Immunomagnetic purification of viable primordial germ cells of Japanese quail (Coturnix japonica)

Initial formation of and sex differences in primordial germ cells in Japanese quail

DEAD-box RNA helicase 4 (DDX4) is posited to be a key maternal germ cell factor regulating avian germ cell formation. We herein showed that the DDX4 gene product of zygotic genome activation associated with the nuclear localization of the cyclin D1 protein in presumptive primordial germ cells (PGCs) plays an essential role in the proliferation of PGCs using a CRISPR/Cas9 system approach combined with in vitro fertilization techniques in Japanese quail. A proteome analysis also revealed molecular-based differences in the features of early male and female 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.

BMP4 activates the Wnt-Lin28A-Blimp1-Wnt pathway to promote primordial germ cell formation via altering H3K4me2

The unique developmental characteristics of chicken primordial germ cells (PGCs) enable them to be used in recovery of endangered bird species, gene editing and the generation of transgenic birds, but the limited number of PGCs greatly limits their application. Studies have shown that the formation of mammalian PGCs is induced by BMP4 signal, but the mechanism underlying chicken PGC formation has not been determined. Here, we confirmed that Wnt signaling activated via BMP4 activates transcription of Lin28A by inducing β-catenin to compete with LSD1 for binding to TCF7L2, causing LSD1 to dissociate from the Lin28A promoter and enhancing H3K4me2 methylation in this region. Lin28A promotes PGC formation by inhibiting gga-let7a-3p maturation to initiate Blimp1 expression. Interestingly, expression of Blimp1 helped sustain Wnt5A expression by preventing LSD1 binding to the Wnt5A promoter. We thus elucidated a positive feedback pathway involving Wnt-Lin28A-Blimp1-Wnt that ensures PGC formation. In summary, our data provide new insight into the development of PGCs in chickens.

Production of germline chimeric quails by transplantation of cryopreserved testicular cells into developing embryos

The germplasm is a resource and tool for the conservation of genetic diversity in animals, including birds. Securing germplasm is limited in most bird species due to difficulties in semen collection and germ cell isolation, lack of germ cell-specific markers, and in vitro culture systems. Here, we report the production of germline chimeric quails by transplant of cryopreserved testicular cells (TCs) into the developing embryo. The testicular germ cell properties were maintained after freeze-thaw, with no significant reduction in cell viability irrespective of storage length. Cryopreserved TCs were transferred into Hamburger Hamilton (HH) stage 14-17 quail embryos, and were demonstrated to migrate into the embryonic gonads with similar efficiency to freshly isolated TCs. Twenty of 81 recipient embryos yielded hatchlings from cryopreserved TCs and the germline transmission efficiency was similar to that of freshly isolated cells. In conclusion, cryopreserved adult quail TCs are capable of (de)differentiation into functional gametes in recipient quail gonads and can generate donor TCs-derived progenies. This system is feasible for the isolation of sufficient germplasm resources from various bird species for conservation purposes.

Primordial germ cell-mediated transgenesis and genome editing in birds

Transgenesis and genome editing in birds are based on a unique germline transmission system using primordial germ cells (PGCs), which is quite different from the mammalian transgenic and genome editing system. PGCs are progenitor cells of gametes that can deliver genetic information to the next generation. Since avian PGCs were first discovered in nineteenth century, there have been numerous efforts to reveal their origin, specification, and unique migration pattern, and to improve germline transmission efficiency. Recent advances in the isolation and in vitro culture of avian PGCs with genetic manipulation and genome editing tools enable the development of valuable avian models that were unavailable before. However, many challenges remain in the production of transgenic and genome-edited birds, including the precise control of germline transmission, introduction of exogenous genes, and genome editing in PGCs. Therefore, establishing reliable germline-competent PGCs and applying precise genome editing systems are critical current issues in the production of avian models. Here, we introduce a historical overview of avian PGCs and their application, including improved techniques and methodologies in the production of transgenic and genome-edited birds, and we discuss the future potential applications of transgenic and genome-edited birds to provide opportunities and benefits for humans.

