wnt and BMP blockers, whereas chondrogenesis was promoted by Wnt inhibitors. Suppressed osteogenic markers by SNRPB knockdown were partly rescued by Wnt/β-catenin pathway activation. Reporter analysis revealed that suppression of SNRPB results in attenuated Wnt pathway and/or enhanced BMP pathway activities. SNRPB knockdown altered splicing of TCF7L2 which impacts Wnt/β-catenin pathway activities. This work helps unravel the mechanism underlying CCMS whereby reduced expression of spliceosomal proteins causes skeletal phenotypes.

Caudal Fgfr1 disruption produces localised spinal mis-patterning and a terminal myelocystocele-like phenotype in mice

Closed spinal dysraphisms are poorly understood malformations classified as neural tube (NT) defects. Several, including terminal myelocystocele, affect the distal spine. We have previously identified a NT closure-initiating point, Closure 5, in the distal spine of mice. Here, we document equivalent morphology of the caudal-most closing posterior neuropore (PNP) in mice and humans. Closure 5 forms in a region of active FGF signalling, and pharmacological FGF receptor blockade impairs its formation in cultured mouse embryos. Conditional genetic deletion of Fgfr1 in caudal embryonic tissues with Cdx2Cre diminishes neuroepithelial proliferation, impairs Closure 5 formation and delays PNP closure. After closure, the distal NT of Fgfr1-disrupted embryos dilates to form a fluid-filled sac overlying ventrally flattened spinal cord. This phenotype resembles terminal myelocystocele. Histological analysis reveals regional and progressive loss of SHH- and FOXA2-positive ventral NT domains, resulting in OLIG2 labelling of the ventral-most NT. The OLIG2 domain is also subsequently lost, eventually producing a NT that is entirely positive for the dorsal marker PAX3. Thus, a terminal myelocystocele-like phenotype can arise after completion of NT closure with localised spinal mis-patterning caused by disruption of FGFR1 signalling.

Myo/Nog Cells: The Jekylls and Hydes of the Lens

Herein, we review a unique and versatile lineage composed of Myo/Nog cells that may be beneficial or detrimental depending on their environment and nature of the pathological stimuli they are exposed to. While we will focus on the lens, related Myo/Nog cell behaviors and functions in other tissues are integrated into the narrative of our research that spans over three decades, examines multiple species and progresses from early stages of embryonic development to aging adults. Myo/Nog cells were discovered in the embryonic epiblast by their co-expression of the skeletal muscle-specific transcription factor MyoD, the bone morphogenetic protein inhibitor Noggin and brain-specific angiogenesis inhibitor 1. They were tracked from the epiblast into the developing lens, revealing heterogeneity of cell types within this structure. Depletion of Myo/Nog cells in the epiblast results in eye malformations arising from the absence of Noggin. In the adult lens, Myo/Nog cells are the source of myofibroblasts whose contractions produce wrinkles in the capsule. Eliminating this population within the rabbit lens during cataract surgery reduces posterior capsule opacification to below clinically significant levels. Parallels are drawn between the therapeutic potential of targeting Myo/Nog cells to prevent fibrotic disease in the lens and other ocular tissues.

The biology of SCUBE

The SCUBE [Signal peptide-Complement C1r/C1s, Uegf, Bmp1 (CUB)-Epithelial growth factor domain-containing protein] family consists of three proteins in vertebrates, SCUBE1, 2 and 3, which are highly conserved in zebrafish, mice and humans. Each SCUBE gene encodes a polypeptide of approximately 1000 amino acids that is organized into five modular domains: (1) an N-terminal signal peptide sequence, (2) nine tandem epidermal growth factor (EGF)-like repeats, (3) a large spacer region, (4) three cysteine-rich (CR) motifs, and (5) a CUB domain at the C-terminus. Murine Scube genes are expressed individually or in combination during the development of various tissues, including those in the central nervous system and the axial skeleton. The cDNAs of human SCUBE orthologs were originally cloned from vascular endothelial cells, but SCUBE expression has also been found in platelets, mammary ductal epithelium and osteoblasts. Both soluble and membrane-associated SCUBEs have been shown to play important roles in physiology and pathology. For instance, upregulation of SCUBEs has been reported in acute myeloid leukemia, breast cancer and lung cancer. In addition, soluble SCUBE1 is released from activated platelets and can be used as a clinical biomarker for acute coronary syndrome and ischemic stroke. Soluble SCUBE2 enhances distal signaling by facilitating the secretion of dual-lipidated hedgehog from nearby ligand-producing cells in a paracrine manner. Interestingly, the spacer regions and CR motifs can increase or enable SCUBE binding to cell surfaces via electrostatic or glycan-lectin interactions. As such, membrane-associated SCUBEs can function as coreceptors that enhance the signaling activity of various serine/threonine kinase or tyrosine kinase receptors. For example, membrane-associated SCUBE3 functions as a coreceptor that promotes signaling in bone morphogenesis. In humans, SCUBE3 mutations are linked to abnormalities in growth and differentiation of both bones and teeth. In addition to studies on human SCUBE function, experimental results from genetically modified mouse models have yielded important insights in the field of systems biology. In this review, we highlight novel molecular discoveries and critical directions for future research on SCUBE proteins in the context of cancer, skeletal disease and cardiovascular disease.

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

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

Loss-of-Function of p21-Activated Kinase 2 Links BMP Signaling to Neural Tube Patterning Defects

Closure of the neural tube represents a highly complex and coordinated process, the failure of which constitutes common birth defects. The serine/threonine kinase p21-activated kinase 2 (PAK2) is a critical regulator of cytoskeleton dynamics; however, its role in the neurulation and pathogenesis of neural tube defects (NTDs) remains unclear. Here, the results show that Pak2^-/- mouse embryos fail to develop dorsolateral hinge points (DLHPs) and exhibit craniorachischisis, a severe phenotype of NTDs. Pak2 knockout activates BMP signaling that involves in vertebrate bone formation. Single-cell transcriptomes reveal abnormal differentiation trajectories and transcriptional events in Pak2^-/- mouse embryos during neural tube development. Two nonsynonymous and one recurrent splice-site mutations in the PAK2 gene are identified in five human NTD fetuses, which exhibit attenuated PAK2 expression and upregulated BMP signaling in the brain. Mechanistically, PAK2 regulates Smad9 phosphorylation to inhibit BMP signaling and ultimately induce DLHP formation. Depletion of pak2a in zebrafish induces defects in the neural tube, which are partially rescued by the overexpression of wild-type, but not mutant PAK2. The findings demonstrate the conserved role of PAK2 in neurulation in multiple vertebrate species, highlighting the molecular pathogenesis of PAK2 mutations in NTDs.

