Genomically integrated transgenes are stably and conditionally expressed in neural crest cell-specific lineages

Leveraging chicken embryos for studying human enhancers

The dynamic activity of complex gene regulatory networks stands at the core of all cellular functions that define cell identity and behaviour. Gene regulatory networks comprise transcriptional enhancers, acted upon by cell-specific transcription factors to control gene expression in a spatial and temporal specific manner. Enhancers are found in the non-coding genome; pathogenic variants can disrupt enhancer activity and lead to disease. Correlating non-coding variants with aberrant enhancer activity remains a significant challenge. Due to their clinical significance, there is a longstanding interest in understanding enhancer function during early embryogenesis. With the onset of the omics era, it is now feasible to identify putative tissue-specific enhancers from epigenome data. However, such predictions in vivo require validation. The early stages of chick embryogenesis closely parallel those of human, offering an accessible in vivo model in which to assess the activity of putative human enhancer sequences. This review explores the unique advantages and recent advancements in employing chicken embryos to elucidate the activity of human transcriptional enhancers and the potential implications of these findings in human disease.

Insights into digit evolution from a fate map study of the forearm using Chameleon, a new transgenic chicken line

The cellular and genetic networks that contribute to the development of the zeugopod (radius and ulna of the forearm, tibia and fibula of the leg) are not well understood, although these bones are susceptible to loss in congenital human syndromes and to the action of teratogens such as thalidomide. Using a new fate-mapping approach with the Chameleon transgenic chicken line, we show that there is a small contribution of SHH-expressing cells to the posterior ulna, posterior carpals and digit 3. We establish that although the majority of the ulna develops in response to paracrine SHH signalling in both the chicken and mouse, there are differences in the contribution of SHH-expressing cells between mouse and chicken as well as between the chicken ulna and fibula. This is evidence that, although zeugopod bones are clearly homologous according to the fossil record, the gene regulatory networks that contribute to their development and evolution are not fixed.

Measuring transcription factor function with cell type-specific somatic transgenesis in chicken embryos

Retroviral-mediated misexpression in chicken embryos has been a powerful research tool for developmental biologists in the last two decades. In the RCASBP retroviral vectors that are widely used for in vivo somatic transgenesis, a coding sequence of interest is under the transcriptional control of a strong viral promoter in the long terminal repeat. While this has proven to be effective for studying secreted signalling proteins, interpretation of the mechanisms of action of nuclear factors is more difficult using this system since it is not clear whether phenotypic effects are cell-autonomous or not, and therefore whether they represent a function of the endogenous protein. Here, we report the consequences of retroviral expression using the RCANBP backbone, in which the transcription factor Dlx5 is expressed under the control of chondrocyte-specific regulatory sequences from the Col2a1 gene. To our knowledge, this is the first demonstration of a tissue-specific phenotype in the chicken embryo.

The cardiac neural crest gene MafB ectopically directs CXCR4 expression in the trunk neural crest

The cardiac neural crest is a subpopulation of cells arising from the caudal hindbrain. The delaminated cardiac neural crest cells migrate to the heart using the CXCR/SDF1 chemokine signaling system. These cells contribute to the formation of the cardiovascular system, including the septation of the outflow tract, which is unique to these cells. Here, we investigated the effect of ectopic expression of the cardiac neural crest gene MafB on trunk neural crest cells. First, we found that MafB has the potential to activate its own cis-regulatory element in enteric and trunk neural crest cells but not in cranial neural crest cells. Forced expression of two cardiac neural crest genes, Ets1 and Sox8, together with or without MafB, induced ectopic Sox10E2 enhancer activity in the trunk region. Finally, we uncovered that the expression of MafB, Ets1 and Sox8 can induce ectopic CXCR4 expression in the trunk neural crest cells, resulting in acquisition of responsiveness to the SDF1 signal. These results demonstrate that MafB, Ets1 and Sox8 are critical components for generation of the identity of the cardiac neural crest, especially the cell migration property.

