Developmental patterning deciphered in avian chimeras

Induced pluripotent stem cells from farm animals

The development of the induced pluripotent stem cells (iPSCs) technology has revolutionized the world on the establishment of pluripotent stem cells (PSCs) across a great variety of animal species. Generation of iPSCs from domesticated animals would provide unrestricted cell resources for the study of embryonic development and cell differentiation of these species, for screening and establishing desired traits for sustainable agricultural production, and as veterinary and preclinical therapeutic tools for animal and human diseases. Induced PSCs from domesticated animals thus harbor enormous scientific, economical, and societal values. Although much progress has been made toward the generation of PSCs from these species, major obstacles remain precluding the exclamation of the establishment of bona fide iPSCs. The most prominent of them remain the inability of these cells to silence exogenous reprogramming factors, the obvious reliance on exogenous factors for their self-renewal, and the restricted development potential in vivo. In this review, we summarize the history and current progress in domestic farm animal iPSC generation, with a focus on swine, ruminants (cattle, ovine, and caprine), horses, and avian species (quails and chickens). We also discuss the problems associated with the farm animal iPSCs and potential future directions toward the complete reprogramming of somatic cells from farm animals.

Molecular Events Controlling Cessation of Trunk Neural Crest Migration and Onset of Differentiation

Neural crest cells (NCC) migrate extensively in vertebrate embryos to populate diverse derivatives including ganglia of the peripheral nervous system. Little is known about the molecular mechanisms that lead migrating trunk NCC to settle at selected sites in the embryo, ceasing their migration and initiating differentiation programs. To identify candidate genes involved in these processes, we profiled genes up-regulated in purified post-migratory compared with migratory NCC using a staged, macroarrayed cDNA library. A secondary screen of in situ hybridization revealed that many genes are specifically enhanced in neural crest-derived ganglia, including macrophage migration inhibitory factor (MIF), a ligand for CXCR4 receptor. Through in vivo and in vitro assays, we found that MIF functions as a potent chemoattractant for NCC. These results provide a molecular profile of genes expressed concomitant with gangliogenesis, thus, offering new markers and potential regulatory candidates involved in cessation of migration and onset of differentiation.

Comparative cytogenomics of poultry: mapping of single gene and repeat loci in the Japanese quail (Coturnix japonica)

Well-characterized molecular and cytogenetic maps are yet to be established in Japanese quail (Coturnix japonica). The aim of the current study was to cytogenetically map and determine linkage of specific genes and gene complexes in Japanese quail through the use of chicken (Gallus gallus) and turkey (Meleagris gallopavo) genomic DNA probes and conduct a comparative study among the three genomes. Chicken and turkey clones were used as probes on mitotic metaphase and meiotic pachytene stage chromosomes of the three species for the purpose of high-resolution fluorescence in situ hybridization (FISH). The genes and complexes studied included telomerase RNA (TR), telomerase reverse transcriptase (TERT), 5S rDNA, 18S-5.8S-28S rDNA (i.e., nucleolus organizer region (NOR)), and the major histocompatibility complex (MHC). The telomeric profile of Japanese quail was investigated through the use of FISH with a TTAGGG-PNA probe. A range of telomeric array sizes were confirmed as found for the other poultry species. Three NOR loci were identified in Japanese quail, and single loci each for TR, TERT, 5S rDNA and the MHC-B. The MHC-B and one NOR locus were linked on a microchromosome in Japanese quail. We confirmed physical linkage of 5S rDNA and the TR gene on an intermediate-sized chromosome in quail, similar to both chicken and turkey. TERT localized to CJA 2 in quail and the orthologous chromosome region in chicken (GGA 2) and in turkey (MGA 3). The cytogenetic profile of Japanese quail was further developed by this study and synteny was identified among the three poultry species.

Mapping the extracellular and membrane proteome associated with the vasculature and the stroma in the embryo

In order to map the extracellular or membrane proteome associated with the vasculature and the stroma in an embryonic organism in vivo, we developed a biotinylation technique for chicken embryo and combined it with mass spectrometry and bioinformatic analysis. We also applied this procedure to implanted tumors growing on the chorioallantoic membrane or after the induction of granulation tissue. Membrane and extracellular matrix proteins were the most abundant components identified. Relative quantitative analysis revealed differential protein expression patterns in several tissues. Through a bioinformatic approach, we determined endothelial cell protein expression signatures, which allowed us to identify several proteins not yet reported to be associated with endothelial cells or the vasculature. This is the first study reported so far that applies in vivo biotinylation, in combination with robust label-free quantitative proteomics approaches and bioinformatic analysis, to an embryonic organism. It also provides the first description of the vascular and matrix proteome of the embryo that might constitute the starting point for further developments.