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

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

Bioengineering the human spinal cord

Three dimensional, self-assembled organoids that recapitulate key developmental and organizational events during embryogenesis have proven transformative for the study of human central nervous system (CNS) development, evolution, and disease pathology. Brain organoids have predominated the field, but human pluripotent stem cell (hPSC)-derived models of the spinal cord are on the rise. This has required piecing together the complex interactions between rostrocaudal patterning, which specifies axial diversity, and dorsoventral patterning, which establishes locomotor and somatosensory phenotypes. Here, we review how recent insights into neurodevelopmental biology have driven advancements in spinal organoid research, generating experimental models that have the potential to deepen our understanding of neural circuit development, central pattern generation (CPG), and neurodegenerative disease along the body axis. In addition, we discuss the application of bioengineering strategies to drive spinal tissue morphogenesis in vitro, current limitations, and future perspectives on these emerging model systems.

Premature Vertebral Mineralization in hmx1-Mutant Zebrafish

H6 family homeobox 1 (HMX1) regulates multiple aspects of craniofacial development, and mutations in HMX1 are linked to an ocular defect termed oculoauricular syndrome of Schorderet–Munier–Franceschetti (OAS) (MIM #612109). Recently, additional altered orofacial features have been reported, including short mandibular rami, asymmetry of the jaws, and altered premaxilla. We found that in two mutant zebrafish lines termed hmx1mut10 and hmx1mut150, precocious mineralization of the proximal vertebrae occurred. Zebrafish hmx1mut10 and hmx1mut150 report mutations in the SD1 and HD domains, which are essential for dimerization and activity of hmx1. In hmx1mut10, the bone morphogenetic protein (BMP) antagonists chordin and noggin1 were downregulated, while bmp2b and bmp4 were highly expressed and specifically localized to the dorsal region prior to the initiation of the osteogenic process. The osteogenic promoters runx2b and spp1 were also upregulated. Supplementation with DMH1—an inhibitor of the BMP signaling pathway—at the specific stage in which bmp2b and bmp4 are highly expressed resulted in reduced vertebral mineralization, resembling the wildtype mineralization progress of the axial skeleton. These results point to a possible role of hmx1 as part of a complex gene network that inhibits bmp2b and bmp4 in the dorsal region, thus regulating early axial skeleton development.

Sonic hedgehog signaling directs patterned cell remodeling during cranial neural tube closure

Neural tube closure defects are a major cause of infant mortality, with exencephaly accounting for nearly one-third of cases. However, the mechanisms of cranial neural tube closure are not well understood. Here, we show that this process involves a tissue-wide pattern of apical constriction controlled by Sonic hedgehog (Shh) signaling. Midline cells in the mouse midbrain neuroepithelium are flat with large apical surfaces, whereas lateral cells are taller and undergo synchronous apical constriction, driving neural fold elevation. Embryos lacking the Shh effector Gli2 fail to produce appropriate midline cell architecture, whereas embryos with expanded Shh signaling, including the IFT-A complex mutants Ift122 and Ttc21b and embryos expressing activated Smoothened, display apical constriction defects in lateral cells. Disruption of lateral, but not midline, cell remodeling results in exencephaly. These results reveal a morphogenetic program of patterned apical constriction governed by Shh signaling that generates structural changes in the developing mammalian brain.

An ontology for developmental processes and toxicities of neural tube closure

In recent years, the development and implementation of animal-free approaches to chemical and pharmaceutical hazard and risk assessment has taken off. Alternative approaches are being developed starting from the perspective of human biology and physiology. Neural tube closure is a vital step that occurs early in human development. Correct closure of the neural tube depends on a complex interplay between proteins along a number of protein concentration gradients. The sensitivity of neural tube closure to chemical disturbance of signalling pathways such as the retinoid pathway, is well known. To map the pathways underlying neural tube closure, literature data on the molecular regulation of neural tube closure were collected. As the process of neural tube closure is highly conserved in vertebrates, the extensive literature available for the mouse was used whilst considering its relevance for humans. Thus, important cell compartments, regulatory pathways, and protein interactions essential for neural tube closure under physiological circumstances were identified and mapped. An understanding of aberrant processes leading to neural tube defects (NTDs) requires detailed maps of neural tube embryology, including the complex genetic signals and responses underlying critical cellular dynamical and biomechanical processes. The retinoid signaling pathway serves as a case study for this ontology because of well-defined crosstalk with the genetic control of neural tube patterning and morphogenesis. It is a known target for mechanistically-diverse chemical structures that disrupt neural tube closure The data presented in this manuscript will set the stage for constructing mathematical models and computer simulation of neural tube closure for human-relevant AOPs and predictive toxicology.

Mouse gastrulation: Coordination of tissue patterning, specification and diversification of cell fate

During mouse embryonic development a mass of pluripotent epiblast tissue is transformed during gastrulation to generate the three definitive germ layers: endoderm, mesoderm, and ectoderm. During gastrulation, a spatiotemporally controlled sequence of events results in the generation of organ progenitors and positions them in a stereotypical fashion throughout the embryo. Key to the correct specification and differentiation of these cell fates is the establishment of an axial coordinate system along with the integration of multiple signals by individual epiblast cells to produce distinct outcomes. These signaling domains evolve as the anterior-posterior axis is established and the embryo grows in size. Gastrulation is initiated at the posteriorly positioned primitive streak, from which nascent mesoderm and endoderm progenitors ingress and begin to diversify. Advances in technology have facilitated the elaboration of landmark findings that originally described the epiblast fate map and signaling pathways required to execute those fates. Here we will discuss the current state of the field and reflect on how our understanding has shifted in recent years.