Pericyte Ontogeny: The Use of Chimeras to Track a Cell Lineage of Diverse Germ Line Origins

The goal of lineage tracing is to understand body formation over time by discovering which cells are the progeny of a specific, identified, ancestral progenitor. Subsidiary questions include unequivocal identification of what they have become, how many descendants develop, whether they live or die, and where they are located in the tissue or body at the end of the window examined. A classical approach in experimental embryology, lineage tracing continues to be used in developmental biology and stem cell and cancer research, wherever cellular potential and behavior need to be studied in multiple dimensions, of which one is time. Each technical approach has its advantages and drawbacks. This chapter, with some previously unpublished data, will concentrate nonexclusively on the use of interspecies chimeras to explore the origins of perivascular (or mural) cells, of which those adjacent to the vascular endothelium are termed pericytes for this purpose. These studies laid the groundwork for our understanding that pericytes derive from progenitor mesenchymal pools of multiple origins in the vertebrate embryo, some of which persist into adulthood. The results obtained through xenografting, like in the methodology described here, complement those obtained through genetic lineage-tracing techniques within a given species.

Stable integration of an optimized inducible promoter system enables spatiotemporal control of gene expression throughout avian development

Precisely altering gene expression is critical for understanding molecular processes of embryogenesis. Although some tools exist for transgene misexpression in developing chick embryos, we have refined and advanced them by simplifying and optimizing constructs for spatiotemporal control. To maintain expression over the entire course of embryonic development we use an enhanced piggyBac transposon system that efficiently integrates sequences into the host genome. We also incorporate a DNA targeting sequence to direct plasmid translocation into the nucleus and a D4Z4 insulator sequence to prevent epigenetic silencing. We designed these constructs to minimize their size and maximize cellular uptake, and to simplify usage by placing all of the integrating sequences on a single plasmid. Following electroporation of stage HH8.5 embryos, our tetracycline-inducible promoter construct produces robust transgene expression in the presence of doxycycline at any point during embryonic development in ovo or in culture. Moreover, expression levels can be modulated by titrating doxycycline concentrations and spatial control can be achieved using beads or gels. Thus, we have generated a novel, sensitive, tunable, and stable inducible-promoter system for high-resolution gene manipulation in vivo.

Blood flow-mediated gene transfer and siRNA-knockdown in the developing vasculature in a spatio-temporally controlled manner in chicken embryos

We describe a method by which early developing vasculature can be gene-manipulated independently of the heart in a spatio-temporally controlled manner. Lipofectamine 2000 or 3000, an easy-to-use lipid reagent, has been found to yield a high efficiency of transfection when co-injected with GFP DNA within a critical range of lipid concentration. By exploiting developmentally changing patterns of vasculature and blood flow, we have succeed in controlling the site of transfection: injection with a lipid-DNA cocktail into the heart before or after the blood circulation starts results in a limited and widely spread patterns of transfection, respectively. Furthermore, a cocktail injection into the right dorsal aorta leads to transgenesis of the right half of embryonic vasculature. In addition, this method combined with the siRNA technique has allowed, for the first time, to knockdown the endogenous expression of VE-cadherin (also called Cdh5), which has been implicated in assembly of nasant blood vessels: when Cah5 siRNA is injected into the right dorsal aorta, pronounced defects in the right half of vasculature are observed without heart defects. Whereas infusion-mediated gene transfection method has previously been reported using lipid reagents that were elaborately prepared on their own, Lipofectamine is an easy-use reagent with no requirement of special expertise. The methods reported here would overcome shortcomings of conventional vascular-transgenic animals, such as mice and zebrafish, in which pan-endothelial enhancer-driven transgenesis often leads to the heart malformation, which, in turn, indirectly affects peripheral vasculature due to flow defects. Since a variety of subtypes in vasculature have increasingly been appreciated, the spatio-temporally controllable gene manipulation described in this study offers a powerful tool to understand how the vasculature is established at the molecular level.

