Zic-associated holoprosencephaly: zebrafish Zic1 controls midline formation and forebrain patterning by regulating Nodal, Hedgehog, and retinoic acid signaling

A 3D molecular map of the cavefish neural plate illuminates eye-field organization and its borders in vertebrates

The vertebrate retinas originate from a specific anlage in the anterior neural plate called the eye field. Its identity is conferred by a set of ‘eye transcription factors’, whose combinatorial expression has been overlooked. Here, we use the dimorphic teleost Astyanax mexicanus, which develops proper eyes in the wild type and smaller colobomatous eyes in the blind cavefish embryos, to unravel the molecular anatomy of the eye field and its variations within a species. Using a series of markers (rx3, pax6a, cxcr4b, zic1, lhx2, emx3 and nkx2.1a), we draw a comparative 3D expression map at the end of gastrulation/onset of neurulation, which highlights hyper-regionalization of the eye field into sub-territories of distinct sizes, shapes, cell identities and combinatorial gene expression levels along the three body axes. All these features show significant variations in the cavefish natural mutant. We also discover sub-domains within the prospective telencephalon and characterize cell identities at the frontiers of the eye field. We propose putative fates for some of the characterized eye-field subdivisions, and suggest the existence of a trade-off between some subdivisions in the two Astyanax morphs on a micro-evolutionary scale.

New SHH and Known SIX3 Variants in a Series of Latin American Patients with Holoprosencephaly

Holoprosencephaly (HPE) is the failure of the embryonic forebrain to develop into 2 hemispheres promoting midline cerebral and facial defects. The wide phenotypic variability and causal heterogeneity make genetic counseling difficult. Heterozygous variants with incomplete penetrance and variable expressivity in the SHH, SIX3, ZIC2, and TGIF1 genes explain ∼25% of the known causes of nonchromosomal HPE. We studied these 4 genes and clinically described 27 Latin American families presenting with nonchromosomal HPE. Three new SHH variants and a third known SIX3 likely pathogenic variant found by Sanger sequencing explained 15% of our cases. Genotype-phenotype correlation in these 4 families and published families with identical or similar driver gene, mutated domain, conservation of residue in other species, and the type of variant explain the pathogenicity but not the phenotypic variability. Nine patients, including 2 with SHH pathogenic variants, presented benign variants of the SHH, SIX3, ZIC2, and TGIF1 genes with potential alteration of splicing, a causal proposition in need of further studies. Finding more families with the same SIX3 variant may allow further identification of genetic or environmental modifiers explaining its variable phenotypic expression.

Chemical-Induced Cleft Palate Is Caused and Rescued by Pharmacological Modulation of the Canonical Wnt Signaling Pathway in a Zebrafish Model

Cleft palate is one of the most frequent birth defects worldwide. It causes severe problems regarding eating and speaking and requires long-term treatment. Effective prenatal treatment would contribute to reducing the risk of cleft palate. The canonical Wnt signaling pathway is critically involved in palatogenesis, and genetic or chemical disturbance of this signaling pathway leads to cleft palate. Presently, preventative treatment for cleft palate during prenatal development has limited efficacy, but we expect that zebrafish will provide a useful high-throughput chemical screening model for effective prevention. To achieve this, the zebrafish model should recapitulate cleft palate development and its rescue by chemical modulation of the Wnt pathway. Here, we provide proof of concept for a zebrafish chemical screening model. Zebrafish embryos were treated with 12 chemical reagents known to induce cleft palate in mammals, and all 12 chemicals induced cleft palate characterized by decreased proliferation and increased apoptosis of palatal cells. The cleft phenotype was enhanced by combinatorial treatment with Wnt inhibitor and teratogens. Furthermore, the expression of tcf7 and lef1 as a readout of the pathway was decreased. Conversely, cleft palate was prevented by Wnt agonist and the cellular defects were also prevented. In conclusion, we provide evidence that chemical-induced cleft palate is caused by inhibition of the canonical Wnt pathway. Our results indicate that this zebrafish model is promising for chemical screening for prevention of cleft palate as well as modulation of the Wnt pathway as a therapeutic target.

Genetics Underlying the Interactions between Neural Crest Cells and Eye Development

The neural crest is a unique, transient stem cell population that is critical for craniofacial and ocular development. Understanding the genetics underlying the steps of neural crest development is essential for gaining insight into the pathogenesis of congenital eye diseases. The neural crest cells play an under-appreciated key role in patterning the neural epithelial-derived optic cup. These interactions between neural crest cells within the periocular mesenchyme and the optic cup, while not well-studied, are critical for optic cup morphogenesis and ocular fissure closure. As a result, microphthalmia and coloboma are common phenotypes in human disease and animal models in which neural crest cell specification and early migration are disrupted. In addition, neural crest cells directly contribute to numerous ocular structures including the cornea, iris, sclera, ciliary body, trabecular meshwork, and aqueous outflow tracts. Defects in later neural crest cell migration and differentiation cause a constellation of well-recognized ocular anterior segment anomalies such as Axenfeld–Rieger Syndrome and Peters Anomaly. This review will focus on the genetics of the neural crest cells within the context of how these complex processes specifically affect overall ocular development and can lead to congenital eye diseases.

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

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

Drosophila Zic family member odd-paired is needed for adult post-ecdysis maturation

Specific neuropeptides regulate in arthropods the shedding of the old cuticle (ecdysis) followed by maturation of the new cuticle. In Drosophila melanogaster, the last ecdysis occurs at eclosion from the pupal case, with a post-eclosion behavioural sequence that leads to wing extension, cuticle stretching and tanning. These events are highly stereotyped and are controlled by a subset of crustacean cardioactive peptide (CCAP) neurons through the expression of the neuropeptide Bursicon (Burs). We have studied the role of the transcription factor Odd-paired (Opa) during the post-eclosion period. We report that opa is expressed in the CCAP neurons of the central nervous system during various steps of the ecdysis process and in peripheral CCAP neurons innerving the larval muscles involved in adult ecdysis. We show that its downregulation alters Burs expression in the CCAP neurons. Ectopic expression of Opa, or the vertebrate homologue Zic2, in the CCAP neurons also affects Burs expression, indicating an evolutionary functional conservation. Finally, our results show that, independently of its role in Burs regulation, Opa prevents death of CCAP neurons during larval development.