Altered Glutaminase 1 Activity During Neurulation and Its Potential Implications in Neural Tube Defects

The neurulation process is regulated by a large amount of genetic and environmental factors that determine the establishment, folding, and fusion of the neural plate to form the neural tube, which develops into the main structure of the central nervous system. A recently described factor involved in this process is glutamate. Through NMDA ionotropic receptor, glutamate modifies intracellular Ca2+ dynamics allowing the oriented cell migration and proliferation, essentials processes in neurulation. Glutamate synthesis depends on the mitochondrial enzyme known as glutaminase 1 (GLS1) that is widely expressed in brain and kidney. The participation of GLS 1 in prenatal neurogenic processes and in the adult brain has been experimentally established, however, its participation in early stages of embryonic development has not been described. The present investigation describes for the first time the presence and functionality of GLS1 in Xenopus laevis embryos during neurulation. Although protein expression levels remains constant, the catalytic activity of GLS1 increases significantly (~66%) between early (stage 12) and middle to late (stages 14-19) neurulation process. Additionally, the use of 6-diazo-5-oxo-L-norleucine (L-DON, competitive inhibitor of glutamine-depend enzymes), reduced significantly the GLS1 specific activity during neurulation (~36%) and induce the occurrence of neural tube defects involving its possible participation in the neural tube closure in Xenopus laevis embryos.

Mouse Models of Neural Tube Defects

During embryonic development, the central nervous system forms as the neural plate and then rolls into a tube in a complex morphogenetic process known as neurulation. Neural tube defects (NTDs) occur when neurulation fails and are among the most common structural birth defects in humans. The frequency of NTDs varies greatly anywhere from 0.5 to 10 in 1000 live births, depending on the genetic background of the population, as well as a variety of environmental factors. The prognosis varies depending on the size and placement of the lesion and ranges from death to severe or moderate disability, and some NTDs are asymptomatic. This chapter reviews how mouse models have contributed to the elucidation of the genetic, molecular, and cellular basis of neural tube closure, as well as to our understanding of the causes and prevention of this devastating birth defect.

Embryology of Spinal Dysraphism and its Relationship to Surgical Treatment

Spinal dysraphism is an umbrella term that encompasses a number of congenital malformations that affect the central nervous system. The etiology of these conditions can be traced back to a specific defect in embryological development, with the more disabling malformations occurring at an earlier gestational age. A thorough understanding of the relevant neuroembryology is imperative for clinicians to select the correct treatment and prevent complications associated with spinal dysraphism. This paper will review the neuroembryology associated with the various forms of spinal dysraphism and provide a clinical-pathological correlation for these congenital malformations.

Spina Bifida: Pathogenesis, Mechanisms, and Genes in Mice and Humans

Spina bifida is among the phenotypes of the larger condition known as neural tube defects (NTDs). It is the most common central nervous system malformation compatible with life and the second leading cause of birth defects after congenital heart defects. In this review paper, we define spina bifida and discuss the phenotypes seen in humans as described by both surgeons and embryologists in order to compare and ultimately contrast it to the leading animal model, the mouse. Our understanding of spina bifida is currently limited to the observations we make in mouse models, which reflect complete or targeted knockouts of genes, which perturb the whole gene(s) without taking into account the issue of haploinsufficiency, which is most prominent in the human spina bifida condition. We thus conclude that the need to study spina bifida in all its forms, both aperta and occulta, is more indicative of the spina bifida in surviving humans and that the measure of deterioration arising from caudal neural tube defects, more commonly known as spina bifida, must be determined by the level of the lesion both in mouse and in man.

An FGF3-BMP Signaling Axis Regulates Caudal Neural Tube Closure, Neural Crest Specification and Anterior-Posterior Axis Extension

During vertebrate axis extension, adjacent tissue layers undergo profound morphological changes: within the neuroepithelium, neural tube closure and neural crest formation are occurring, while within the paraxial mesoderm somites are segmenting from the presomitic mesoderm (PSM). Little is known about the signals between these tissues that regulate their coordinated morphogenesis. Here, we analyze the posterior axis truncation of mouse Fgf3 null homozygotes and demonstrate that the earliest role of PSM-derived FGF3 is to regulate BMP signals in the adjacent neuroepithelium. FGF3 loss causes elevated BMP signals leading to increased neuroepithelium proliferation, delay in neural tube closure and premature neural crest specification. We demonstrate that elevated BMP4 depletes PSM progenitors in vitro, phenocopying the Fgf3 mutant, suggesting that excessive BMP signals cause the Fgf3 axis defect. To test this in vivo we increased BMP signaling in Fgf3 mutants by removing one copy of Noggin, which encodes a BMP antagonist. In such mutants, all parameters of the Fgf3 phenotype were exacerbated: neural tube closure delay, premature neural crest specification, and premature axis termination. Conversely, genetically decreasing BMP signaling in Fgf3 mutants, via loss of BMP receptor activity, alleviates morphological defects. Aberrant apoptosis is observed in the Fgf3 mutant tailbud. However, we demonstrate that cell death does not cause the Fgf3 phenotype: blocking apoptosis via deletion of pro-apoptotic genes surprisingly increases all Fgf3 defects including causing spina bifida. We demonstrate that this counterintuitive consequence of blocking apoptosis is caused by the increased survival of BMP-producing cells in the neuroepithelium. Thus, we show that FGF3 in the caudal vertebrate embryo regulates BMP signaling in the neuroepithelium, which in turn regulates neural tube closure, neural crest specification and axis termination. Uncovering this FGF3-BMP signaling axis is a major advance toward understanding how these tissue layers interact during axis extension with important implications in human disease.

During embryological development, the vertebrate embryo undergoes profound growth in a head-to-tail direction. During this process, formation of different structures within adjacent tissue layers must occur in a coordinated fashion. Insights into how these adjacent tissues molecularly communicate with each other is essential to understanding both basic embryology and the underlying causes of human birth defects. Mice lacking Fgf3, which encodes a secreted signaling factor, have long been known to have premature axis termination, but the underlying mechanism has not been studied until now. Through a series of complex genetic experiments, we show that FGF3 is an essential factor for coordination of neural tube development and axis extension. FGF3 is secreted from the mesodermal layer, which is the major driver of extending the axis, and negatively regulates expression of another class of secreted signaling molecules in the neuroepithelium, BMPs. In the absence of FGF3, excessive BMP signals cause a delay in neural tube closure, premature specification of neural crest cells and negatively affect the mesoderm, causing a premature termination of the embryological axis. Our work suggests that FGF3 may be a player in the complex etiology of the human birth defect, spina bifida, the failure of posterior neural tube closure.