Novel MYC-driven medulloblastoma models from multiple embryonic cerebellar cells

Group3 medulloblastoma (MBG3) that predominantly occur in young children are usually associated with MYC amplification and/or overexpression, frequent metastasis and a dismal prognosis. Physiologically relevant MBG3 models are currently lacking, making inferences related to their cellular origin thus far limited. Using in utero electroporation, we here report that MBG3 mouse models can be developed in situ from different multipotent embryonic cerebellar progenitor cells via conditional expression of Myc and loss of Trp53 function in several Cre driver mouse lines. The Blbp-Cre driver that targets embryonic neural progenitors induced tumors exhibiting a large-cell/anaplastic histopathology adjacent to the fourth ventricle, recapitulating human MBG3. Enforced co-expression of luciferase together with Myc and a dominant-negative form of Trp53 revealed that GABAergic neuronal progenitors as well as cerebellar granule cells give rise to MBG3 with their distinct growth kinetics. Cross-species gene expression analysis revealed that these novel MBG3 models shared molecular characteristics with human MBG3, irrespective of their cellular origin. We here developed MBG3 mouse models in their physiological environment and we show that oncogenic insults drive this MB subgroup in different cerebellar lineages rather than in a specific cell of origin.

CRISPR/Cas9 in the Chicken Embryo

Genome editing is driving a revolution in the biomedical sciences that carries the promise for future treatments of genetic diseases. The CRISPR/Cas9 system of RNA-guided genome editing has been successfully applied to modify the genome of a wide spectrum of organisms. We recently showed that this technique can be combined with in vivo electroporation to inhibit the function of genes of interest in somatic cells of the developing chicken embryo. We present here a simplified version of the previously described technique that leads to effective gene loss-of-function.

Genetic Manipulation of the Avian Urogenital System Using In Ovo Electroporation

One of the advantages of the avian embryo as an experimental model is its in ovo development and hence accessibility for genetic manipulation. Electroporation has been used extensively in the past to study gene function in chicken and quail embryos . Readily accessible tissues such as the neural tube, somites, and limb bud, in particular, have been targeted. However, more inaccessible tissues, such as the embryonic urogenital system , have proven more challenging to study. Here, we describe the use of in ovo electroporation of TOL2 vectors or RCASBP avian viral vectors for the rapid functional analysis of genes involved in avian sex determination and urogenital development . In the context of the developing urogenital system , these vectors have inherent advantages and disadvantages, which will be considered here. Either vector can both be used for mis-expressing a gene and for targeting endogenous gene knockdown via expression of short hairpin RNAs (shRNAs). Both of these vectors integrate into the genome and are hence spread throughout developing tissues. Going forward, electroporation could be combined with CRISPR/Cas9 technology for targeted genome editing in the avian urogenital system .

Melanosome transfer to keratinocyte in the chicken embryonic skin is mediated by vesicle release associated with Rho-regulated membrane blebbing

During skin pigmentation in amniotes, melanin synthesized in the melanocyte is transferred to keratinocytes by a particle called the melanosome. Previous studies, mostly using dissociated cultured cells, have proposed several different models that explain how the melanosome transfer is achieved. Here, using a technique that labels the plasma membrane of melanocytes within a three-dimensional system that mimics natural tissues, we have visualized the plasma membrane of melanocytes with EGFP in chicken embryonic skin. Confocal time-lapse microscopy reveals that the melanosome transfer is mediated, at least in part, by vesicles produced by plasma membrane. Unexpectedly, the vesicle release is accompanied by the membrane blebbing of melanocytes. Blebs that have encapsulated a melanosome are pinched off to become vesicles, and these melanosome-containing vesicles are finally engulfed by neighboring keratinocytes. For both the membrane blebbing and vesicle release, Rho small GTPase is essential. We further show that the membrane vesicle-mediated melanosome transfer plays a significant role in the skin pigmentation. Given that the skin pigmentation in inter-feather spaces in chickens is similar to that in inter-hair spaces of humans, our findings should have important consequences in cosmetic medicine.