Electroporation: past, present and future

Gene transfer by electroporation has become an indispensable method for the study of developmental biology. The technique is applied not only in chick embryos but also in mice and other organisms. Here, a short history and perspectives of electroporation for gene transfer in vertebrates are described.

Neural crest and somitic mesoderm as paradigms to investigate cell fate decisions during development

The dorsal domains of the neural tube and somites are transient embryonic epithelia; they constitute the source of neural crest progenitors that generate the peripheral nervous system, pigment cells and ectomesenchyme, and of the dermomyotome that develops into myocytes, dermis and vascular cells, respectively. Based on the variety of derivatives produced by each type of epithelium, a classical yet still highly relevant question is whether these embryonic epithelia are composed of homogeneous multipotent progenitors or, alternatively, of subsets of fate-restricted cells. Growing evidence substantiates the notion that both the dorsal tube and the dermomyotome are heterogeneous epithelia composed of multipotent as well as fate-restricted precursors that emerge as such in a spatio-temporally regulated manner. Elucidation of the state of commitment of the precedent progenitors is of utmost significance for deciphering the mechanisms that regulate fate segregation during embryogenesis. In addition, it will contribute to understanding the nature of well documented neural crest-somite interactions shown to modulate the timing of neural crest cell emigration, their segmental migration, and myogenesis.

Interordinal chimera formation between medaka and zebrafish for analyzing stem cell differentiation

Chimera formation is a standard test for pluripotency of stem cells in vivo. Interspecific chimera formation between distantly related organisms offers also an attractive approach for propagating endangered species. Parameters influencing interspecies chimera formation have remained poorly elucidated. Here, we report interordinal chimera formation between medaka and zebrafish, which separated ∼320 million years ago and exhibit a more than 2-fold difference in developmental speed. We show that, on transplantation into zebrafish blastulae, both noncultivated blastomeres and long-term cultivated embryonic stem (ES) cells of medaka adopted the zebrafish developmental program and differentiated into physiologically functional cell types including pigment cells, blood cells, and cardiomyocytes. We also show that medaka ES cells express differentiation gene markers during chimeric embryogenesis. Therefore, the evolutionary distance and different embryogenesis speeds do not produce donor-host incompatibility to compromise chimera formation between medaka and zebrafish, and molecular markers are valuable for analyzing lineage commitment and cell differentiation in interspecific chimeric embryos.

Avian-induced pluripotent stem cells derived using human reprogramming factors

Avian species are important model animals for developmental biology and disease research. However, unlike in mice, where clonal lines of pluripotent stem cells have enabled researchers to study mammalian gene function, clonal and highly proliferative pluripotent avian cell lines have been an elusive goal. Here we demonstrate the generation of avian induced pluripotent stem cells (iPSCs), the first nonmammalian iPSCs, which were clonally isolated and propagated, important attributes not attained in embryo-sourced avian cells. This was accomplished using human pluripotency genes rather than avian genes, indicating that the process in which mammalian and nonmammalian cells are reprogrammed is a conserved process. Quail iPSCs (qiPSCs) were capable of forming all 3 germ layers in vitro and were directly differentiated in culture into astrocytes, oligodendrocytes, and neurons. Ultimately, qiPSCs were capable of generating live chimeric birds and incorporated into tissues from all 3 germ layers, extraembryonic tissues, and potentially the germline. These chimera competent qiPSCs and in vitro differentiated cells offer insight into the conserved nature of reprogramming and genetic tools that were only previously available in mammals.

Dynamic lineage analysis of embryonic morphogenesis using transgenic quail and 4D multispectral imaging

We describe the development of transgenic quail that express various fluorescent proteins in targeted manners and their use as a model system that integrates advanced imaging approaches with conventional and emerging molecular genetics technologies. We also review the progression and complications of past fate mapping techniques that led us to generate transgenic quail, which permit dynamic imaging of amniote embryogenesis with unprecedented subcellular resolution.