Derivation of keratinocytes from chicken embryonic stem cells: establishment and characterization of differentiated proliferative cell populations

A common challenge in avian cell biology is the generation of differentiated cell-lines, especially in the keratinocyte lineage. Only a few avian cell-lines are available and very few of them show an interesting differentiation profile. During the last decade, mammalian embryonic stem cell-lines were shown to differentiate into almost all lineages, including keratinocytes. Although chicken embryonic stem cells had been obtained in the 1990s, few differentiation studies toward the ectodermal lineage were reported. Consequently, we explored the differentiation of chicken embryonic stem cells toward the keratinocyte lineage by using a combination of stromal induction, ascorbic acid, BMP4 and chicken serum. During the induction period, we observed a downregulation of pluripotency markers and an upregulation of epidermal markers. Three homogenous cell populations were derived, which were morphologically similar to chicken primary keratinocytes, displaying intracellular lipid droplets in almost every pavimentous cell. These cells could be serially passaged without alteration of their morphology and showed gene and protein expression profiles of epidermal markers similar to chicken primary keratinocytes. These cells represent an alternative to the isolation of chicken primary keratinocytes, being less cumbersome to handle and reducing the number of experimental animals used for the preparation of primary cells.

Modeling anterior development in mice: diet as modulator of risk for neural tube defects

Head morphogenesis is a complex process that is controlled by multiple signaling centers. The most common defects of cranial development are craniofacial defects, such as cleft lip and cleft palate, and neural tube defects, such as anencephaly and encephalocoele in humans. More than 400 genes that contribute to proper neural tube closure have been identified in experimental animals, but only very few causative gene mutations have been identified in humans, supporting the notion that environmental influences are critical. The intrauterine environment is influenced by maternal nutrition, and hence, maternal diet can modulate the risk for cranial and neural tube defects. This article reviews recent progress toward a better understanding of nutrients during pregnancy, with particular focus on mouse models for defective neural tube closure. At least four major patterns of nutrient responses are apparent, suggesting that multiple pathways are involved in the response, and likely in the underlying pathogenesis of the defects. Folic acid has been the most widely studied nutrient, and the diverse responses of the mouse models to folic acid supplementation indicate that folic acid is not universally beneficial, but that the effect is dependent on genetic configuration. If this is the case for other nutrients as well, efforts to prevent neural tube defects with nutritional supplementation may need to become more specifically targeted than previously appreciated. Mouse models are indispensable for a better understanding of nutrient-gene interactions in normal pregnancies, as well as in those affected by metabolic diseases, such as diabetes and obesity.

Amniotic fluid and serum biomarkers from women with neural tube defect-affected pregnancies: a case study for myelomeningocele and anencephaly: clinical article

OBJECT

The authors sought to identify novel biomarkers for early detection of neural tube defects (NTDs) in human fetuses.

METHODS

Amniotic fluid and serum were drawn from women in the second trimester of pregnancy. The study group included 2 women pregnant with normal fetuses and 4 with fetuses displaying myelomeningocele (n = 1), anencephaly (n = 1), holoprosencephaly (n = 1), or encephalocele (n = 1). Amniotic fluid stem cells (AFSCs) were isolated and cultured. The cells were immunostained for the stem cell markers Oct4, CD133, and Sox2; the epigenetic biomarkers H3K4me2, H3K4me3, H3K27me2, H3K27me3, H3K9Ac, and H3K18Ac; and the histone modifiers KDM6B (a histone H3K27 demethylase) and Gcn5 (a histone acetyltransferase). The levels of 2 markers for neural tube development, bone morphogenetic protein-4 (BMP4) and sonic hedgehog (Shh), were measured in amniotic fluid and serum using an enzyme-linked immunosorbent assay.

RESULTS

The AFSCs from the woman pregnant with a fetus affected by myelomeningocele had higher levels of H3K4me2, H3K4me3, H3K27me2, and H3K27me3 and lower levels of KDM6B than the AFSCs from the women with healthy fetuses. The levels of H3K9ac, H3K18ac, and Gcn5 were also decreased in the woman with the fetus exhibiting myelomeningocele. In AFSCs from the woman carrying an anencephalic fetus, levels of H3K27me3, along with those of H3K9Ac, H3K18ac, and Gcn5, were increased, while that of KDM6B was decreased. Compared with the normal controls, the levels of BMP4 in amniotic fluid and serum from the woman with a fetus with myelomeningocele were increased, whereas levels of Shh were increased in the woman pregnant with a fetus displaying anencephaly.

CONCLUSIONS

The levels of epigenetic marks, such as H3K4me, H3K27me3, H3K9Ac, and H3K18A, in cultured AFSCs in combination with levels of key developmental proteins, such as BMP4 and Shh, are potential biomarkers for early detection and identification of NTDs in amniotic fluid and maternal serum.

Apicobasal polarity and neural tube closure

During development, a flat neural plate rolls up and closes to form a neural tube. This process, called neural tube closure, is complex and requires morphogenetic events to occur along multiple axes of the neural plate. Recent studies suggest that cell and tissue polarity play a major role in neural tube morphogenesis. While the planar cell polarity pathway is known to be involved in this process, a role for the apicobasal polarity pathway has only recently begun to be elucidated. These studies show that bone morphogenetic proteins can regulate the apicobasal polarity pathway in the neural plate in a cell cycle dependent manner. This dynamically modulates apical junctions in the neural plate, resulting in cell and tissue shape changes that help bend, shape and close the neural tube.

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

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

Copy number variation analysis implicates the cell polarity gene glypican 5 as a human spina bifida candidate gene

Neural tube defects (NTDs) are common birth defects of complex etiology. Family and population-based studies have confirmed a genetic component to NTDs. However, despite more than three decades of research, the genes involved in human NTDs remain largely unknown. We tested the hypothesis that rare copy number variants (CNVs), especially de novo germline CNVs, are a significant risk factor for NTDs. We used array-based comparative genomic hybridization (aCGH) to identify rare CNVs in 128 Caucasian and 61 Hispanic patients with non-syndromic lumbar-sacral myelomeningocele. We also performed aCGH analysis on the parents of affected individuals with rare CNVs where parental DNA was available (42 sets). Among the eight de novo CNVs that we identified, three generated copy number changes of entire genes. One large heterozygous deletion removed 27 genes, including PAX3, a known spina bifida-associated gene. A second CNV altered genes (PGPD8, ZC3H6) for which little is known regarding function or expression. A third heterozygous deletion removed GPC5 and part of GPC6, genes encoding glypicans. Glypicans are proteoglycans that modulate the activity of morphogens such as Sonic Hedgehog (SHH) and bone morphogenetic proteins (BMPs), both of which have been implicated in NTDs. Additionally, glypicans function in the planar cell polarity (PCP) pathway, and several PCP genes have been associated with NTDs. Here, we show that GPC5 orthologs are expressed in the neural tube, and that inhibiting their expression in frog and fish embryos results in NTDs. These results implicate GPC5 as a gene required for normal neural tube development.

