Proliferation of chick primordial germ cells cultured on stroma cells from the germinal ridge

Designing A Transgenic Chicken: Applying New Approaches toward A Promising Bioreactor

Specific developmental characteristics of the chicken make it an attractive model for the generation of transgenic organisms. Chicken possess a strong potential for recombinant protein production and can be used as a powerful bioreactor to produce pharmaceutical and nutritional proteins. Several transgenic chickens have been generated during the last two decades via viral and non-viral transfection. Culturing chicken primordial germ cells (PGCs) and their ability for germline transmission ushered in a new stage in this regard. With the advent of CRISPR/Cas9 system, a new phase of studies for manipulating genomes has begun. It is feasible to integrate a desired gene in a predetermined position of the genome using CRISPR/Cas9 system. In this review, we discuss the new approaches and technologies that can be applied to generate a transgenic chicken with regards to recombinant protein productions.

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.

Chick stem cells: current progress and future prospects

Chick embryonic stem cells (cESCs) can be derived from cells obtained from stage X embryos (blastoderm stage); these have the ability to contribute to all somatic lineages in chimaeras, but not to the germ line. However, lines of stem cells that are able to contribute to the germ line can be established from chick primordial germ cells (cPGCs) and embryonic germ cells (cEGCs). This review provides information on avian stem cells, emphasizing different sources of cells and current methods for derivation and culture of pluripotent cells from chick embryos. We also review technologies for isolation and derivation of chicken germ cells and the production of transgenic birds.

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Chick embryonic stem cells (cESCs) can be derived from a variety of sources.

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cESCs can contribute to all somatic cell types but not to the germ line.

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germ cells can be isolated from early embryos, embryonic blood and gonads.

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germ cells can establish self-renewing lines and contribute to the germline.

Expression dynamics of pluripotency genes in chicken primordial germ cells before and after colonization of the genital ridges

Mammalian species utilize an inductive mechanism of germ cell specification, diverting the fate of some of somatic cells toward pluripotency and germ-cell totipotency. It is not known if avian species utilize a similar mechanism nor if, analogous to mammalian primordial germ cells (PGCs), pluripotency genes are continuously upregulated in migrating and genital ridge-colonizing avian PGCs. Thus, this study was conducted to quantify and to analyze the expression profile of pluripotency genes at different stages of chicken PGCs development at Hamburger and Hamilton (HH) stage 14, when the majority of PGCs have entered into the bloodstream; at HH stage 18, when PGCs have resided for 8-12 hr in the bloodstream; and at HH stage 28, when the majority of PGCs are found in the genital ridge. The transcription for Oct4, Sox2, and Nanog continuously decreased from HH stage 14 to HH stage 28. In addition, equal amounts of total RNA could be isolated from chicken PGCs at each stage of development, indicating that the observed drop of transcription of pluripotency genes is not a consequence of transcriptional repression in general. Decreased expression for all three proteins was also observed at HH stage 28. Furthermore, in comparison to blood PGCs, those residing in the gonad have lost their full capacity for colony formation. Our results indicate that, in contrast to mammalian PGCs, chicken PGCs continuously downregulate the expression of pluripotency genes and show a progressive loss of pluripotency-associated features during different stages of germ-line migration.

Primordial germ cell-mediated chimera technology produces viable pure-line Houbara bustard offspring: potential for repopulating an endangered species

BACKGROUND

The Houbara bustard (Chlamydotis undulata) is a wild seasonal breeding bird populating arid sandy semi-desert habitats in North Africa and the Middle East. Its population has declined drastically during the last two decades and it is classified as vulnerable. Captive breeding programmes have, hitherto, been unsuccessful in reviving population numbers and thus radical technological solutions are essential for the long term survival of this species. The purpose of this study was to investigate the use of primordial germ cell-mediated chimera technology to produce viable Houbara bustard offspring.

METHODOLOGY/PRINCIPAL FINDINGS

Embryonic gonadal tissue was dissected from Houbara bustard embryos at eight days post-incubation. Subsequently, Houbara tissue containing gonadal primordial germ cells (gPGCs) was injected into White Leghorn chicken (Gallus gallus domesticus) embryos, producing 83/138 surviving male chimeric embryos, of which 35 chimeric roosters reached sexual maturity after 5 months. The incorporation and differentiation of Houbara gPGCs in chimeric chicken testis were assessed by PCR with Houbara-specific primers and 31.3% (5/16) gonads collected from the injected chicken embryos showed the presence of donor Houbara cells. A total of 302 semen samples from 34 chimeric roosters were analyzed and eight were confirmed as germline chimeras. Semen samples from these eight roosters were used to artificially inseminate three female Houbara bustards. Subsequently, 45 Houbara eggs were obtained and incubated, two of which were fertile. One egg hatched as a male live born Houbara; the other was female but died before hatching. Genotyping confirmed that the male chick was a pure-line Houbara derived from a chimeric rooster.

CONCLUSION

This study demonstrates for the first time that Houbara gPGCs can migrate, differentiate and eventually give rise to functional sperm in the chimeric chicken testis. This approach may provide a promising tool for propagation and conservation of endangered avian species that cannot breed in captivity.