Avian Biotechnology

Primordial germ cells (PGCs) generate new individuals through differentiation, maturation and fertilization. This means that the manipulation of PGCs is directly linked to the manipulation of individuals, making PGCs attractive target cells in the animal biotechnology field. A unique biological property of avian PGCs is that they circulate temporarily in the vasculature during early development, and this allows us to access and manipulate avian germ lines. Following the development of a technique for transplantation, PGCs have become central to avian biotechnology, in contrast to the use of embryo manipulation and subsequent transfer to foster mothers, as in mammalian biotechnology. Today, avian PGC transplantation combined with recent advanced manipulation techniques, including cell purification, cryopreservation, depletion, and long-term culture in vitro, have enabled the establishment of genetically modified poultry lines and ex-situ conservation of poultry genetic resources. This chapter introduces the principles, history, and procedures of producing avian germline chimeras by transplantation of PGCs, and the current status of avian germline modification as well as germplasm cryopreservation. Other fundamental avian reproductive technologies are described, including artificial insemination and embryo culture, and perspectives of industrial applications in agriculture and pharmacy are considered, including poultry productivity improvement, egg modification, disease resistance impairment and poultry gene "pharming" as well as gene banking.

Avian Primordial Germ Cells

Germ cells transmit genetic information to the next generation through gametogenesis. Primordial germ cells (PGCs) are the first germ-cell population established during development, and are the common origins of both oocytes and spermatogonia. Unlike in other species, PGCs in birds undergo blood circulation to migrate toward the genital ridge, and are one of the major biological properties of avian PGCs. Germ cells enter meiosis and arrest at prophase I during embryogenesis in females, whereas in males they enter mitotic arrest during embryogenesis and enter meiosis only after birth. In chicken, gonadal sex differentiation occurs as early as embryonic day 6, but meiotic initiation of female germ cells starts from a relatively late stage (embryonic day 15.5). Retinoic acid controls meiotic entry in developing chicken gonads through the expressions of retinaldehyde dehydrogenase 2, a major retinoic acid synthesizing enzyme, and cytochrome P450 family 26, subfamily B member 1, a major retinoic acid-degrading enzyme. The other major biological property of avian PGCs is that they can be propagated in vitro for the long term, and this technique is useful for investigating proliferation mechanisms. The main factor involved in chicken PGC proliferation is fibroblast growth factor 2, which activates the signaling of MEK/ERK and thus promotes the cell cycle and anti-apoptosis. Furthermore, the activation of PI3K/Akt signaling is indispensable for the proliferation and survival of chicken PGCs.

Poultry genetic resource conservation using primordial germ cells

The majority of poultry genetic resources are maintained in situ in living populations. However, in situ conservation of poultry genetic resources always carries the risk of loss owing to pathogen outbreaks, genetic problems, breeding cessation, or natural disasters. Cryobanking of germplasm in birds has been limited to the use of semen, preventing conservation of the W chromosome and mitochondrial DNA. A further challenge is posed by the structure of avian eggs, which restricts the cryopreservation of ova and fertilized embryos, a technique widely used for mammalian species. By using a unique biological property and accessibility of avian primordial germ cells (PGCs), precursor cells for gametes, which temporally circulate in the vasculature during early development, an avian PGC transplantation technique has been established. To date, several techniques for PGC manipulation including purification, cryopreservation, depletion, and long-term culture have been developed in chickens. PGC transplantation combined with recent advanced PGC manipulation techniques have enabled ex situ conservation of poultry genetic resources in their complete form. Here, the updated technologies for avian PGC manipulation are introduced, and then the concept of a poultry PGC-bank is proposed by considering the biological properties of avian PGCs.