Bone morphogenetic proteins regulate hinge point formation during neural tube closure by dynamic modulation of apicobasal polarity

BACKGROUND

A critical event in neural tube closure is the formation of median hinge points (MHPs) and dorsolateral hinge points (DLHPs). Together, they buckle the ventral midline and elevate and juxtapose the neural folds for proper neural tube closure. Dynamic cell behaviors occur at hinge points (HPs), but their molecular regulation is largely unexplored. Bone morphogenetic proteins (BMPs) have been implicated in a variety of neural tube closure defects, although the underlying mechanisms are poorly understood.

METHODS

In this study, we used in vivo electroporations, high-resolution microscopy, and biochemical analyses to explore the role of BMP signaling in chick midbrain neural tube closure.

RESULTS

We identified a cell-cycle-dependent BMP gradient in the midbrain neural plate, which results in low-level BMP activity at the MHP. We show that although BMP signaling does not have a role in midbrain cell-fate specification, its attenuation is necessary and sufficient for MHP formation and midbrain closure. BMP blockade induces MHP formation by regulating apical constriction and basal nuclear migration. Furthermore, BMP signaling is critically important for maintaining epithelial organization by biochemically interacting with apicobasal polarity proteins (e.g., PAR3). As a result, prolonged BMP blockade disrupts apical junctions, desegregating the apical (PAR3(+), ZO1(+)) and basolateral (LGL(+)) compartments. Direct apical LGL-GFP misexpression in turn is sufficient to induce ectopic HPs.

CONCLUSIONS

BMPs have a critical role in maintaining epithelial organization, a role that is conserved across species and tissue types. Its cell-cycle-dependent modulation in the neural plate dynamically regulates apicobasal polarity and helps to bend, shape, and close the neural tube.

Functional interactions between Fat family cadherins in tissue morphogenesis and planar polarity

The atypical cadherin fat (ft) was originally discovered as a tumor suppressor in Drosophila and later shown to regulate a form of tissue patterning known as planar polarity. In mammals, four ft homologs have been identified (Fat1-4). Recently, we demonstrated that Fat4 plays a role in vertebrate planar polarity. Fat4 has the highest homology to ft, whereas other Fat family members are homologous to the second ft-like gene, ft2. Genetic studies in flies and mice imply significant functional differences between the two groups of Fat cadherins. Here, we demonstrate that Fat family proteins act both synergistically and antagonistically to influence multiple aspects of tissue morphogenesis. We find that Fat1 and Fat4 cooperate during mouse development to control renal tubular elongation, cochlear extension, cranial neural tube formation and patterning of outer hair cells in the cochlea. Similarly, Fat3 and Fat4 synergize to drive vertebral arch fusion at the dorsal midline during caudal vertebra morphogenesis. We provide evidence that these effects depend on conserved interactions with planar polarity signaling components. In flies, the transcriptional co-repressor Atrophin (Atro) physically interacts with Ft and acts as a component of Fat signaling for planar polarity. We find that the mammalian orthologs of atro, Atn1 and Atn2l, modulate Fat4 activity during vertebral arch fusion and renal tubular elongation, respectively. Moreover, Fat4 morphogenetic defects are enhanced by mutations in Vangl2, a 'core' planar cell polarity gene. These studies highlight the wide range and complexity of Fat activities and suggest that a Fat-Atrophin interaction is a conserved element of planar polarity signaling.

LRP2 is an auxiliary SHH receptor required to condition the forebrain ventral midline for inductive signals

Sonic hedgehog (SHH) is a regulator of forebrain development that acts through its receptor, patched 1. However, little is known about cellular mechanisms at neurulation, whereby SHH from the prechordal plate governs specification of the rostral diencephalon ventral midline (RDVM), a major forebrain organizer. We identified LRP2, a member of the LDL receptor gene family, as a component of the SHH signaling machinery in the RDVM. LRP2 acts as an apical SHH-binding protein that sequesters SHH in its target field and controls internalization and cellular trafficking of SHH/patched 1 complexes. Lack of LRP2 in mice and in cephalic explants results in failure to respond to SHH, despite functional expression of patched 1 and smoothened, whereas overexpression of LRP2 variants in cells increases SHH signaling capacity. Our data identify a critical role for LRP2 in SHH signaling and reveal the molecular mechanism underlying forebrain anomalies in mice and patients with Lrp2 defects.

PDGF mediates TGFβ-induced migration during development of the spinous process

Mechanisms mediating closure of the dorsal vertebrae are not clear. Previously, we showed that deletion of TGFβ type II receptor (Tgfbr2) in sclerotome in mice results in failure in the formation of the spinous process, mimicking spina bifida occulta, a common malformation in humans. In this study, we aimed to determine whether missing dorsal structures in Tgfbr2 mutant mice were due to defects in mesenchymal migration and to clarify mechanism of TGFβ-mediated migration. First, we showed that gross alterations in dorsal vertebrae were apparent by E16.5days in Tgfbr2 mutants. In addition, histological staining showed that the mesenchyme adjacent to the developing cartilage was thin compared to controls likely due to reduced proliferation and migration of these cells. Next, we used a chemotaxis migration assay to show that TGFβ promotes migration in mixed cultures of embryonic sclerotome and associated mesenchyme. TGFβ stimulated expression of PDGF ligands and receptors in the cultures and intact PDGF signaling was required for TGFβ-mediated migration. Since PDGF ligands are expressed in the sclerotome-derived cartilage where Tgfbr2 is deleted and the receptors are predominantly expressed in the adjacent mesenchyme, we propose that TGFβ acts on the sclerotome to regulate expression of PDGF ligands, which then act on the associated mesenchyme in a paracrine fashion to mediate proliferation, migration and subsequent differentiation of the adjacent sclerotome.