Fundc1 is necessary for proper body axis formation during embryogenesis in zebrafish

FUN14 domain-containing protein 1 (FUNDC1) is a mitochondrial outer membrane protein which is responsible for hypoxia-induced mitophagy in mammalian cells. Knockdown of fundc1 is known to cause severe defects in the body axis of a rare minnow. To understand the role of Fundc1 in embryogenesis, we used zebrafish in this study. We used bioimaging to locate zebrafish Fundc1 (DrFundc1) with MitoTracker, a marker of mitochondria, and/or CellLight Lysosomes-GFP, a label of lysosomes, in the transfected ovary cells of grass carp. The use of Western blotting detected DrFundc1 as a component of mitochondrial proteins with endogenous COX IV, LC3B, and FUNDC1 in transgenic human embryonic kidney 293 T cells. DrFundc1 induced LC3B activation. The ectopic expression of Drfundc1 caused cell death and apoptosis as well as impairing cell proliferation in the 293 T cell line, as detected by Trypan blue, terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) and incorporation of BrdU. DrFundc1 up-regulated expression of both autophagy- and apoptosis-related genes, including ATG5, ATG7, LC3B, BECLIN1, and BAX in transgenic 293 T cells. A knockdown of Drfundc1 using short hairpin RNA (shRNA) led to midline bifurcation with two notochords and two spinal cords in zebrafish embryos. Co-injection of Drfundc1 mRNA repaired defects resulting from shRNA. Knockdown of Drfundc1 resulted in up- or down-regulation of genes related to autophagy and apoptosis, as well as decreased expression of neural genes such as cyclinD1, pax2a, opl, and neuroD1. In summary, DrFundc1 is a mitochondrial protein which is involved in mitophagy and is critical for typical body axis development in zebrafish.

Localization of Tfap2β, Casq2, Penk, Zic1, and Zic3 Expression in the Developing Retina, Muscle, and Sclera of the Embryonic Mouse Eye

Optic development involves sequential interactions between several different tissue types, including the overlying ectoderm, adjacent mesoderm, and neural crest mesenchyme and the neuroectoderm. In an ongoing expression screen, we identified that Tfap2β, Casq2, Penk, Zic1, and Zic3 are expressed in unique cell types in and around the developing eye. Tfap2β, Zic1, and Zic3 are transcription factors, Casq2 is a calcium binding protein and Penk is a neurotransmitter. Tfap2β, Zic1, and Zic3 have reported roles in brain and craniofacial development, while Casq2 and Penk have unknown roles. These five genes are expressed in the major tissue types in the eye, including the muscles, nerves, cornea, and sclera. Penk expression is found in the sclera and perichondrium. At E12.5 and E15.5, the extra-ocular muscles express Casq2, the entire neural retina expresses Zic1, and Zic3 is expressed in the optic disk and lip of the optic cup. The expression of Tfap2β expanded from corneal epithelium to the neural retina between E12.5 to E15.5. These genes are expressed in similar domains as Hedgehog (Gli1, and Ptch1) and the Wnt (Lef1) pathways. The expression patterns of these five genes warrant further study to determine their role in eye morphogenesis.

The transcription factor odd-paired regulates temporal identity in transit-amplifying neural progenitors via an incoherent feed-forward loop

Neural progenitors undergo temporal patterning to generate diverse neurons in a chronological order. This process is well-studied in the developing Drosophila brain and conserved in mammals. During larval stages, intermediate neural progenitors (INPs) serially express Dichaete (D), grainyhead (Grh) and eyeless (Ey/Pax6), but how the transitions are regulated is not precisely understood. Here, we developed a method to isolate transcriptomes of INPs in their distinct temporal states to identify a complete set of temporal patterning factors. Our analysis identifies odd-paired (opa), as a key regulator of temporal patterning. Temporal patterning is initiated when the SWI/SNF complex component Osa induces D and its repressor Opa at the same time but with distinct kinetics. Then, high Opa levels repress D to allow Grh transcription and progress to the next temporal state. We propose that Osa and its target genes opa and D form an incoherent feedforward loop (FFL) and a new mechanism allowing the successive expression of temporal identities.

The brain consists of billions of neurons that come in a range of shapes and sizes, with different types of neurons specialized to perform different tasks. Despite their diversity, all of these neurons originate from a single population known as neural stem cells. As the brain develops, each neural stem cell divides to produce two daughter cells: one remains a stem cell, which can then divide again, and the other becomes a neuron.

A longstanding question in developmental biology is how a limited pool of neural stem cells can generate so many different types of neurons. The answer seems to lie in a process known as temporal identity, whereby neural stem cells of different ages give rise to different types of neurons. This requires neural stem cells to keep track of their own age, but it is still unclear how they can do so.

Abdusselamoglu et al. have now uncovered part of the underlying mechanism behind temporal identity by studying fruit flies, an insect in which the early stages of brain development are similar to the ones in mammals. A method was developed to sort fly neural stem cells into groups based on their age. Comparing these groups revealed that a protein called Opa make neural stem cells switch from being 'young' to being 'middle-aged'. Another protein, Osa activates Opa, which in turn represses a protein called Dichaete. As Dichaete is mainly active in young neural stem cells, the actions of Osa and Opa push neural stem cells into middle age.

Fruit flies are therefore a valuable system with which to study the mechanisms that regulate neural stem cell aging. Revealing how the brain generates different types of neurons could help us study the way these cells organize themselves into complex circuits. This knowledge could then be harnessed to understand how these processes go wrong and disrupt development.

Nodal and Hedgehog synergize in gill slit formation during development of the cephalochordate Branchiostoma floridae

The larval pharynx of the cephalochordate Branchiostoma (amphioxus) is asymmetrical. The mouth is on the left, and endostyle and gill slits are on the right. At the neurula, Nodal and Hedgehog (Hh) expression becomes restricted to the left. To dissect their respective roles in gill slit formation, we inhibited each pathway separately for 20 min at intervals during the neurula stage, before gill slits penetrate, and monitored the effects on morphology and expression of pharyngeal markers. The results pinpoint the short interval spanning the gastrula/neurula transition as the critical period for specification and positioning of future gill slits. Thus, reduced Nodal signaling shifts the gill slits ventrally, skews the pharyngeal domains of Hh, Pax1/9, Pax2/5/8, Six1/2 and IrxC towards the left, and reduces Hh and Tbx1/10 expression in endoderm and mesoderm, respectively. Nodal auto-regulates. Decreased Hh signaling does not affect gill slit positions or Hh or Nodal expression, but it does reduce the domain of Gli, the Hh target, in the pharyngeal endoderm. Thus, during the neurula stage, Nodal and Hh cooperate in gill slit development - Hh mediates gill slit formation and Nodal establishes their left-right position.