Germline-competent stem cell in avian species and its application

Germ cells are the only cell type in the body that can transfer genetic information to the next generation. Germline-competent stem cells can self-renew and contribute to the germ cell lineage giving rise to pluripotent stem cells under specific conditions. Hence far, studies on germline-competent stem cells have contributed to the generation of avian model systems and the conservation of avian genetic resources. In this review, we focus on previous studies on germline-competent stem cells from avian species, mainly chicken germline-competent stem cells, which have been well established and characterized. We discuss different sources of germline-competent stem cells and recent advances for the future applications in birds.

Efficient system for preservation and regeneration of genetic resources in chicken: concurrent storage of primordial germ cells and live animals from early embryos of a rare indigenous fowl (Gifujidori)

The unique accessibility of chicken primordial germ cells (PGCs) during early development provides the opportunity to combine the reproduction of live animals with genetic conservation. Male and female Gifujidori fowl (GJ) PGCs were collected from the blood of early embryos, and cryopreserved in liquid nitrogen for >6 months until transfer. Manipulated GJ embryos were cultured until hatching; fertility tests indicated that they had normal reproductive abilities. Embryos from two lines of White Leghorn (24HS, ST) were used as recipients for chimera production following blood removal. The concentration of PGCs in the early embryonic blood of 24HS was significantly higher than in ST (P < 0.05). Frozen–thawed GJ PGCs were microinjected into the bloodstream of same-sex recipients. Offspring originating from GJ PGCs in ST recipients were obtained with a higher efficiency than those originating from GJ PGCs in 24HS recipients (23.3% v. 3.1%). Additionally, GJ progeny were successfully regenerated by crossing germline chimeras of the ST group. In conclusion, the cryogenic preservation of PGCs from early chicken embryos was combined with the conservation of live animals.

A novel method to isolate primordial germ cells and its use for the generation of germline chimeras in chicken

A novel method was developed to isolate chick primordial germ cells (PGCs) from circulating embryonic blood. This is a very simple and rapid method for the isolation of circulating PGCs (cPGCs) using an ammonium chloride-potassium (ACK) buffer for lysis of the red blood cells. The PGCs were purified as in vitro culture proceeded. Most of the initial red blood cells were removed in the first step using the ACK lysis buffer. The purity of the cPGCs after ACK treatment was 57.1%, and the recovery rate of cPGCs from whole blood was 90.3%. The ACK process removed only red blood cells and it did not affect cPGC morphology. In the second step, the red blood cells disappeared as the culture progressed. At 7 days of in vitro culture, the purity of the PGCs was 92.9%. Most of these cells expressed germline-specific antibodies, such as those against chicken vasa homolog (CVH). The cultured PGCs expressed the Cvh and Dazl genes. Chimeric chickens were produced from these cultured PGCs, and the donor cells were detected in the gonads, suggesting that the PGCs had biological function. In conclusion, this novel isolation system for PGCs should be easier to use than previous methods. The results of the present study suggest that this novel method will become a powerful tool for germline manipulation in the chicken.

Avian Germplasm Preservation: Embryonic Stem Cells or Primordial Germ Cells?

Presently, avian genetic resources are best maintained as living collections of birds. Unfortunately, these stocks have been under constant pressure to be destroyed because of the decline in the number of Poultry Science Departments and pressures to cut costs at land grant institutions. Cryopreservation of semen is often suggested as a means to bank avian germplasm. However, this is only applicable for single-gene traits and does not allow for full reconstitution of the genetics of the original line. Over the last 15 yr, advances in the manipulation of the early chick embryo, manipulation of primordial germ cells (PGC), and the culture of embryonic stem cells (ESC) suggests that cryopreservation of blastodermal cells, ESC, or PGC might offer a means to preserve the entire genome of highly selected, specialized stocks of poultry. Freezing each of these cell types is possible with varying degrees of efficiency. Similarly, the effectiveness of generating germ line chimeras using blastodermal cells, ESC, or PGC also varies greatly. Other factors that must be considered include the choice of the recipient lines to develop the germ line chimeras and the number of individuals needed to reconstitute the line. Finally, the low efficiency rate of reconstitution and the high cost associated with current technologies makes these approaches prohibitive. Significant challenges remain to be overcome before the entire genome of poultry stocks can be routinely cryoperserved and reconstituted.