CRISPR/Cas9 Targets Chicken Embryonic Somatic Cells In Vitro and In Vivo and generates Phenotypic Abnormalities

Chickens are an invaluable model for studying human diseases, physiology and especially development, but have lagged in genetic applications. With the advent of Programmable Engineered Nucleases, genetic manipulation has become efficient, specific and rapid. Here, we show that the CRISPR/Cas9 system can precisely edit the chicken genome. We generated HIRA, TYRP1, DICER, MBD3, EZH2, and 6 other gene knockouts in two chicken cell lines using the CRISPR/Cas9 system, with no off-target effects detected. We also showed that very large deletions (>75 kb) could be achieved. We also achieved targeted modification by homology-directed repair (HDR), producing MEN2A and MEN2B mutations of the RET gene. We also targeted DGCR8 in neural cells of the chicken embryo by in vivo electroporation. After FACS isolation of transfected cells, we observed appropriate sequence changes in DGCR8. Wholemount and frozen section antibody labelling showed reduction of DGCR8 levels in transfected cells. In addition, there was reduced expression levels of DGCR8-associated genes DROSHA, YPEL1 and NGN2. We also observed morphological differences in neural tissue and cardiac-related tissues of transfected embryos. These findings demonstrate that precisely targeted genetic manipulation of the genome using the CRISPR/Cas9 system can be extended to the highly adaptable in vivo chicken embryo model.

CRISPR mediated somatic cell genome engineering in the chicken

Gene-targeted knockout technologies are invaluable tools for understanding the functions of genes in vivo. CRISPR/Cas9 system of RNA-guided genome editing is revolutionizing genetics research in a wide spectrum of organisms. Here, we combined CRISPR with in vivo electroporation in the chicken embryo to efficiently target the transcription factor PAX7 in tissues of the developing embryo. This approach generated mosaic genetic mutations within a wild-type cellular background. This series of proof-of-principle experiments indicate that in vivo CRISPR-mediated cell genome engineering is an effective method to achieve gene loss-of-function in the tissues of the chicken embryo and it completes the growing genetic toolbox to study the molecular mechanisms regulating development in this important animal model.

In ovo gene manipulation of melanocytes and their adjacent keratinocytes during skin pigmentation of chicken embryos

During skin pigmentation in avians and mammalians, melanin is synthesized in the melanocytes, and subsequently transferred to adjacently located keratinocytes, leading to a wide coverage of the body surface by melanin-containing cells. The behavior of melanocytes is influenced by keratinocytes shown mostly by in vitro studies. However, it has poorly been investigated how such intercellular cross-talk is regulated in vivo because of a lack of suitable experimental models. Using chicken embryos, we developed a method that enables in vivo gene manipulations of melanocytes and keratinocytes, where these cells are separately labeled by different genes. Two types of gene transfer techniques were combined: one was a retrovirus-mediated gene infection into the skin/keratinocytes, and the other was the in ovo DNA electroporation into neural crest cells, the origin of melanocytes. Since the Replication-Competent Avian sarcoma-leukosis virus long terminal repeat with Splice acceptor (RCAS) infection was available only for the White leghorn strain showing little pigmentation, melanocytes prepared from the Hypeco nera (pigmented) were back-transplanted into embryos of White leghorn. Prior to the transplantation, enhanced green fluorescent protein (EGFP)(+) Neo(r+) -electroporated melanocytes from Hypeco nera were selectively grown in G418-supplemented medium. In the skin of recipient White leghorn embryos infected with RCAS-mOrange, mOrange(+) keratinocytes and transplanted EGFP(+) melanocytes were frequently juxtaposed each other. High-resolution confocal microscopy also revealed that transplanted melanocytes exhibited normal behaviors regarding distribution patterns of melanocytes, dendrite morphology, and melanosome transfer. The method described in this study will serve as a useful tool to understand the mechanisms underlying intercellular regulations during skin pigmentation in vivo.