Cellular rearrangement of the prechordal plate contributes to eye degeneration in the cavefish

Astyanax mexicanus consists of two different populations: a sighted surface-dwelling form (surface fish) and a blind cave-dwelling form (cavefish). In the cavefish, embryonic expression of sonic hedgehog a (shha) in the prechordal plate is expanded towards the anterior midline, which has been shown to contribute to cavefish specific traits such as eye degeneration, enhanced feeding apparatus, and specialized brain anatomy. However, it is not clear how this expanded expression is achieved and which signaling pathways are involved. Nodal signaling has a crucial role for expression of shh and formation of the prechordal plate. In this study, we report increased expression of prechordal plate marker genes, nodal-related 2 (ndr2) and goosecoid (gsc) in cavefish embryos at the tailbud stage. To investigate whether Nodal signaling is responsible for the anterior expansion of the prechordal plate, we used an inhibitor of Nodal signaling and showed a decreased anterior expansion of the prechordal plate and increased pax6 expression in the anterior midline in treated cavefish embryos. Later in development, the lens and optic cup of treated embryos were significantly larger than untreated embryos. Conversely, increasing Nodal signaling in the surface fish embryo resulted in the expansion of anterior prechordal plate and reduction of pax6 expression in the anterior neural plate together with the formation of small lenses and optic cups later in development. These results confirmed that Nodal signaling has a crucial role for the anterior expansion of the prechordal plate and plays a significant role in cavefish eye development. We showed that the anterior expansion of the prechordal plate was not due to increased total cell number, suggesting the expansion is achieved by changes in cellular distribution in the prechordal plate. In addition, the distribution of presumptive prechordal plate cells in Spemann's organiser was also altered in the cavefish. These results suggested that changes in the cellular arrangement of Spemann's organiser in early gastrulae could have an essential role in the anterior expansion of the prechordal plate contributing to eye degeneration in the cavefish.

Toxicity implications for early life stage Japanese medaka (Oryzias latipes) exposed to oxyfluorfen

We investigated the potential toxic effects of Oxyfluorfen (OXY), an herbicide used in agriculture, on the embryo-larval development of Japanese medaka fish (Oryzias latipes). Embryos (1-day postfertilization) and larvae (2-day posthatch) were exposed to OXY (0.5-8 mg/L) for 96 h and evaluated for mortality and hatching on embryos, and the mortality and growth on larvae during depuration. It was observed that the embryo-mortality was inconsistently altered by OXY; only the 2 mg/L group showed significant reduction on embryo survivability. However, larval-mortality was concentration-dependent and OXY exposure induced scoliosis-like phenotypic features in the surviving larvae; the calculated LC₅₀ was 5.238 mg/L. Our data further indicated that larval skeleton, both axial and appendicular, was the potential target site of OXY. Skeletal growth in larvae exposed to 2 mg/L was inhibited significantly until 1 week of depuration with regard to the linear lengths of neurocranium, Meckel's cartilage, caudal vertebrae (first 10) in the axial skeletons, and pectoral fin and urostyle in the appendicular skeletons. Moreover, the total protein content remained unaltered by OXY after 96 h exposure; while the RNA concentration was reduced significantly in larvae exposed to 2 mg/L. Expression analysis of several genes by quantitative real-time RT-PCR (RT-qPCR) showed significant upregulation of zic5, a zinc-finger type transcription regulator, at the transcription level. This study indicated that the scoliosis induced by OXY in Japanese medaka larvae was the result of stunted skeletal growth, probably because of deregulation of zinc-finger type transcription regulators, at the genomic level.

Link between the causative genes of holoprosencephaly: Zic2 directly regulates Tgif1 expression

One of the causal genes for holoprosencephaly (HPE) is ZIC2 (HPE5). It belongs to the zinc finger protein of the cerebellum (Zic) family of genes that share a C2H2-type zinc finger domain, similar to the GLI family of genes. In order to clarify the role of Zic2 in gene regulation, we searched for its direct target genes using chromatin immunoprecipitation (ChIP). We identified TGIF1 (HPE4), another holoprosencephaly-causative gene in humans. We identified Zic2-binding sites (ZBS) on the 5′ flanking region of Tgif1 by in vitro DNA binding assays. ZBS were essential for Zic2-dependent transcriptional activation in reporter gene assays. Zic2 showed a higher affinity to ZBS than GLI-binding sequences. Zic2-binding to the cis-regulatory element near the Tgif1 promoter may be involved in the mechanism underlying forebrain development and incidences of HPE.

ZIC3 in Heterotaxy

Mutation of ZIC3 causes X-linked heterotaxy, a syndrome in which the laterality of internal organs is disrupted. Analysis of model organisms and gene expression during early development suggests ZIC3-related heterotaxy occurs due to defects at the earliest stage of left-right axis formation. Although there are data to support abnormalities of the node and cilia as underlying causes, it is unclear at the molecular level why loss of ZIC3 function causes such these defects. ZIC3 has putative roles in a number of developmental signalling pathways that have distinct roles in establishing the left-right axis. This complicates the understanding of the mechanistic basis of Zic3 in early development and left-right patterning. Here we summarise our current understanding of ZIC3 function and describe the potential role ZIC3 plays in important signalling pathways and their links to heterotaxy.

Mis-expression of grainyhead-like transcription factors in zebrafish leads to defects in enveloping layer (EVL) integrity, cellular morphogenesis and axial extension

The grainyhead-like (grhl) transcription factors play crucial roles in craniofacial development, epithelial morphogenesis, neural tube closure, and dorso-ventral patterning. By utilising the zebrafish to differentially regulate expression of family members grhl2b and grhl3, we show that both genes regulate epithelial migration, particularly convergence-extension (CE) type movements, during embryogenesis. Genetic deletion of grhl3 via CRISPR/Cas9 results in failure to complete epiboly and pre-gastrulation embryonic rupture, whereas morpholino (MO)-mediated knockdown of grhl3 signalling leads to aberrant neural tube morphogenesis at the midbrain-hindbrain boundary (MHB), a phenotype likely due to a compromised overlying enveloping layer (EVL). Further disruptions of grhl3-dependent pathways (through co-knockdown of grhl3 with target genes spec1 and arhgef19) confirm significant MHB morphogenesis and neural tube closure defects. Concomitant MO-mediated disruption of both grhl2b and grhl3 results in further extensive CE-like defects in body patterning, notochord and somite morphogenesis. Interestingly, over-expression of either grhl2b or grhl3 also leads to numerous phenotypes consistent with disrupted cellular migration during gastrulation, including embryo dorsalisation, axial duplication and impaired neural tube migration leading to cyclopia. Taken together, our study ascribes novel roles to the Grhl family in the context of embryonic development and morphogenesis.