Development and maturation of the spinal cord: implications of molecular and genetic defects

The human central nervous system (CNS) may be the most complex structure in the universe. Its development and appropriate specification into phenotypically and spatially distinct neural subpopulations involves a precisely orchestrated response, with thousands of transcriptional regulators combining with epigenetic controls and specific temporal cues in perfect synchrony. Understandably, our insight into the sophisticated molecular mechanisms which underlie spinal cord development are as yet limited. Even less is known about abnormalities of this process - putative genetic and molecular causes of well-described defects have only begun to emerge in recent years. Nonetheless, modern scientific techniques are beginning to demonstrate common patterns and principles amid the tremendous complexity of spinal cord development and maldevelopment. These advances are important, given that developmental anomalies of the spinal cord are an important cause of mortality and morbidity (Sadler, 2000); it is hoped that research advances will lead to better methods to detect, treat, and prevent these lesions.

Bone morphogenetic proteins regulate neural tube closure by interacting with the apicobasal polarity pathway

During neural tube closure, specialized regions called hinge points (HPs) display dynamic and polarized cell behaviors necessary for converting the neural plate into a neural tube. The molecular bases of such cell behaviors (e.g. apical constriction, basal nuclear migration) are poorly understood. We have identified a two-dimensional canonical BMP activity gradient in the chick neural plate that results in low and temporally pulsed BMP activity at the ventral midline/median hinge point (MHP). Using in vivo manipulations, high-resolution imaging and biochemical analyses, we show that BMP attenuation is necessary and sufficient for MHP formation. Conversely, BMP overexpression abolishes MHP formation and prevents neural tube closure. We provide evidence that BMP modulation directs neural tube closure via the regulation of apicobasal polarity. First, BMP blockade produces partially polarized neural cells, which retain contact with the apical and basal surfaces but where basolateral proteins (LGL) become apically localized and apical junctional proteins (PAR3, ZO1) become targeted to endosomes. Second, direct LGL misexpression induces ectopic HPs identical to those produced by noggin or dominant-negative BMPR1A. Third, BMP-dependent biochemical interactions occur between the PAR3-PAR6-aPKC polarity complex and phosphorylated SMAD5 at apical junctions. Finally, partially polarized cells normally occur at the MHP, their frequencies inversely correlated with the BMP activity gradient in the neural plate. We propose that spatiotemporal modulation of the two-dimensional BMP gradient transiently alters cell polarity in targeted neuronal cells. This ensures that the neural plate is flexible enough to be focally bent and shaped into a neural tube, while retaining overall epithelial integrity.

Myo/Nog cell regulation of bone morphogenetic protein signaling in the blastocyst is essential for normal morphogenesis and striated muscle lineage specification

Cells that express MyoD mRNA, the G8 antigen and the bone morphogenetic protein (BMP) inhibitor noggin (Nog) are present in the epiblast before gastrulation. Ablation of "Myo/Nog" cells in the blastocyst results in an expansion of canonical BMP signaling and prevents the expression of noggin and follistatin before and after the onset of gastrulation. Once eliminated in the epiblast, they are neither replaced nor compensated for as development progresses. Older embryos lacking Myo/Nog cells exhibit severe axial malformations. Although Wnts and Sonic hedgehog are expressed in ablated embryos, skeletal muscle progenitors expressing Pax3 are missing in the somites. Pax3+ cells do emerge adjacent to Wnt3a+ cells in vitro; however, few undergo skeletal myogenesis. Ablation of Myo/Nog cells also results in ectopically placed cardiac progenitors and cardiomyocytes in the somites. Reintroduction of Myo/Nog cells into the epiblast of ablated embryos restores normal patterns of BMP signaling, morphogenesis and skeletal myogenesis, and inhibits the expression of cardiac markers in the somites. This study demonstrates that Myo/Nog cells are essential regulators of BMP signaling in the early epiblast and are indispensable for normal morphogenesis and striated muscle lineage specification.

Rab3d is required for Xenopus anterior neurulation by regulating Noggin secretion

Rab3d is a member of the Ras-related small GTPase family of secretory Rab, Rab3. In this study, we showed that Xenopus Rab3d is expressed specifically in the anterior border of the neural plate when the neural plate converges and folds to initiate neural tube formation. Morpholino-mediated knockdown of Rab3d resulted in neurulation defects both in neural plate convergence and folding. Interestingly, perturbation of BMP signaling rescued neurulation defects of Rab3d morphants, suggesting that Rab3d inhibits BMP signaling during neurulation. By secretion assay in the Xenopus animal cap, we found that Rab3d specifically regulates secretion of a BMP antagonist, Noggin, but not Chordin and Wnts. We also found that Rab3d is co-localized with Noggin and that this interaction is dependent on the GTP/GDP cycle of Rab3d. Collectively, these findings suggest that Rab3d-mediated secretion regulation of a BMP antagonist, Noggin, is one of the mechanisms of BMP antagonism during Xenopus anterior neurulation.

Bone morphogenetic protein signaling is required in the dorsal neural folds before neurulation for the induction of spinal neural crest cells and dorsal neurons

Bone Morphogenetic Protein (BMP) activity has been implicated as a key regulator of multiple aspects of dorsal neural tube development. BMP signaling in the dorsal-most neuroepithelial cells presumably plays a critical role. We use tissue-specific gene ablation to probe the roles of BMPR1A, the type 1 BMP receptor that is seemingly the best candidate to mediate the activities of BMPs on early dorsal neural development. We use two different Cre lines expressed in the dorsal neural folds, one prior to spinal neurulation and one shortly afterward, together with a Bmpr1a conditional null mutation. Our findings indicate that BMPR1A signaling in the dorsal neural folds is important for hindbrain neural tube closure, but suggest it is dispensable for spinal neurulation. Our results also demonstrate a requirement for BMP signaling in patterning of dorsal neural tube cell fate and in neural crest cell formation, and imply a critical period shortly before neural tube closure.

Association study of BMP4, IL6, Leptin, MMP3, and MTNR1B gene promoter polymorphisms and adolescent idiopathic scoliosis

STUDY DESIGN

a genetic association study was performed on 126 patients with adolescent idiopathic scoliosis and 197 healthy controls from independent Hungarian pedigrees.

OBJECTIVE

to reveal implication of promoter polymorphisms of bone morphogenetic protein 4 (BMP4), interleukin-6 (IL6), leptin, matrix metalloproteinase-3 (MMP3), melatonin 1B receptor (MTNR1B) genes in adolescent idiopathic scoliosis (AIS). Combinatorial association of these candidate genes was also studied to detect additive effect of certain single-nucleotide polymorphism (SNP) patterns.