Angiogenesis in the developing spinal cord: blood vessel exclusion from neural progenitor region is mediated by VEGF and its antagonists

Blood vessels in the central nervous system supply a considerable amount of oxygen via intricate vascular networks. We studied how the initial vasculature of the spinal cord is formed in avian (chicken and quail) embryos. Vascular formation in the spinal cord starts by the ingression of intra-neural vascular plexus (INVP) from the peri-neural vascular plexus (PNVP) that envelops the neural tube. At the ventral region of the PNVP, the INVP grows dorsally in the neural tube, and we observed that these vessels followed the defined path at the interface between the medially positioned and undifferentiated neural progenitor zone and the laterally positioned differentiated zone. When the interface between these two zones was experimentally displaced, INVP faithfully followed a newly formed interface, suggesting that the growth path of the INVP is determined by surrounding neural cells. The progenitor zone expressed mRNA of vascular endothelial growth factor-A whereas its receptor VEGFR2 and FLT-1 (VEGFR1), a decoy for VEGF, were expressed in INVP. By manipulating the neural tube with either VEGF or the soluble form of FLT-1, we found that INVP grew in a VEGF-dependent manner, where VEGF signals appear to be fine-tuned by counteractions with anti-angiogenic activities including FLT-1 and possibly semaphorins. These results suggest that the stereotypic patterning of early INVP is achieved by interactions between these vessels and their surrounding neural cells, where VEGF and its antagonists play important roles.

The avian embryo as a model system for skeletal myogenesis

This review will focus on the use of the chicken and quail as model systems to analyze myogenesis and as such will emphasize the experimental approaches that are strongest in these systems-the amenability of the avian embryo to manipulation and in ovo observation. During somite differentiation, a wide spectrum of developmental processes occur such as cellular differentiation, migration, and fusion. Cell lineage studies combined with recent advancements in cell imaging allow these biological phenomena to be readily observed and hypotheses tested extremely rapidly-a strength that is restricted to the avian system. A clear weakness of the chicken in the past has been genetic approaches to modulate gene function. Recent advances in the electroporation of expression vectors, siRNA constructs, and use of tissue specific reporters have opened the door to increasingly sophisticated experiments that address questions of interest not only to the somite/muscle field in particular but also fundamental to biology in general. Importantly, an ever-growing body of evidence indicates that somite differentiation in birds is indistinguishable to that of mammals; therefore, these avian studies complement the complex genetic models of the mouse.

Parameters affecting efficiency of in ovo electroporation of the avian neural tube and crest

BACKGROUND

Many variations in avian in ovo transfection of the neural tube/crest have been reported, but never compared quantitatively.

RESULTS

Genome integrating pT2K-CAGGS-GFP and pCAGGS-T2TP transposase plasmids were co-electroporated into quail E2 embryo trunk neural tube and the proportion of GFP-expressing neural cells was counted 1 and 7 days later. Electroporation efficiency increased with plasmid concentration and pulse number but plateaued at, respectively, above 1.25 µg/µL and 3 pulses. Bilateral electroporation transfected more cells than unilateral but less than that anticipated by doubling the unilateral treatment. Holding the concentration of GFP plasmid constant and varying the transposase plasmid concentration revealed an optimum ratio of, in this case, 4:1 (1.2 µg/µL:0.3 µg/µL). Leaving transfected embryos to E9 confirmed that expression was maintained in vivo with the transposase system, but declined with non-integrated plasmid. Transfection of neural crest cells was low if electroporated less than 6-8 hr before emigration. We propose this indicates loss of epithelial integrity well prior to exit. We suggest this event be termed epithelio-mesenchymal transition sensu stricto, whereas the term delamination be reserved for the later emigration from the neural epithelium.