Zebrafish zic2 controls formation of periocular neural crest and choroid fissure morphogenesis

The vertebrate retina develops in close proximity to the forebrain and neural crest-derived cartilages of the face and jaw. Coloboma, a congenital eye malformation, is associated with aberrant forebrain development (holoprosencephaly) and with craniofacial defects (frontonasal dysplasia) in humans, suggesting a critical role for cross-lineage interactions during retinal morphogenesis. ZIC2, a zinc-finger transcription factor, is linked to human holoprosencephaly. We have previously used morpholino assays to show zebrafish zic2 functions in the developing forebrain, retina and craniofacial cartilage. We now report that zebrafish with genetic lesions in zebrafish zic2 orthologs, zic2a and zic2b, develop with retinal coloboma and craniofacial anomalies. We demonstrate a requirement for zic2 in restricting pax2a expression and show evidence that zic2 function limits Hh signaling. RNA-seq transcriptome analysis identified an early requirement for zic2 in periocular neural crest as an activator of alx1, a transcription factor with essential roles in craniofacial and ocular morphogenesis in human and zebrafish. Collectively, these data establish zic2 mutant zebrafish as a powerful new genetic model for in-depth dissection of cell interactions and genetic controls during craniofacial complex development.

Phosphatidylinositol glycan anchor biosynthesis, class X containing complex promotes cancer cell proliferation through suppression of EHD2 and ZIC1, putative tumor suppressors

We identified phosphatidylinositol glycan anchor biosynthesis, class X (PIGX), which plays a critical role in the biosynthetic pathway of glycosylphosphatidylinositol (GPI)-anchor motif, to be upregulated highly and frequently in breast cancer cells. Knockdown of PIGX as well as reticulocalbin 1 (RCN1) and reticulocalbin 2 (RCN2), which we found to interact with PIGX and was indicated to regulate calcium-dependent activities, significantly suppressed the growth of breast cancer cells. We also identified PIGX to be a core protein in an RCN1/PIGX/RCN2 complex. Microarray analysis revealed that the expression of two putative tumor suppressor genes, Zic family member 1 (ZIC1) and EH-domain containing 2 (EHD2), were upregulated commonly in cells in which PIGX, RCN1, or RCN2 was knocked down, suggesting that this RCN1/PIGX/RCN2 complex could negatively regulate the expression of these two genes and thereby contribute to human breast carcinogenesis. Our results imply that PIGX may be a good candidate molecule for development of novel anticancer drugs for breast cancer.

Initiation of cyp26a1 Expression in the Zebrafish Anterior Neural Plate by a Novel Cis-Acting Element

Early patterning of the vertebrate neural plate involves a complex hierarchy of inductive interactions orchestrated by signalling molecules and their antagonists. The morphogen retinoic acid, together with the Cyp26 enzymes which degrade it, play a central role in this process. The cyp26a1 gene expressed in the anterior neural plate thus contributes to the fine modulation of the rostrocaudal retinoic acid gradient. Despite this important role of cyp26a1 in early brain formation, the mechanisms that control its expression in the anterior neural plate are totally unknown. Here, we present the isolation of a 310-base-pair DNA element adjacent to cyp26a1 promoter, displaying enhancer activity restricted to the anterior neural plate of the zebrafish gastrula. We show that unlike that of cyp26a1, expression driven by this cyp26a1 anterior neural plate element (cANE) is independent of retinoic acid. Through deletion analysis, we identify a 12-nucleotide motif essential for cANE activity. A consensus bipartite binding site for SoxB:Oct transcription factors overlaps with this motif. Mutational analysis suggests that SoxB binding is essential for its activity. We discuss the contribution of this study to the elucidation of the regulatory hierarchy involved in early neural plate patterning.

The chick embryo as a model for the effects of prenatal exposure to alcohol on craniofacial development

Prenatal exposure to ethanol results in fetal alcohol spectrum disorder (FASD), a syndrome characterised by a broad range of clinical manifestations including craniofacial dysmorphologies and neurological defects. The characterisation of the mechanisms by which ethanol exerts its teratogenic effects is difficult due to the pleiotropic nature of its actions. Different experimental model systems have been employed to investigate the aetiology of FASD. Here, I will review studies using these different model organisms that have helped to elucidate how ethanol causes the craniofacial abnormalities characteristic of FASD. In these studies, ethanol was found to impair the prechordal plate-an important embryonic signalling centre-during gastrulation and to negatively affect the induction, migration and survival of the neural crest, a cell population that generates the cartilage and most of the bones of the skull. At the cellular level, ethanol appears to inhibit Sonic hedgehog signalling, alter levels of retionoic acid activity, trigger a Ca(2+)-CamKII-dependent pathway that antagonises WNT signalling, affect cytoskeletal dynamics and increase oxidative stress. Embryos of the domestic chick Gallus gallus domesticus have played a central role in developing a working model for the effects of ethanol on craniofacial development because they are easily accessible and because key steps in craniofacial development are particularly well established in the avian embryo. I will finish this review by highlighting some potential future avenues of fetal alcohol research.

Zic1 controls placode progenitor formation non-cell autonomously by regulating retinoic acid production and transport

All cranial placode progenitors arise from a common precursor field anterior to the neural plate, the pre-placodal region (PPR). We showed that transcription factor Zic1, expressed at the anterior neural plate, is necessary and sufficient to promote placode fate. Here we reveal the non-cell autonomous activity of Zic1 and implicate retinoic acid (RA) signalling as a key player in cranial placode progenitor specification. In a screen for genes activated by Zic1, we identify several factors involved in RA metabolism and function. Among them we show that retinaldehyde dehydrogenase 2 (RALDH2) and lipocalin-type prostaglandin D2 synthase (LPGDS), which, respectively, regulate the synthesis and transport of RA, directly participate in the establishment of the PPR. We propose that RALDH2 and LPGDS induction by Zic1 at the anterior neural plate allows for the localized production and transport of RA, which in turn activates a cranial placode developmental programme in neighbouring cells.

Advances in the targeted therapy of liposarcoma

Liposarcoma (LPS) is the most common type of soft-tissue sarcoma. Complete surgical resection is the only curative means for localized disease; however, both radiation and conventional cytotoxic chemotherapy remain controversial for metastatic or unresectable disease. An increasing number of trials with novel targeted therapy of LPS have provided encouraging data during recent years. This review will provide an overview of the advances in our understanding of LPS and summarize the results of recent trials with novel therapies targeting different genetic and molecular aberrations for different subtypes of LPS.