SUMMARY OF BACKGROUND DATA

it was previously unraveled that IL6, MMP3, and MTNR1B genes could be considered as predisposition genes of AIS. Since BMP4 and leptin play a central role in bone formation and remodeling and are in direct interaction with melatonin, IL6, and MMP3, these also can be potential predisposition genes.

METHODS

the genotyping was determined by polymerase chain reaction-restriction fragment length polymorphism.

RESULTS

at a single gene level, no significant differences were found for allele and genotype frequencies of the polymorphisms of these genes between cases or controls; therefore, the formerly detected association of IL6, MMP3, and MTNR1B with AIS was not confirmed in the Hungarian population by independent SNP analysis. However, significantly increased AIS risk was observed at particular combinations of genotypes of paired SNPs of the candidate genes.

CONCLUSIONS

the genetic effect of promoter polymorphisms of BMP4, IL6, leptin, MMP3, and MTNR1B can be synergistic for susceptibility to AIS. The combinatorial effect can modulate the final biological impact of many susceptibility polymorphisms; therefore, this should be considered at the comparison of results from case-control studies of different populations.

BMP antagonism protects Nodal signaling in the gastrula to promote the tissue interactions underlying mammalian forebrain and craniofacial patterning

Holoprosencephaly (HPE) is the most common forebrain and craniofacial malformation syndrome in humans. The genetics of HPE suggest that it often stems from a synergistic interaction of mutations in independent loci. In mice, several combinations of mutations in Nodal signaling pathway components can give rise to HPE, but it is not clear whether modest deficits of Nodal signaling along with lesions in other pathways might also cause such defects. We find that HPE results from simultaneous reduction of Nodal signaling and an organizer BMP (bone morphogenetic protein) antagonist, either Chordin or Noggin. These defects result from reduced production of tissues that promote forebrain and craniofacial development. Nodal promotes the expression of genes in the anterior primitive streak that are important for the development of these tissues, whereas BMP inhibits their expression. Pharmacological and transgenic manipulation of these signaling pathways suggests that BMP and Nodal antagonize each other prior to intracellular signal transduction. Biochemical experiments in vitro indicate that secreted Bmp2 and Nodal can form extracellular complexes, potentially interfering with receptor activation. Our results reveal that the patterning of forebrain and medial craniofacial elements requires a fine balance between BMP and Nodal signaling during primitive streak development, and provide a potential mechanistic basis for a new multigenic model of HPE.

Bone morphogenetic protein 4 mediates myocardial ischemic injury through JNK-dependent signaling pathway

Bone morphogenetic protein (BMP) signaling regulates embryonic development of many organ systems and defective BMP signaling has been implicated in adult disorders of many of these systems. However, its relevance in cardiac disease has not been reported. Here we demonstrate for the first time that Bmp4 activity promotes cellular apoptosis following ischemia-reperfusion (I/R) injury induced myocardial infarction (MI). Bmp4 heterozygous null mice (Bmp4(+/)(-)) demonstrated reduced infarct size, less myocardial apoptosis and down-regulation of pro-apoptotic proteins relative to wild-type mice following I/R injury. This was associated with reduction in I/R induced BMP4 levels in the left ventricular infarcted region. Furthermore, treatment of neonatal cardiomyocytes with BMP4 resulted in time and dose-dependent increase in cellular apoptosis and activation of the JNK MAP kinase pathway. In contrast, while JNK activation was significantly attenuated in Bmp4(+/)(-) mice and following Smad1 inhibition in myocytes, inhibition of JNK with a specific inhibitory peptide, TAT-JBD(20,) blocked BMP4 induced apoptosis. In vivo treatment of mice with Noggin, an endogenous extracellular BMP antagonist, or dorsomorphin, a small molecule inhibitor of BMP signaling, reduced infarct size, and inhibited pro-apoptotic signaling accompanied by an inhibition of Smad1 phosphorylation and JNK activation. These studies identify a novel role for Bmp4 in the pathogenesis of myocardial infarction and illustrate the use of a small molecule inhibitor of BMP signaling for treatment of acute I/R injury.

Human embryonic stem-cell derivatives for full reconstruction of the pluristratified epidermis: a preclinical study

BACKGROUND

Cell therapy for large burns is dependent upon autologous epidermis reconstructed in vitro. However, the effectiveness of current procedures is limited by the delay needed to culture the patient's own keratinocytes. To assess whether the keratinocyte progeny of human embryonic stem cells (hESCs) could be used to form a temporary skin substitute for use in patients awaiting autologous grafts, we investigated the cells' capability of constructing a pluristratified epidermis.

METHODS

hESCs from lines H9 and SA01 were seeded at least in triplicate on fibroblast feeder cells for 40 days in a medium supplemented with bone morphogenetic protein 4 and ascorbic acid. Molecular characterisation of cell differentiation was done throughout the process by quantitative PCR, fluorescence-activated cell sorting, and immunocytochemical techniques. Keratinocyte molecular differentiation and functional capacity to construct a human epidermis were assessed in vitro and in vivo.

FINDINGS

From hESCs, we generated a homogeneous population of cells that showed phenotypic characteristics of basal keratinocytes. Expression levels of genes encoding keratin 14, keratin 5, integrin alpha6, integrin beta4, collagen VII, and laminin 5 in these cells were similar to those in basal keratinocytes. After seeding on an artificial matrix, keratinocytes derived from hESCs (K-hESCs) formed a pluristratified epidermis. Keratin-14 immunostaining was seen in the basal compartment, with keratin 10 present in layers overlying the basal layer. Involucrin and filaggrin, late markers of epidermal differentiation, were detected in the uppermost layers only. 12 weeks after grafting onto five immunodeficient mice, epidermis derived from K-hESCs had a structure consistent with that of mature human skin. Human involucrin was appropriately located in spinous and granular layers and few Ki67-positive cells were detected in the basal layer.

INTERPRETATION

hESCs can be differentiated into basal keratinocytes that are fully functional--ie, able to construct a pluristratified epidermis. This resource could be developed to provide temporary skin substitutes for patients awaiting autologous grafts.

FUNDING

Institut National de la Santé et de la Recherche Médicale, University Evry Val d'Essonne, Association Française contre les Myopathies, Fondation René Touraine, and Genopole.

