In vitro improvement of quail primordial germ cell expansion through activation of TGF‐beta signaling pathway

First study on repeatable culture of primordial germ cells from various embryonic regions with giant feeder cells in Japanese quail (Coturnix japonica)

Japanese quail (JQ, Coturnix japonica) is a farmed animal with a high economic value and has been used extensively as an avian model for research. Germline chimera production based on cryopreserved primordial germ cells (PGCs) is possible for conservation management of quail breeds as successful isolation has been reported of PGCs from their blood and gonads. However, the repeatable cultivation protocol has not been elucidated yet, which has hindered technological development. The current study characterized cultivation of pregonadal PGCs isolated from embryonic parts; embryonic blood (cPGCs), whole embryonic tissues (tPGCs), parts of tail buds (tbPGCs), and a mixture of blood and tail bud tissues (ctbPGCs). The results showed that the cultivation system required the presence of specific embryonic cells to act as a feeder for JQ-PGCs and that such a system facilitated more successful cultivation, as shown by the percentages of isolation and cultivation in tbPGCs (100%, 100%, respectively), tPGCs (60%, 55%, respectively), and ctbPGCs (60%, 30%, respectively), but not in cPGCs (0%) cultured on a mitomycin-treated JQ feeder cell-line. Once the co-culture system had been established, the PGCs could be propagated for at least 5 months. These PGCs expressed germ cell-specific markers (DAZL and CVH) and could colonize embryonic gonads. Conclusively, the isolation of pregonadal PGCs and their long-term cultivation in vitro requires a unique embryonic cell, giant cell feeder, that is indispensable for the proliferation of PGCs. Characterization of cell signaling sustaining a mutual interaction between the PGCs and the specific feeder cells will elucidate a superior environment for in vitro cultivation, as well as support the minimal transfer of used xenobiotics in chimera production.

The constitutively active pSMAD2/3 relatively improves the proliferation of chicken primordial germ cells

In many multicellular organisms, mature gametes originate from primordial germ cells (PGCs). Improvements in the culture of PGCs are important not only for developmental biology research, but also for preserving endangered species, and for genome editing and transgenic animal technologies. SMAD2/3 appear to be powerful regulators of gene expression; however, their potential positive impact on the regulation of PGC proliferation has not been taken into consideration. Here, the effect of TGF-β signaling as the upstream activator of SMAD2/3 transcription factors was evaluated on chicken PGCs' proliferation. For this, chicken PGCs at stages 26-28 Hamburger-Hamilton were obtained from the embryonic gonadal regions and cultured on different feeders or feeder-free substrates. The results showed that TGF-β signaling agonists (IDE1 and Activin-A) improved PGC proliferation to some extent while treatment with SB431542, the antagonist of TGF-β, disrupted PGCs' proliferation. However, the transfection of PGCs with constitutively active SMAD2/3 (SMAD2/3CA) resulted in improved PGC proliferation for more than 5 weeks. The results confirmed the interactions between overexpressed SMAD2/3CA and pluripotency-associated genes NANOG, OCT4, and SOX2. According to the results, the application of SMAD2/3CA could represent a step toward achieving an efficient expansion of avian PGCs.

Strategies for the Generation of Gene Modified Avian Models: Advancement in Avian Germline Transmission, Genome Editing, and Applications

Avian models are valuable for studies of development and reproduction and have important implications for food production. Rapid advances in genome-editing technologies have enabled the establishment of avian species as unique agricultural, industrial, disease-resistant, and pharmaceutical models. The direct introduction of genome-editing tools, such as the clustered regularly interspaced short palindromic repeats (CRISPR) system, into early embryos has been achieved in various animal taxa. However, in birds, the introduction of the CRISPR system into primordial germ cells (PGCs), a germline-competent stem cell, is considered a much more reliable approach for the development of genome-edited models. After genome editing, PGCs are transplanted into the embryo to establish germline chimera, which are crossed to produce genome-edited birds. In addition, various methods, including delivery by liposomal and viral vectors, have been employed for gene editing in vivo. Genome-edited birds have wide applications in bio-pharmaceutical production and as models for disease resistance and biological research. In conclusion, the application of the CRISPR system to avian PGCs is an efficient approach for the production of genome-edited birds and transgenic avian models.