An adverse outcome pathway framework for neural tube and axial defects mediated by modulation of retinoic acid homeostasis

Developmental toxicity can be caused through a multitude of mechanisms and can therefore not be captured through a single simple mechanistic paradigm. However, it may be possible to define a selected group of overarching mechanisms that might allow detection of the vast majority of developmental toxicants. Against this background, we have explored the usefulness of retinoic acid mediated regulation of neural tube and axial patterning as a general mechanism that, when perturbed, may result in manifestations of developmental toxicity that may cover a large part of malformations known to occur in experimental animals and in man. Through a literature survey, we have identified key genes in the regulation of retinoic acid homeostasis, as well as marker genes of neural tube and axial patterning, that may be used to detect developmental toxicants in in vitro systems. A retinoic acid-neural tube/axial patterning adverse outcome pathway (RA-NTA AOP) framework was designed. The framework was tested against existing data of flusilazole exposure in the rat whole embryo culture, the zebrafish embryotoxicity test, and the embryonic stem cell test. Flusilazole is known to interact with retinoic acid homeostasis, and induced common and unique NTA marker gene changes in the three test systems. Flusilazole-induced changes were similar in directionality to gene expression responses after retinoic acid exposure. It is suggested that the RA-NTA framework may provide a general tool to define mechanistic pathways and biomarkers of developmental toxicity that may be used in alternative in vitro assays for the detection of embryotoxic compounds.

zic-1 Expression in Planarian neoblasts after injury controls anterior pole regeneration

Mechanisms that enable injury responses to prompt regenerative outgrowth are not well understood. Planarians can regenerate essentially any tissue removed by wounding, even after decapitation, due to robust regulation of adult pluripotent stem cells of the neoblast population. Formation of pole signaling centers involving Wnt inhibitors or Wnt ligands promotes head or tail regeneration, respectively, and this process requires the use of neoblasts early after injury. We used expression profiling of purified neoblasts to identify factors needed for anterior pole formation. Using this approach, we identified zic-1, a Zic-family transcription factor, as transcriptionally activated in a subpopulation of neoblasts near wound sites early in head regeneration. As head regeneration proceeds, the Wnt inhibitor notum becomes expressed in the newly forming anterior pole in zic-1-expressing cells descended from neoblasts. Inhibition of zic-1 by RNAi resulted in a failure to express notum at the anterior pole and to regenerate a head, but did not affect tail regeneration. Both injury and canonical Wnt signaling inhibition are required for zic-1 expression, and double-RNAi experiments suggest zic-1 inhibits Wnt signaling to allow head regeneration. Analysis of neoblast fate determinants revealed that zic-1 controls specification of notum-expressing cells from foxD-expressing neoblasts to form the anterior pole, which organizes subsequent outgrowth. Specialized differentiation programs may in general underlie injury-dependent formation of tissue organizing centers used for regenerative outgrowth.

Some animals are capable of regenerating organs damaged or removed by injury, and this ability likely requires precise control of secreted proteins that promote growth. Planarians are flatworms that can regenerate any missing tissues by regulating the activity of adult stem cells that can produce any specialized cell type. We identify the zic-1 gene as activated in planarian stem cells by injury and needed for head regeneration after decapitation. This gene's product likely acts as a transcription factor to produce cells that secrete a growth-promoting protein, NOTUM, at the tip of the regenerating tissue outgrowth to organize and enable head regeneration. These results suggest that regeneration requires specialized uses of stem cell descendants to orchestrate new tissue production following injury.

Rare nasal cleft in a patient with holoprosencephaly due to a mutation in the ZIC2 gene

BACKGROUND

Holoprosencephaly (HPE) is a spectrum of midline malformations of the prosencephalon generally reflected in a continuum of midline facial anomalies. Patients with mutation in the ZIC2 gene usually present a normal or mildly dysmorphic face associated with a severe brain malformation. Here we present a rare unilateral nasal cleft (Tessier cleft n. 1) with holoprosencephaly in a patient with a ZIC2 mutation.

CASE

The male newborn presented with alobar HPE, microcephaly, ocular hypertelorism, upslanting palpebral fissures, a bulky nose with a left paramedian alar cleft. Mutational screening for HPE genes revealed the occurrence of a frameshift mutation in the ZIC2 gene. The mutation was inherited from the father who presented only mild ocular hypotelorism but had an affected child with HPE from his first marriage.

CONCLUSION

The occurrence of oral clefts is common in patients with HPE, but unusual in patients with mutation in the ZIC2 gene. To our knowledge, clefts of the nasal alae have been reported only once or twice in patients with ZIC2 mutations. In documented patients from the literature, only 2% of individuals with described pathogenic mutations in the ZIC2 gene (3/171) presented facial clefts, one of them a nasal cleft, while common oral clefts were observed in 27% of individuals (7/26) described with nonpathogenic ZIC2 mutations or presenting a concomitant mutation in another HPE gene. When compared with the general population, nasal clefts are common in ZIC2 mutations and these mutations must be searched for in undiagnosed cases.

Virulence of infecting Helicobacter pylori strains and intensity of mononuclear cell infiltration are associated with levels of DNA hypermethylation in gastric mucosae

DNA methylation changes are known to occur in gastric cancers and in premalignant lesions of the gastric mucosae. In order to examine variables associated with methylation levels, we quantitatively evaluated DNA methylation in tumors, non-tumor gastric mucosae, and in gastric biopsies at promoters of 5 genes with methylation alterations that discriminate gastric cancers from non-tumor epithelia (EN1, PCDH10, RSPO2, ZIC1, and ZNF610). Among Colombian subjects at high and low risk for gastric cancer, biopsies from subjects from the high-risk region had significantly higher levels of methylation at these 5 genes than samples from subjects in the low risk region (p ≤ 0.003). When results were stratified by Helicobacter pylori infection status, infection with a cagA positive, vacA s1m1 strain was significantly associated with highest methylation levels, compared with other strains (p = 0.024 to 0.001). More severe gastric inflammation and more advanced precancerous lesions were also associated with higher levels of DNA methylation (p ≤ 0.001). In a multivariate model, location of residence of the subject and the presence of cagA and vacA s1m1 in the H. pylori strain were independent variables associated with higher methylation in all 5 genes. High levels of mononuclear cell infiltration were significantly related to methylation in PCDH10, RSPO2, and ZIC1 genes. These results indicate that for these genes, levels of methylation in precancerous lesions are related to H. pylori virulence, geographic region and measures of chronic inflammation. These genes seem predisposed to sustain significant quantitative changes in DNA methylation at early stages of the gastric precancerous process.

The role of Zic transcription factors in regulating hindbrain retinoic acid signaling

Background

The reiterated architecture of cranial motor neurons aligns with the segmented structure of the embryonic vertebrate hindbrain. Anterior-posterior identity of cranial motor neurons depends, in part, on retinoic acid signaling levels. The early vertebrate embryo maintains a balance between retinoic acid synthetic and degradative zones on the basis of reciprocal expression domains of the retinoic acid synthesis gene aldhehyde dehydrogenase 1a2 (aldh1a2) posteriorly and the oxidative gene cytochrome p450 type 26a1 (cyp26a1) in the forebrain, midbrain, and anterior hindbrain.

Results

This manuscript investigates the role of zinc finger of the cerebellum (zic) transcription factors in regulating levels of retinoic acid and differentiation of cranial motor neurons. Depletion of zebrafish Zic2a and Zic2b results in a strong downregulation of aldh1a2 expression and a concomitant reduction in activity of a retinoid-dependent transgene. The vagal motor neuron phenotype caused by loss of Zic2a/2b mimics a depletion of Aldh1a2 and is rescued by exogenously supplied retinoic acid.