A critical role for sFRP proteins in maintaining caudal neural tube closure in mice via inhibition of BMP signaling

Both the BMP and Wnt pathways have been implicated in directing aspects of dorsal neural tube closure and cell fate specification. However, the mechanisms that control the diverse responses to these signals are poorly understood. In this study, we provide genetic and functional evidence that the secreted sFRP1 and sFRP2 proteins, which have been primarily implicated as negative regulators of Wnt signaling, can also antagonize BMP signaling in the caudal neural tube and that this function is critical to maintain proper neural tube closure and dorsal cell fate segregation. Our studies thus reveal a novel role for specific sFRP proteins in balancing the response of cells to two critical extracellular signaling pathways.

Mice defective in Trpm6 show embryonic mortality and neural tube defects

The syndrome of hypomagnesemia with secondary hypocalcemia is caused by defective TRPM6. This protein is an ion channel that also contains a kinase in its C-terminus. It is usually diagnosed in childhood and, without treatment with supplemental Mg, affected children suffer from mental retardation, seizures and retarded development. We developed a mouse lacking Trpm6 in order to understand in greater detail the function of this protein. In contrast to our expectations, Trpm6(-/-) mice almost never survived to weaning. Many mice died by embryonic day 12.5. Most that survived to term had neural tube defects consisting of both exencephaly and spina bifida occulta, an unusual combination. Feeding dams a high Mg diet marginally improved offspring survival to weaning. The few Trpm6(-/-) mice that survived were fertile but matings between Trpm6(-/-) mice produced no viable pregnancies. Trpm6(+/-) mice had normal electrolytes except for modestly low plasma [Mg]. In addition, some Trpm6(+/-) mice died prematurely. Absence of Trpm6 produces an apparently different phenotype in mice than in humans. The presence of neural tube defects identifies a previously unsuspected role of Trpm6 in effecting neural tube closure. This genetic defect produces one of very few mouse models of spina bifida occulta. These results point to a critical role of Trpm6 in development and suggest an important role in neural tube closure.

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.

Mechanistic insights into folate supplementation from Crooked tail and other NTD-prone mutant mice

Despite two decades of research since Smithells and colleagues began exploring its benefits, the mechanisms through which folic acid supplementation supports neural tube closure and early embryonic development are still unclear. The greatest progress toward a molecular-genetic understanding of folate effects on neural tube defect (NTD) pathogenesis has come from animal models. The number of NTD-associated mouse mutants accumulated and studied over the past decade has illuminated the complexity of both genetic factors contributing to NTDs and also NTD-gene interactions with folate metabolism. This article discusses insights gained from mouse models into how folate supplementation impacts neurulation. A case is made for renewed efforts to systematically screen the folate responsiveness of the scores of NTD-associated mouse mutations now identified. Designed after Crooked tail, supplementation studies of additional mouse mutants could build the molecular network maps that will ultimately enable tailoring of therapeutic regimens to individual families.

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

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

LUZP deficiency affects neural tube closure during brain development

LUZP is a leucine zipper-containing protein predominantly expressed in the brain. The functional significance of LUZP remains unknown. To explore the role of LUZP in brain development, a knockout mouse strain with a lacZ knock-in (Luzp-KO/lacZ-KI) has been established. LacZ reporter expression driven by the endogenous Luzp promoter was detected in the neuroepithelium and the cardiac tissue. Luzp(-/-) mice exhibited perinatal death, presumably due to the accompanied complex cardiovascular defects. Luzp(-/-) embryos displayed a cranial neural tube closure defect (NTD), with exposed brain tissues. Ectopic expression of Sonic-hedgehog, which is a protein known to be involved in neural tube closure, and elevated apoptosis were observed in the dorsal lateral neuroepithelium of the NTD Luzp(-/-) hindbrain. These findings assign a novel function of LUZP in the embryonic development of brain.

BMP antagonism is required in both the node and lateral plate mesoderm for mammalian left-right axis establishment

In mouse, left-right (L-R) patterning depends on asymmetric expression of Nodal around the node, leading to Nodal expression specifically in the left lateral plate mesoderm (LPM). Bone morphogenetic protein (BMP) signaling is also involved, but the mechanistic relationship with Nodal expression remains unclear. We find that BMP signal transduction is higher in the right LPM, although Bmp4, which is required for L-R patterning, is expressed symmetrically. By contrast, the BMP antagonists noggin (Nog) and chordin (Chrd) are expressed at higher levels in the left LPM. In Chrd;Nog double mutants, BMP signaling is elevated on both sides, whereas Nodal expression is absent. Ectopic expression of Nog in the left LPM of double mutants restores Nodalexpression. Ectopic Bmp4 expression in the left LPM of wild-type embryos represses Nodal transcription, whereas ectopic Nogin the right LPM leads to inappropriate Nodal expression. These data indicate that chordin and noggin function to limit BMP signaling in the left LPM, thereby derepressing Nodal expression. In the node, they promote peripheral Nodal expression and proper node morphology, potentially in concert with Notch signaling. These results indicate that BMP antagonism is required in both the node and LPM to facilitate L-R axis establishment in the mammalian embryo.

Neural plate morphogenesis during mouse neurulation is regulated by antagonism of Bmp signalling

Dorsolateral bending of the neural plate, an undifferentiated pseudostratified epithelium, is essential for neural tube closure in the mouse spinal region. If dorsolateral bending fails, spina bifida results. In the present study, we investigated the molecular signals that regulate the formation of dorsolateral hinge points (DLHPs). We show that Bmp2expression correlates with upper spinal neurulation (in which DLHPs are absent); that Bmp2-null embryos exhibit premature, exaggerated DLHPs;and that the local release of Bmp2 inhibits neural fold bending. Therefore,Bmp signalling is necessary and sufficient to inhibit DLHPs. By contrast, the Bmp antagonist noggin is expressed dorsally in neural folds containing DLHPs,noggin-null embryos show markedly reduced dorsolateral bending and local release of noggin stimulates bending. Hence, Bmp antagonism is both necessary and sufficient to induce dorsolateral bending. The local release of Shh suppresses dorsal noggin expression, explaining the absence of DLHPs at high spinal levels, where notochordal expression of Shh is strong. DLHPs`break through' at low spinal levels, where Shh expression is weaker. Zic2 mutant embryos fail to express Bmp antagonists dorsally and lack DLHPs, developing severe spina bifida. Our findings reveal a molecular mechanism based on antagonism of Bmp signalling that underlies the regulation of DLHP formation during mouse spinal neural tube closure.