CONCLUSIONS

Co-electroporation in ovo must take into account plasmid(s) concentration and ratio, pulse number, pulse directionality, and timing.

Interepithelial signaling with nephric duct is required for the formation of overlying coelomic epithelial cell sheet

In most organs of the body, epithelial tissues are supported by their own basement membrane and underlying stroma, the latter being regarded as a complex of amorphous cells, extracellular matrices, and soluble factors. We demonstrate here that an epithelial tube can serve as a component of stroma that supports the formation of epithelial cell sheet derived from a different origin. During development of the mesonephros in chicken embryos, the intermediate mesoderm (IMM), which contains the Wolffian duct (WD) and its associated tubules, is overlain by a sheet of epithelial cells derived from lateral plate (coelomic) mesoderm. We describe that in normal embryos, epitheliogenesis of IMM tubes and the adjacent coelomic cell sheet proceed in a coordinated manner. When the WD was surgically ablated, the overlying coelomic epithelium exhibited aberrant morphology accompanied by a punctated basement membrane. Furthermore, the WD-ablated coelomic epithelium became susceptible to latent external stress; electroporation of Rac1 resulted in epithelial-to-mesenchymal transitions (EMTs) within the coelomic epithelium. The distorted coelomic epithelium was rescued by implanting fibronectin-producing cells in place of the WD, suggesting that fibronectin provided by WD has an important role acting interepithelially. This notion was corroborated further by directly visualizing a translocation of EGFP-tagged fibronectin from fibronectin-producing to -receiving epithelia in vivo. Our findings provide a novel insight into interepithelial signaling that also might occur in adult tissues to protect against EMT and suggest a possible new target for anticancer therapeutic strategy.

An impairment of long distance SOX10 regulatory elements underlies isolated Hirschsprung disease

A deletion encompassing several SOX10 enhancers was recently identified in a patient presenting with Waardenburg syndrome type 4 (WS4), which is defined as a combination of Hirschsprung disease (HSCR, intestinal aganglionosis) and WS (deafness and pigmentation defects). The expression patterns of some of the known SOX10 enhancers in animal models led to the speculation that endophenotypes of WS4 may be linked to mutations within some of these sequences. The present study investigated deletions and point mutations within four SOX10 enhancers in 144 unexplained isolated HSCR cases. One deletion and two point mutations affecting binding sites for known neural crest transcription factors were identified. In vitro functional analysis revealed that the first point mutation disrupts autoregulation by SOX10, whereas the second affects AP2a and SOX10 synergistic activity. The present findings suggest that the mutations within SOX10 enhancers contribute to isolated HSCR.

Insights into neural crest development and evolution from genomic analysis

The neural crest is an excellent model system for the study of cell type diversification during embryonic development due to its multipotency, motility, and ability to form a broad array of derivatives ranging from neurons and glia, to cartilage, bone, and melanocytes. As a uniquely vertebrate cell population, it also offers important clues regarding vertebrate origins. In the past 30 yr, introduction of recombinant DNA technology has facilitated the dissection of the genetic program controlling neural crest development and has provided important insights into gene regulatory mechanisms underlying cell migration and differentiation. More recently, new genomic approaches have provided a platform and tools that are changing the depth and breadth of our understanding of neural crest development at a “systems” level. Such advances provide an insightful view of the regulatory landscape of neural crest cells and offer a new perspective on developmental as well as stem cell and cancer biology.

Long-term, inducible gene loss-of-function in the chicken embryo

The use of shRNAmir to down-regulate the expression of genes of interest is a powerful tool for studying gene function during early chick development. However, because of the limitations of electroporation-mediated transgenesis, the down-regulation of genes expressed at late stages of development in specific tissues is difficult to perform. By combining electroporation of a doxycycline-inducible, miR30-based shRNA plasmid with the Tol2 genomic integration system, we are now able to down-regulate the expression of any gene of interest at defined stage of chicken development.