Conclusion

Zic transcription factors function in patterning hindbrain motor neurons through their regulation of embryonic retinoic acid signaling.

Quantitative proteomics reveals diverse roles of miR-148a from gastric cancer progression to neurological development

MicroRNAs (miRNAs) are noncoding RNAs that control gene expression either by degradation of mRNAs or inhibition of protein translation. miR-148a has been reported to have the impacts on tumor progression. Here, a quantitative proteomics combined with stable isotope labeling was applied to identify the global profile of miR-148a-regulated downstream proteins. The data have been deposited to the ProteomeXchange with identifier PXD000190. A total of 2938 proteins were quantified, and 55 proteins were considered to be regulated by miR-148a. We found that not only proteins associated with cancer progression but also molecules involved in neural development were regulated by miR-148a. This study is the first to identify the function of miR-148a in neural development by using a proteomic approach. Analysis of a public clinical database also showed that the patients with neural diseases could display abnormal expression of miR-148a. Moreover, silencing of miR-148a led to the abnormal morphology and decreased expression of neuron-related markers in the developing brain of zebrafish. These results provided important insight into the regulation of neurological development elicited by miR-148a.

Spatiotemporal expression of Zic genes during vertebrate inner ear development

BACKGROUND

Inner ear development involves signaling from surrounding tissues, including the adjacent hindbrain, periotic mesenchyme, and notochord. These signals include SHH, FGFs, BMPs, and WNTs from the hindbrain and SHH from the notochord. Zic genes, which are expressed in the dorsal neural tube and act during neural development, have been implicated as effectors of these pathways. This report examines whether Zic genes' involvement in inner ear development is a tenable hypothesis based on their expression patterns.

RESULTS

In the developing inner ear of both the chick and mouse, all of the Zic genes were expressed in the dorsal neural tube and variably in the periotic mesenchyme, but expression of the Zic genes in the otic epithelium was not found. The onset of expression differed among the Zic genes; within any given region surrounding the otic epithelium, multiple Zic genes were expressed in the same place at the same time.

CONCLUSIONS

Zic gene expression in the region of the developing inner ear is similar between mouse and chick. Zic expression domains overlap with sites of WNT and SHH signaling during otocyst patterning, suggesting a role for Zic genes in modulating signaling from these pathways.

Zebrafish Zic2a and Zic2b regulate neural crest and craniofacial development

Holoprosencephaly (HPE), the most common malformation of the human forebrain, is associated with defects of the craniofacial skeleton. ZIC2, a zinc-finger transcription factor, is strongly linked to HPE and to a characteristic set of dysmorphic facial features in humans. We have previously identified important functions for zebrafish Zic2 in the developing forebrain. Here, we demonstrate that ZIC2 orthologs zic2a and zic2b also regulate the forming zebrafish craniofacial skeleton, including the jaw and neurocranial cartilages, and use the zebrafish to study Zic2-regulated processes that may contribute to the complex etiology of HPE. Using temporally controlled Zic2a overexpression, we show that the developing craniofacial cartilages are sensitive to Zic2 elevation prior to 24hpf. This window of sensitivity overlaps the critical expansion and migration of the neural crest (NC) cells, which migrate from the developing neural tube to populate vertebrate craniofacial structures. We demonstrate that zic2b influences the induction of NC at the neural plate border, while both zic2a and zic2b regulate NC migratory onset and strongly contribute to chromatophore development. Both Zic2 depletion and early ectopic Zic2 expression cause moderate, incompletely penetrant mispatterning of the NC-derived jaw precursors at 24hpf, yet by 2dpf these changes in Zic2 expression result in profoundly mispatterned chondrogenic condensations. We attribute this discrepancy to an additional role for Zic2a and Zic2b in patterning the forebrain primordium, an important signaling source during craniofacial development. This hypothesis is supported by evidence that transplanted Zic2-deficient cells can contribute to craniofacial cartilages in a wild-type background. Collectively, these data suggest that zebrafish Zic2 plays a dual role during craniofacial development, contributing to two disparate aspects of craniofacial morphogenesis: (1) neural crest induction and migration, and (2) early patterning of tissues adjacent to craniofacial chondrogenic condensations.

ZIC1 modulates cell-cycle distributions and cell migration through regulation of sonic hedgehog, PI(3)K and MAPK signaling pathways in gastric cancer

Background

ZIC1, a vital transcription factor with zinc finger domains, has been implicated in the process of neural development. We previously showed that ZIC1 may function as a tumour suppressor in gastrointestinal cancers. However, the molecular mechanism underlying ZIC1 participation in tumour progression remains unknown.

Methods

The role of ZIC1 on cell proliferation and migration was examined. The regulation of sonic hedgehog (Shh), phosphoinositide 3-kinase (PI₃K) and mitogen-activated protein kinase (MAPK) signaling pathways after ectopic expression of ZIC1 in gastric cancer cells were evaluated.

Results

Overexpression of ZIC1 contributes to the inhibition of cell proliferation migration and cell-cycle distribution in gastric cancer. The modulation of G1/S checkpoint by ZIC1 is mainly mediated through the regulation of cyclin-dependent kinases (p21 ^Waf1/Cip1, p27 ^Kip1 and cyclin D1). In addition, ZIC1 can inactivate the level of phospholated Akt and Erk1/2, and transcriptionally regulate sonic hedgehog (Shh) signaling, thus leading to regulate the expression of p21 ^Waf1/Cip1 and cyclin D1. Finally, we have systemically identified ZIC1 downstream targets by cDNA microarray analysis and revealed that 132 genes are down-regulated and 66 genes are up-regulated after transfection with ZIC1 in gastric cancer cells. These candidate genes play critical roles in cell proliferation, cell cycle and cell motility.

Conclusions

Overexpression of ZIC1 results in inactivation of Shh, PI₃K and MAPK signaling pathways, as well as regulation of multiple downstream targets which are essential for the development and progression of gastric cancer. ZIC1 serves as a potential therapeutic target for gastric cancer.

Expression pattern and functions of autophagy-related gene atg5 in zebrafish organogenesis

The implications of autophagy-related genes in serious neural degenerative diseases have been well documented. However, the functions and regulation of the family genes in embryonic development remain to be rigorously studied. Here, we report on for the first time the important role of atg5 gene in zebrafish neurogenesis and organogenesis as evidenced by the spatiotemporal expression pattern and functional analysis. Using morpholino oligo knockdown and mRNA overexpression, we demonstrated that zebrafish atg5 is required for normal morphogenesis of brain regionalization and body plan as well as for expression regulation of neural gene markers: gli1, huC, nkx2.2, pink1, β-synuclein, xb51 and zic1. We further demonstrated that ATG5 protein is involved in autophagy by LC3-II/LC3I ratio and rapamycin-induction experiments, and that ATG5 is capable of regulating expression of itself gene in the manner of a feedback inhibition loop. In addition, we found that expression of another autophagy-related gene, atg12, is maintained at a higher constant level like a housekeeping gene. This indicates that the formation of the ATG12–ATG5 conjugate may be dependent on ATG5 protein generation and its splicing, rather than on ATG12 protein in zebrafish. Importantly, in the present study, we provide a mechanistic insight into the regulation and functional roles of atg5 in development of zebrafish nervous system.