Comparative single-cell transcriptomic analysis reveals differences in signaling pathways in gonadal primordial germ cells between chicken (Gallus gallus) and zebra finch (Taeniopygia guttata)

Primordial germ cells (PGCs) have been used in avian genetic resource conservation and transgenic animal production. Despite their potential applications to numerous avian taxa facing extinction due to habitat loss and degradation, research has largely focused on poultry, such as chickens, in part owing to the difficulty in obtaining intact PGCs from other species. Recently, phenotypic differences between PGCs of chicken and zebra finch, a wild bird with vocal learning, in early embryonic development have been reported. In this study, we used advanced single-cell RNA sequencing (scRNA-seq) technology to evaluate zebra finch and chicken PGCs and surrounding cells, and to identify species-specific characteristics. We constructed single-cell transcriptome landscapes of chicken gonadal PGCs for a comparison with previously reported scRNA-seq data for zebra finch. We identified interspecific differences in several signaling pathways in gonadal PGCs and somatic cells. In particular, NODAL and insulin signaling pathway activity levels were higher in zebra finch than in chickens, whereas activity levels of the downstream FGF signaling pathway, involved in the proliferation of chicken PGCs, were higher in chickens. This study is the first cross-species single-cell transcriptomic analysis targeting birds, revealing differences in germ cell development between phylogenetically distant Galliformes and Passeriformes. Our results provide a basis for understanding the reproductive physiology of avian germ cells and for utilizing PGCs in the restoration of endangered birds and the production of transgenic birds.

Bend family proteins mark chromatin boundaries and synergistically promote early germ cell differentiation

Understanding the regulatory networks for germ cell fate specification is necessary to developing strategies for improving the efficiency of germ cell production in vitro. In this study, we developed a coupled screening strategy that took advantage of an arrayed bi-molecular fluorescence complementation (BiFC) platform for protein-protein interaction screens and epiblast-like cell (EpiLC)-induction assays using reporter mouse embryonic stem cells (mESCs). Investigation of candidate interaction partners of core human pluripotent factors OCT4, NANOG, KLF4 and SOX2 in EpiLC differentiation assays identified novel primordial germ cell (PGC)-inducing factors including BEN-domain (BEND/Bend) family members. Through RNA-seq, ChIP-seq, and ATAC-seq analyses, we showed that Bend5 worked together with Bend4 and helped mark chromatin boundaries to promote EpiLC induction in vitro. Our findings suggest that BEND/Bend proteins represent a new family of transcriptional modulators and chromatin boundary factors that participate in gene expression regulation during early germline development.

Production of quail (Coturnix japonica) germline chimeras by transfer of Ficoll-enriched spermatogonial stem cells

Due to the absence of long-term in vitro germline competent stem cell maintenance systems and efficient methods for germline transmission, efforts to develop an effective transgenic system in quail has remained limited. To overcome this limitation, here we produced germline chimeric quails through transplantation of spermatogonial stem cells (SSCs) enriched by density gradient methods utilizing Ficoll-Paque PLUS (Ficoll), Percoll and sucrose solution as a practical strategy for germline transmission in quail. For all gradient methods, testicular cells were separated as two fractions, and the expression levels of SSC-specific genes (GFRA1, ITGA6, ITGB1) and pluripotency genes (NANOG, POUV) were examined. As a result, quantitative reverse transcription-polymerase chain reaction (qRT-PCR) and RNA probe hybridization analysis revealed that the upper fraction that was separated by Ficoll showed the highest expression of SSC-specific and pluripotency genes among all fractions. Cells in the upper Ficoll gradient fraction also displayed reduced heterochromatin distribution, as observed in differentiated spermatogonia using transmission electron microscopy (TEM). These results indicate that SSCs were enriched in the upper fraction by Ficoll density gradient centrifugation. Subsequent transplantation experiments revealed that the efficiency of germline transmission to donor-derived gametes in the germline chimeras with transplanted SSCs and whole testicular cells was 0-13.2% and 0-4.4%, respectively. Collectively, these results demonstrate that quail SSCs were easily enriched with a density gradient method and that this method is a feasible and practical way to preserve the germplasm of quail. Furthermore, we can expect to apply this method in research examining the production of transgenic quail and preservation of avian species.

Reproductive Science as an Essential Component of Conservation Biology: New Edition

The previous edition of this book mainly provided a snapshot of the state of the art in terms of species-specific reproductive biology and emerging technologies. The influence of environmental changes on reproductive fitness was introduced but not fully explored. The objectives of this second edition were to (1) emphasize the need for holistic and global efforts to understand and sustain reproduction in a constantly changing environment and (2) provide more knowledge in the reproductive physiology of different taxa. The first section of the book is dedicated to survival and adaptation of species in a changing environment (including chapters on environmental impacts in different taxa, as well as the role of microbiomes). The second section focuses on progress in understanding, assisting or even suppressing reproduction in wild species, keeping in mind the influence of environmental factors as well. It contains chapters from the previous edition that were updated (reproduction in elephants, koalas, marsupials, amphibians, and corals), new chapters on species such as sharks and rays, and contributions about the increasing role of reproductive manipulations, such as assisted reproduction and contraception. While the present book emphasizes the overarching issue of environmental impacts on reproduction (resulting in infertility, subfecundity, or fitness), it also highlights the challenges of maintaining wild species in captivity, including those associated with ensuring good welfare. Captive environments can influence reproduction in a multitude of ways, some unexpected, such as the selection of unwanted genetic traits, an essential dimension to be considered to ensure the success of conservation breeding programs. Lastly, new approaches, such as the use of allostatic load indexes and reproductive microbiome analyses also will be closely examined for the first time in rare and endangered species to address conservation issues.