Enriched gonadal migration of donor-derived gonadal primordial germ cells by immunomagnetic cell sorting in birds

This study was conducted to evaluate whether immunomagnetic treatment could improve the retrieval and migration capacity of avian gonadal primordial germ cells (gPGCs) collected from gonads in 5.5-day-old chick and 5-day-old quail embryos, respectively. Collected gPGCs were loaded into a magnetic-activated cell sorter (MACS) after being conjugated with specific gPGC antibodies and either MACS-treated or non-treated cells in each species were subsequently transferred to the recipient embryos. MACS treatment significantly (P < 0.05) increased the population ratio of gPGCs in gonadal cells retrieved (0.74 to 33.4% in the chicken and 2.68 to 45.1% in the quail). This was due to decreased number of non-gPGCs in total cell population. MACS treatment further enhanced gonadal migration of gPGCs transferred in both species (10% vs. 80-85% in the chicken and 10-15% vs. 70-80% in the quail). Increase in the number of microinjected cells up to 600 cells/embryo did not eliminate such promoting effect. In conclusion, MACS treatment greatly increased the population ratio of avian gPGCs in gonadal cells, resulting improved gonadal migration in recipient embryos.

Improved germline transmission in chicken chimeras produced by transplantation of gonadal primordial germ cells into recipient embryos

In the avian species, germline chimera production could be possible by transfer of donor germ cells into the blood vessel of recipient embryos. This study was conducted to establish an efficient transfer system of chicken gonadal primordial germ cells (gPGCs) for producing the chimeras having a high capacity of germline transmission. Gonadal PGCs retrieved from 5.5-day-old embryos (stage 28) of Korean Ogol chicken (KOC with i/i gene) were transferred into the dorsal aorta of 2.5-day-old embryos (stage 17) of White Leghorn chicken (WL with I/I gene). Prospective evaluations of whether culture duration (0, 5, or 10 days) and subsequent Ficoll separation of gPGCs before transfer affected chimera production and germline transmission in the chimeras were made while retrospective analysis was conducted for examining the effect of chimera sexuality. A testcross analysis by artificial insemination of presumptive chimeras with adult KOC was performed for evaluating each treatment effect. First, comparison was made for evaluating whether experimental treatments could improve chimera production, but none of the treatments were significantly (P = 0.6831) influenced (5.1%-14.4%). Second, it was determined whether each treatment could enhance germline transmission in produced chimeras. More (P < 0.0001) progenies with black feathers (i/i) were produced in the germline chimeras derived from the transfer of 10-day-cultured gPGCs than from the transfer of 0- or 5-day-cultured gPGCs (0.6%-7.8% vs. 10.7%-49.7%). Ficoll separation was negatively affected (P < 0.0001), whereas there was no effect in chimera sexuality (P = 0.6011). In conclusion, improved germline transmission of more than a 45% transmission rate was found in chicken chimeras produced by transfer of 10-day-cultured gPGCs being separated without Ficoll treatment.

Restriction of proliferation of primordial germ cells by the irradiation of Japanese quail embryos with soft X-rays

Primordial germ cells (PGCs) are the progenitor cells for the gametes. Avian PGCs are located in the central region of the area pellucida at the blastoderm stage. Shortly after further incubation, they migrate to the extra-embryonic germinal crescent, and then as soon as the blood vessels form, they enter the circulation and finally settle in the gonadal primordium. We have developed a simple method using soft X-ray irradiation (18 kV power, 20 cm distance) to reduce the number of PGCs in Japanese quail embryos, which should be useful in preparing recipient embryos for PGC-transfer studies. When embryos were exposed to the soft X-rays for 40 s before incubation, the concentration of circulating PGCs was less than one-fifth that in controls after 2 days of incubation. Embryos at day 6 of incubation contained approximately half the number of PGCs compared to controls when they were exposed before or at day 2 of incubation. Irradiation for 40 s is recommended taking into consideration the restriction of proliferation of PGCs, and viability and hatchability.