Gli protein nuclear localization signal

Drosophila cubitus interruptus (Ci) and its vertebrate homologues, the glioblastoma (Gli) protein family, are the transcription factors belonging to the metazoan Gli/Glis/Zic ZF protein superfamily that shares similar five tandemly repeated C2H2-type zinc finger (ZF) motifs. Nuclear transport of Gli/Ci proteins is regulated by hedgehog (Hh) signaling and is an essential part of the Hh signal transduction pathway. Gli/Ci proteins possess a nuclear localization signal (NLS) and a nuclear export signal (NES), both of which are key signatures for controlling nucleocytoplasmic shuttling. The NLS of the Gli/Ci proteins has been mapped to the fifth ZF domain and its C-terminal side. It contains two clusters of basic residues (classical bipartite-type), which are conserved in metazoan Gli/Ci homologues, but which partially deviate from the intra-ZF domain NLSs in the Glis and Zic proteins. Recently, Importin α3 was identified as a nuclear transport protein for Ci. When we modeled the 3D structure of the Gli NLS-Importin α complex, the two basic clusters were predicted to fit in the two binding interfaces of Importin α. The mechanisms controlling the function of NLSs and NESs involve the elimination of the NES by Hh signaling-dependent protein cleavage in the Ci and the Gli3 proteins, and the phosphorylation of a threonine residue close to the NLS in Gli1. Both processes depend on the activity of protein kinase A, which has a critical role in Hh signaling in fly wing discs. In addition, the Roadkill protein, a substrate recognition component of E3 ubiquitin ligase, competes with the Ci protein to interact with Importin α3 resulting in inhibition of Ci protein nuclear import.

ZIC1 is downregulated through promoter hypermethylation, and functions as a tumor suppressor gene in colorectal cancer

The transcription factor, Zinc finger of the cerebellum (ZIC1), plays a crucial role in vertebrate development. Recently, ZIC1 has also been found to participate in the progression of human cancers, including medulloblastomas, endometrial cancers, and mesenchymal neoplasms. However, the function of ZIC1 in colon cancer progression has not been defined. In this study, we demonstrate ZIC1 to be silenced or significantly downregulated in colon cancer cell lines. These effects were reversed by demethylation treatment with 5-aza-2'-deoxycytidine (Aza). ZIC1 expression is also significantly downregulated in primary colorectal cancer tissues relative to adjacent non-tumor tissues (p = 0.0001). Furthermore, methylation of ZIC1 gene promoter is frequently detected in primary tumor tissues (85%, 34/40), but not in adjacent non-tumor tissues. Ectopic expression of ZIC1 suppresses cell proliferation and induces apoptosis, which is associated with MAPK and PI(3)K/Akt pathways, as well as the Bcl-xl/Bad/Caspase3 cascade. To identify target candidates of ZIC1, we employed cDNA microarray and found that 337 genes are downregulated and 95 genes upregulated by ectopic expression of ZIC1, which were verified by 10 selected gene expressions by qRT-PCR. Taken together, our results suggest that ZIC1 may potentially function as a tumor suppressor gene, which is downregulated through promoter hypermethylation in colorectal cancers.

The germinal zones of the basal ganglia but not the septum generate GABAergic interneurons for the cortex

Cortical interneurons originate from subpallial precursors and migrate into the cortex during development. Using genetic lineage tracing in transgenic mice we examine the contribution of two germinal zones, the septum and the lateral ganglionic eminence/caudal ganglionic eminence (LGE/CGE) to interneurons of the cortex. We find that the septal neuroepithelium does not generate interneurons for the neocortex. There is, however, clear migration of cells from the LGE/CGE to the cortex. Comparison of the dynamics of cortical colonization by the two major cohorts of interneurons originating in the medial ganglionic eminence (MGE) and the LGE/CGE has shown differences in the timing of migration and initial route of entry into the cortex. LGE/CGE-derived interneurons enter the cortex later than the MGE-derived ones. They invade the cortex through the subventricular/intermediate zone route and only later disperse within the cortical plate and the marginal zone. During the first postnatal week MGE interneurons move extensively to acquire their laminar position within the cortical plate whereas LGE/CGE-derived cells remain largely within the upper layers of the cortex. The two populations intermingle in the adult cortex but have distinct neurochemical properties and different overall distributions. LGE/CGE-derived interneurons account for one third of the total GABAergic interneuron population in the adult cortex.

Aberrant forebrain signaling during early development underlies the generation of holoprosencephaly and coloboma

In this review, we highlight recent literature concerning the signaling mechanisms underlying the development of two neural birth defects, holoprosencephaly and coloboma. Holoprosencephaly, the most common forebrain defect, occurs when the cerebral hemispheres fail to separate and is typically associated with mispatterning of embryonic midline tissue. Coloboma results when the choroid fissure in the eye fails to close. It is clear that Sonic hedgehog (Shh) signaling regulates both forebrain and eye development, with defects in Shh, or components of the Shh signaling cascade leading to the generation of both birth defects. In addition, other intercellular signaling pathways are known factors in the incidence of holoprosencephaly and coloboma. This review will outline recent advances in our understanding of forebrain and eye embryonic pattern formation, with a focus on zebrafish studies of Shh and retinoic acid pathways. Given the clear overlap in the mechanisms that generate both diseases, we propose that holoprosencephaly and coloboma can represent mild and severe aspects of single phenotypic spectrum resulting from aberrant forebrain development. This article is part of a Special Issue entitled Zebrafish Models of Neurological Diseases.

Expression of the zic1, zic2, zic3, and zic4 genes in early chick embryos

Background

The zic genes encode a family of transcription factors with important roles during early development. Since little is known about zic gene expression in chick embryos, we have characterized the expression patterns of the zic1, zic2, zic3, and zic4 (zic1-4) genes during neurulation and somitogenesis.

Findings

We used in situ hybridization to analyze the expression patterns of the zic1-4 genes during early chick development (HH stages 7-19). The zic1-3 genes showed both overlapping and gene-specific expression patterns along the length of the dorsal neural tube and in the dorsal parts of the somites. In addition, unique expression domains of zic genes included: zic2 in the neural plate, periotic mesoderm and limb buds; zic3 in the paraxial mesoderm surrounding the neural plate, in presomitic mesoderm and in the most recently formed epithelial somites; zic2 and zic3 in developing eyes. zic4 expression was limited to dorsal fore- and midbrain regions and, unlike the expression of the zic1-3 genes, zic4 expression was not detected in the hindbrain and trunk. This was in contrast to more extensive zic4 expression in other vertebrates.