Experimental approaches for gene regulatory network construction: the chick as a model system

Setting up the body plan during embryonic development requires the coordinated action of many signals and transcriptional regulators in a precise temporal sequence and spatial pattern. The last decades have seen an explosion of information describing the molecular control of many developmental processes. The next challenge is to integrate this information into logic "wiring diagrams" that visualize gene actions and outputs, have predictive power and point to key control nodes. Here, we provide an experimental workflow on how to construct gene regulatory networks using the chick as model system.

Dorsal aorta formation: separate origins, lateral-to-medial migration, and remodeling

Blood vessel formation is a highly dynamic tissue-remodeling event that can be observed from early development in vertebrate embryos. Dorsal aortae, the first functional intra-embryonic blood vessels, arise as two separate bilateral vessels in the trunk and undergo lateral-to-medial translocation, eventually fusing into a single large vessel at the midline. After this dramatic remodeling, the dorsal aorta generates hematopoietic stem cells. The dorsal aorta is a good model to use to increase our understanding of the mechanisms controlling the establishment and remodeling of larger blood vessels in vivo. Because of the easy accessibility to the developing circulatory system, quail and chick embryos have been widely used for studies on blood vessel formation. In particular, the mapping of endothelial cell origins has been performed using quail-chick chimera analysis, revealing endothelial, vascular smooth muscle, and hematopoietic cell progenitors of the dorsal aorta. The avian embryo model also allows conditional gene activation/inactivation and direct observation of cell behaviors during dorsal aorta formation. This allows a better understanding of the molecular mechanisms underlying specific morphogenetic events during dynamic dorsal aorta formation from a cell behavior perspective.

Screening of MITF and SOX10 regulatory regions in Waardenburg syndrome type 2

Waardenburg syndrome (WS) is a rare auditory-pigmentary disorder that exhibits varying combinations of sensorineural hearing loss and pigmentation defects. Four subtypes are clinically defined based on the presence or absence of additional symptoms. WS type 2 (WS2) can result from mutations within the MITF or SOX10 genes; however, 70% of WS2 cases remain unexplained at the molecular level, suggesting that other genes might be involved and/or that mutations within the known genes escaped previous screenings. The recent identification of a deletion encompassing three of the SOX10 regulatory elements in a patient presenting with another WS subtype, WS4, defined by its association with Hirschsprung disease, led us to search for deletions and point mutations within the MITF and SOX10 regulatory elements in 28 yet unexplained WS2 cases. Two nucleotide variations were identified: one in close proximity to the MITF distal enhancer (MDE) and one within the U1 SOX10 enhancer. Functional analyses argued against a pathogenic effect of these variations, suggesting that mutations within regulatory elements of WS genes are not a major cause of this neurocristopathy.

The dorsal aorta initiates a molecular cascade that instructs sympatho-adrenal specification

The autonomic nervous system, which includes the sympathetic neurons and adrenal medulla, originates from the neural crest. Combining avian blood vessel-specific gene manipulation and mouse genetics, we addressed a long-standing question of how neural crest cells (NCCs) generate sympathetic and medullary lineages during embryogenesis. We found that the dorsal aorta acts as a morphogenetic signaling center that coordinates NCC migration and cell lineage segregation. Bone morphogenetic proteins (BMPs) produced by the dorsal aorta are critical for the production of the chemokine stromal cell-derived factor-1 (SDF -1) and Neuregulin 1 in the para-aortic region, which act as chemoattractants for early migration. Later, BMP signaling is directly involved in the sympatho-medullary segregation. This study provides insights into the complex developmental signaling cascade that instructs one of the earliest events of neurovascular interactions guiding embryonic development.