Improvement of hatchability of chicken eggs injected by blastoderm cells

In our first experiment, we studied the effect of injection method of blastoderm cells (BC) into the subgerminal cavity of White Leghorn embryos on hatchability of chicken chimeras. Freshly laid eggs were injected through a hole made in the equatorial plane of the eggshell (Method A). In Method B, eggs were stored pointed end down for 5 to 7 d prior to injection, and a hole was cut in the blunt end of the eggshell. An advantage of Method B was that the early embryonic mortality was reduced (P < or = 0.01) and resulted in higher hatchability (41.0%; 43/105) than Method A (9.8%; 14/143). In the second experiment, we studied chicken hatchabililty as influenced by windowing (no hole, Group 1; hole in the equatorial plane, Group 2; hole in the blunt end of egg, Groups 3 and 4) and egg turning (Groups 1 and 4) or not (Groups 2 and 3) during incubation. The hatchability percentages were as follows: 67.9 (Group 1) 0.0, (Group 2) 23.3, (Group 3), and 56.8 (Group 4). A statistically significant difference (P < or = 0.05) was noted between Group 1 or 4 and the other groups. We found no statistically significant differences in the weight changes (g) but did note certain differences in the egg weight loss (%) among different egg treatments. In the third experiment, we investigated the influence of origins of BC donors: Rhode Island Red (RIR), Barred Plymouth Rock (BPR), and Green-legged Partridgelike (GP) on hatchability of putative and somatic chimera chickens. The hatchability of chimeras was dependent on the adequate assortment of BC of the donor and ranged from 7.4% (RIR) to 56.1% (GP). In the case of BC injection of the GP breed, good hatchability was accompanied by very high percentage (86.9; 20/23) of somatic chimeras.

Spermiogenesis in commercial poultry species: anatomy and control

Spermatogenesis is a complicated process dependent upon several factors. Formation of a testis requires the interaction of gene-products and hormones (androgens) on pluripotent tissue. In birds, the female is the heterogametic (ZW) sex, but W chromosomal genes do not influence gonadal development in a way similar to the SRY gene on the mammalian Y chromosome. However, autosomal genes such as SRY-like HMG box gene 9 (SOX9) may influence gonadal development. Hormones affect development; male gonads subjected to estrogen form an ovotestis, whereas ovaries exposed to aromatase inhibitors form an atypical testis. Sertoli cell numbers are set early in spermiogenesis, possibly under the influence of follicle-stimulating hormone and thyroid hormone, and this may determine the number of gonial cells that can be supported. Sertoli cells make a number of substances that affect testicular development and function, particularly anti-Müllerian hormone, which inhibits female oviduct formation from the Müllerian anlage, inhibits aromatase activity to stop estrogen production, and possibly stimulates androgen production by Leydig cells. Undifferentiated primordial germ cells (PGC) migrate to the testis and are converted to spermatogonia by factors from gonadal ridge tissue and androgens. The PGC of males in the ovary form oocytes of Z genotype, whereas the female PGC in males form mostly Z sperm (with a few of W genotype). Transmission electron microscopy micrographs of turkey testis are presented, and control of spermatogenesis by hormones and cytokines is discussed. This discussion includes follicle-stimulating hormone, luteinizing hormone, inhibin, activin, follistatin, tumor necrosis factor-alpha, growth factors such as transforming growth factor-beta, interleukins, and interferon. Although information concerning paracrine and autocrine regulation of the avian testis by these substances is sparse, much can be learned from mammalian studies, in which putative roles of each of these substances have been established. How Sertoli cells cause directed apoptosis of spermatogonia using the Fas-ligand, Fas-receptor pathway is reviewed, as well as ways to circumvent this process. A possible role for ubiquitin concerning prevention of heat-induced damage to the testis is presented.