Conclusions

The zic1-3 genes were expressed in both overlapping and unique domains within the neural tube, somites and other ectoderm and mesoderm-derived structures in the future head and trunk. zic4 expression, however, was limited to dorso-anterior regions of the future brain. This is the first comprehensive study of zic1-4 gene expression in chick embryos during neurulation and somitogenesis.

B1 SOX coordinate cell specification with patterning and morphogenesis in the early zebrafish embryo

The B1 SOX transcription factors SOX1/2/3/19 have been implicated in various processes of early embryogenesis. However, their regulatory functions in stages from the blastula to early neurula remain largely unknown, primarily because loss-of-function studies have not been informative to date. In our present study, we systematically knocked down the B1 sox genes in zebrafish. Only the quadruple knockdown of the four B1 sox genes sox2/3/19a/19b resulted in very severe developmental abnormalities, confirming that the B1 sox genes are functionally redundant. We characterized the sox2/3/19a/19b quadruple knockdown embryos in detail by examining the changes in gene expression through in situ hybridization, RT–PCR, and microarray analyses. Importantly, these phenotypic analyses revealed that the B1 SOX proteins regulate the following distinct processes: (1) early dorsoventral patterning by controlling bmp2b/7; (2) gastrulation movements via the regulation of pcdh18a/18b and wnt11, a non-canonical Wnt ligand gene; (3) neural differentiation by regulating the Hes-class bHLH gene her3 and the proneural-class bHLH genes neurog1 (positively) and ascl1a (negatively), and regional transcription factor genes, e.g., hesx1, zic1, and rx3; and (4) neural patterning by regulating signaling pathway genes, cyp26a1 in RA signaling, oep in Nodal signaling, shh, and mdkb. Chromatin immunoprecipitation analysis of the her3, hesx1, neurog1, pcdh18a, and cyp26a1 genes further suggests a direct regulation of these genes by B1 SOX. We also found an interesting overlap between the early phenotypes of the B1 sox quadruple knockdown embryos and the maternal-zygotic spg embryos that are devoid of pou5f1 activity. These findings indicate that the B1 SOX proteins control a wide range of developmental regulators in the early embryo through partnering in part with Pou5f1 and possibly with other factors, and suggest that the B1 sox functions are central to coordinating cell fate specification with patterning and morphogenetic processes occurring in the early embryo.

In the developing embryo, various processes such as cell fate specification, embryo patterning, and morphogenesis take place concurrently. The embryo must control gene expression in order to coordinate these processes and thereby enable the proper organization of its structures. The B1 sox transcription factor genes, exemplified by the “stem cell gene” sox2, are thought to play a key role in these embryonic processes from the blastoderm stage to the neural stage. However, the precise regulatory functions of these genes are largely unknown due to the lack of loss-of-function studies. In our current study, we took advantage of the zebrafish system and successfully depleted B1 sox activity from the early embryo using antisense knockdown technology. This approach enabled us to further uncover the regulatory functions of B1 sox in early embryos. We found that the activity of the B1 sox genes is required for the expression of a wide range of developmental regulators including transcription factors, signaling pathway components, and cell adhesion molecules. These findings suggest that the B1 sox functions are central to coordinating diverse embryonic processes, particularly those that occur during the development of the primordium of the central nervous system.

Genesis of teratogen-induced holoprosencephaly in mice

Evidence from mechanical, teratological, and genetic experimentation demonstrates that holoprosencephaly (HPE) typically results from insult prior to the time that neural tube closure is completed and occurs as a consequence of direct or indirect insult to the rostral prechordal cells that induce the forebrain or insult to the median forebrain tissue, itself. Here, we provide an overview of normal embryonic morphogenesis during the critical window for HPE induction, focusing on the morphology and positional relationship of the developing brain and subjacent prechordal plate and prechordal mesoderm cell populations. Subsequent morphogenesis of the HPE spectrum is then examined in selected teratogenesis mouse models. The temporal profile of Sonic Hedgehog expression in rostral embryonic cell populations and evidence for direct or indirect perturbation of the Hedgehog pathway by teratogenic agents in the genesis of HPE is highlighted. Emerging opportunities based on recent insights and new techniques to further characterize the mechanisms and pathogenesis of HPE are discussed.

The molecular genetics of holoprosencephaly

Holoprosencephaly (HPE) has captivated the imagination of Man for millennia because its most extreme manifestation, the single-eyed cyclopic newborn infant, brings to mind the fantastical creature Cyclops from Greek mythology. Attempting to understand this common malformation of the forebrain in modern medical terms requires a systematic synthesis of genetic, cytogenetic, and environmental information typical for studies of a complex disorder. However, even with the advances in our understanding of HPE in recent years, there are significant obstacles remaining to fully understand its heterogeneity and extensive variability in phenotype. General lessons learned from HPE will likely be applicable to other malformation syndromes. Here we outline the common, and rare, genetic and environmental influences on this conserved developmental program of forebrain development and illustrate the similarities and differences between these malformations in humans and those of animal models.

Zebrafish zic2a patterns the forebrain through modulation of Hedgehog-activated gene expression

Holoprosencephaly (HPE) is the most common congenital malformation of the forebrain in human. Several genes with essential roles during forebrain development have been identified because they cause HPE when mutated. Among these are genes that encode the secreted growth factor Sonic hedgehog (Shh) and the transcription factors Six3 and Zic2. In the mouse, Six3 and Shh activate each other's transcription, but a role for Zic2 in this interaction has not been tested. We demonstrate that in zebrafish, as in mouse, Hh signaling activates transcription of six3b in the developing forebrain. zic2a is also activated by Hh signaling, and represses six3b non-cell-autonomously, i.e. outside of its own expression domain, probably through limiting Hh signaling. Zic2a repression of six3b is essential for the correct formation of the prethalamus. The diencephalon-derived optic stalk (OS) and neural retina are also patterned in response to Hh signaling. We show that zebrafish Zic2a limits transcription of the Hh targets pax2a and fgf8a in the OS and retina. The effects of Zic2a depletion in the forebrain and in the OS and retina are rescued by blocking Hh signaling or by increasing levels of the Hh antagonist Hhip, suggesting that in both tissues Zic2a acts to attenuate the effects of Hh signaling. These data uncover a novel, essential role for Zic2a as a modulator of Hh-activated gene expression in the developing forebrain and advance our understanding of a key gene regulatory network that, when disrupted, causes HPE.