A tightly controlled conditional knockdown system using the Tol2 transposon-mediated technique

Background

Gene knockdown analyses using the in utero electroporation method have helped reveal functional aspects of genes of interest in cortical development. However, the application of this method to analyses in later stages of brain development or in the adult brain is still difficult because the amount of injected plasmids in a cell decreases along with development due to dilution by cell proliferation and the degradation of the plasmids. Furthermore, it is difficult to exclude the influence of earlier knockdown effects.

Methodology/Principal Findings

We developed a tightly controlled conditional knockdown system using a newly constructed vector, pT2K-TBI-shRNAmir, based on a Tol2 transposon-mediated gene transfer methodology with the tetracycline-inducible gene expression technique, which allows us to maintain a transgene for a long period of time and induce the knockdown of the gene of interest. We showed that expression of the endogenous amyloid precursor protein (APP) was sharply decreased by our inducible, stably integrated knockdown system in PC12 cells. Moreover, we induced an acute insufficiency of Dab1 with our system and observed that radial migration was impaired in the developing cerebral cortex. Such inhibitory effects on radial migration were not observed without induction, indicating that our system tightly controlled the knockdown, without any expression leakage in vivo.

Conclusions/Significance

Our system enables us to investigate the brain at any of the later stages of development or in the adult by utilizing a knockdown technique with the aid of the in utero electroporation gene transfer methodology. Furthermore, we can perform knockdown analyses free from the influence of undesired earlier knockdown effects.

Neural crest progenitors and stem cells: from early development to adulthood

In the vertebrate embryo, the neural crest forms transiently in the dorsal neural primordium to yield migratory cells that will invade nearly all tissues and later, will differentiate into bones and cartilages, neurons and glia, endocrine cells, vascular smooth muscle cells and melanocytes. Due to the amazingly diversified array of cell types it produces, the neural crest is an attractive model system in the stem cell field. We present here in vivo and in vitro studies of single cell fate, which led to the discovery and the characterization of stem cells in the neural crest of avian and mammalian embryos. Some of the key issues in neural crest cell diversification are discussed, such as the time of segregation of mesenchymal vs. neural/melanocytic lineages, and the origin and close relationships between the glial and melanocytic lineages. An overview is also provided of the diverse types of neural crest-like stem cells and progenitors, recently identified in a growing number of adult tissues in animals and humans. Current and future work, in which in vivo lineage studies and the use of injury models will complement the in vitro culture analysis, should help in unraveling the properties and function of neural crest-derived progenitors in development and disease.

Alu-mediated deletion of SOX10 regulatory elements in Waardenburg syndrome type 4

Waardenburg syndrome type 4 (WS4) is a rare neural crest disorder defined by the combination of Waardenburg syndrome (sensorineural hearing loss and pigmentation defects) and Hirschsprung disease (intestinal aganglionosis). Three genes are known to be involved in this syndrome, that is, EDN3 (endothelin-3), EDNRB (endothelin receptor type B), and SOX10. However, 15-35% of WS4 remains unexplained at the molecular level, suggesting that other genes could be involved and/or that mutations within known genes may have escaped previous screenings. Here, we searched for deletions within recently identified SOX10 regulatory sequences and describe the first characterization of a WS4 patient presenting with a large deletion encompassing three of these enhancers. Analysis of the breakpoint region suggests a complex rearrangement involving three Alu sequences that could be mediated by a FosTes/MMBIR replication mechanism. Taken together with recent reports, our results demonstrate that the disruption of highly conserved non-coding elements located within or at a long distance from the coding sequences of key genes can result in several neurocristopathies. This opens up new routes to the molecular dissection of neural crest disorders.

In ovo eye electroporation

Electroporation has been used successfully to introduce macromolecules such as DNA into the chick embryo for at least 15 years. Purified plasmid DNA is microinjected into embryo and then a series of low voltage electrical pulses are applied to the embryo which allows naked DNA to enter cells. Following entrance into the cytoplasm, the DNA is transported to the nucleus where it is transiently expressed. This powerful technique is useful for studies involving overexpression, misexpression, and knockdown of genes of interest at a variety of developmental timepoints.