Gamma-irradiation depletes endogenous germ cells and increases donor cell distribution in chimeric chickens

The production of chimeric birds is an important tool for the investigation of vertebrate development, the conservation of endangered birds, and the development of various biotechnological applications. This study examined whether gamma (γ)-irradiation depletes endogenous primordial germ cells and enhances the efficiency of somatic chimerism in chickens. An optimal irradiation protocol for stage X embryos was determined after irradiation at various doses (0, 100, 300, 500, 600, 700, and 2,000 rad). Exposure to 500 rad of γ-irradiation for 73 s significantly decreased the number of primordial germ cells (P < 0.0001). Somatic chimera hatchlings were then produced by transferring blastodermal cells from a Korean Oge into either an irradiated (at 500 rad) or intact stage X White Leghorn embryo. An analysis of feather color pattern and polymerase chain reaction-based species-specific amplification of various tissues of the hatchlings confirmed chimerism in most organs of the chick produced from the irradiated recipient; a lesser degree of chimerism was observed in the non-irradiated control recipient. In conclusion, the exposure of chick embryos to an optimized dose of γ-irradiation effectively depleted germ cells and yielded greater somatic chimerism than non-irradiated control embryos. This technique can be applied to interspecies reproduction or the production of transgenic birds.

Polypyrimidine tract-binding protein is required for the repression of gene expression by all-trans retinoic acid

All-trans retinoic acid is a key regulator of early development. High concentrations of retinoic acid interfere with differentiation and migration of neural crest cells. Here we report that a dinucleotide repeat in the cis-element of Snail2 (previously known as Slug) gene plays a role in repression by all-trans retinoic acid. We analyzed the cis-acting regulatory regions of the Xenopus Snail2 gene, whose expression is repressed by all-trans retinoic acid. The analysis identified a TG/CA repeat as a necessary element for the repression. By performing a yeast one-hybrid screen, we found that a polypyrimidine tract-binding protein (PTB), which is known to be a regulator of the alternative splicing of pre-messenger RNA, binds to the TG/CA repeat. Overexpression and knockdown experiments for PTB in HEK293 cells and Xenopus embryos indicated that PTB is required for repression by retinoic acid. The green fluorescent protein-PTB fusion protein was localized in the nucleus of 293T cells. In situ hybridization for PTB in Xenopus embryos showed that PTB is expressed at the regions including neural crest at the early stages. Our results indicate that PTB plays a role in the repression of gene expression by retinoic acid through binding to the TG/CA repeats.

Developmental stages of the Japanese quail

Developmental biology research has used various avian species as model organisms for studying morphogenesis, with the chick embryo being used by the majority of groups. The focus on the chick embryo led Hamburger and Hamilton to develop their definitive staging series nearly 60 years ago and this series is still the mainstay of all laboratories working with avian embryos. The focus on the chick embryo has somewhat overshadowed the importance of another avian embryo that has proved to be equally powerful, the Japanese quail. Since the late 1960s, chimeras have been produced using chick and quail embryos and this technique has revolutionized the approach taken to the investigation of the cellular and molecular interactions that occur during development. Reviews of the literature demonstrate that many research groups are using the quail embryo in a number of established and new ways, and this species has become a primary animal model in developmental biology. Some staging of quail has been performed but this has been incomplete and variations in descriptions, stages and incubation timings mean that comparisons with the chick are not always easily made. There appears to be general agreement that, at the early stages of embryogenesis, there is little developmental difference between chick and quail embryos, although the basis for this has not been established experimentally. The accelerated ontogeny of quail embryos at mid to late stages of development means that registration with the chick is lost. We have therefore developed a definitive developmental stage series for Japanese quail so that differences are fully characterized, misconceptions or assumptions are avoided, and the results of comparative studies are not distorted.

Draxin, an axon guidance protein, affects chick trunk neural crest migration

The neural crest is a multipotent population of migratory cells that arises in the central nervous system and subsequently migrates along defined stereotypic pathways. In the present work, we analyzed the role of a repulsive axon guidance protein, draxin, in the migration of neural crest cells. Draxin is expressed in the roof plate of the chick trunk spinal cord and around the early migration pathway of neural crest cells. Draxin modulates chick neural crest cell migration in vitro by reducing the polarization of these cells. When exposed to draxin, the velocity of migrating neural crest cells was reduced, and the cells changed direction so frequently that the net migration distance was also reduced. Overexpression of draxin also caused some early migrating neural crest cells to change direction to the dorsolateral pathway in the chick trunk region, presumably due to draxin's inhibitory activity. These results demonstrate that draxin, an axon guidance protein, can also affect trunk neural crest migration in the chick embryo.

Neural crest contribution to forebrain development

The neural crest (NC), a defining feature of vertebrate embryo, generates most of the skeletal tissues encasing the developing forebrain and provides the prosencephalon with functional vasculature and meninges. Recent findings show that early in development, the cephalic NC is also essential for the pre-otic neural tube closure and promotes the development of the prosencephalic alar plate by regulating the morphogenetic activities of forebrain organizers.