Identification, Culture, and Characterization of Germline Stem Cell-Like Cells in Chicken Testes1

We recently succeeded in inducing germline transmission by transferring chicken testicular cells into heterologous testes. This study was designed subsequently to identify pluripotent cells in the testicular cells, which would induce the germline transmission. Testicular cells retrieved from juvenile (4-wk-old) or adult (24-wk-old) White Leghorn (WL) chickens were stained with germ cell-specific markers anti-SSEA1, anti-SSEA3, anti-SSEA4, anti-EMA1, anti-ITGA6, and anti-ITGB1 antibodies; 2C9; and lectin-Solanum tuberosum agglutinin (STA). The percentages of the cells that were positive for each marker were within the ranges of 0.33% -0.44% and 0.029%-0.072% of the total testicular cell population in the juvenile and adult, respectively, and significant (P < 0.0002) differences were detected between the ages. When 1 x 10(6) testicular cells were cultured in Dulbecco minimum essential medium-based medium supplemented with leukemia inhibitory factor (LIF), basic fibroblast growth factor (FGF2), and/or insulinlike growth factor 1 (IGF1), colony formation was detected only in LIF++FGF2-containing or LIF+FGF2+IGF1-containing medium during primary culture, and the supplementation of LIF+FGF2+IGF1 was the most efficient for maintaining the colony-forming cells through subculture. The established cells retrieved at the end of the primary culture or the 20th subpassage were positive for chicken germ cell-specific periodic acid-Schiff (PAS), EMA1, 2C9, SSEA1, SSEA3, SSEA4, ITGA6, and ITGB1; and lectin-STA markers (evaluated after 11th subpassage). Double staining of lectin-STA with anti-SSEA1, anti-SSEA3, anti-SSEA4, anti-ITGA6, and anti-ITGB1 also was possible. They differentiated spontaneously into embryoid bodies after being cultured in LIF-free medium. We conclude that germline stem cell-like cells are present in chicken testicular cells retrieved from both juvenile and adult testes, which can be identified with the specific markers for primordial germ cells or embryonic germ cells.

Activation of protein kinases A and C promoted proliferation of chicken primordial germ cells

Many growth factors or cytokines regulate cell proliferation via different intracellular signaling pathways. The mechanisms remained quite unclear in avian primordial germ cells (PGCs). In the present study, two major protein kinases, PKA and PKC, were investigated to be involved in signal transduction of PGC proliferation. PGCs were isolated from genital ridge of 3.5-day chicken embryos and primary culture was performed with 5% fetal calf serum (FCS)-supplemented medium 199. After culture for 24 h, PGCs were subcultured on chicken embryonic fibroblast feeder (CEF) and the cells were characterized by histochemical stainings of alkaline phosphatase (ALP) and periodic acid-Schiff (PAS) reagent as well as immunocytochemical stainings of c-kit and stage-specific embryonic antigen-1 (SSEA-I). In addition, cells were challenged with adenylate cyclase activator forskolin (FRSK) and PKC activator phorbol-12-myristate-13-acetate (PMA) alone or in combinations with PKA inhibitor H(89) and PKC inhibitor H(7), respectively. Results showed that subcultured PGCs on CEF displayed positive histochemical and immunocytochemical stainings for ALP, PAS, c-kit and SSEA-I and manifested intensive proliferating activity by colony formation. Downstream activation of PKA by FRSK (10(-7) to 10(-5)M) significantly promoted the proliferation of PGCs by increasing colony number (ALP-stained) in a dose-dependant manner. PMA (10(-8)M) also increased PGC colony number (P<0.05). However, the proliferating effects elicited by FRSK or PMA could be inhibited by the respective protein kinase inhibitor H(89) or H(7). Therefore, the above results suggest that activation of intracellular protein kinases A and C by external factors may promote proliferation of cultured PGCs and PKA represents the most likely mediator of PGC proliferation in embryonic chickens.

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.

Avian pluripotent stem cells

Pluripotent embryonic stem cells are undifferentiated cells capable of proliferation and self-renewal and have the capacity to differentiate into all somatic cell types and the germ line. They provide an in vitro model of early embryonic differentiation and are a useful means for targeted manipulation of the genome. Pluripotent stem cells in the chick have been derived from stage X blastoderms and 5.5 day gonadal primordial germ cells (PGCs). Blastoderm-derived embryonic stem cells (ESCs) have the capacity for in vitro differentiation into embryoid bodies and derivatives of the three primary germ layers. When grafted onto the chorioallantoic membrane, the ESCs formed a variety of differentiated cell types and attempted to organize into complex structures. In addition, when injected into the unincubated stage X blastoderm, the ESCs can be found in numerous somatic tissues and the germ line. The potential give rise to somatic and germ line chimeras is highly dependent upon the culture conditions and decreases with passage. Likewise, PGC-derived embryonic germ cells (EGCs) can give rise to simple embryoid bodies and can undergo some differentiation in vitro. Interestingly, chicken EG cells contribute to somatic lineages when injected into the stage X blastoderm, but only germ line chimeras have resulted from EGCs injected into the vasculature of the stage 16 embryo. To date, no lines of transgenic chickens have been generated using ESCs or EGCs. Nevertheless, progress towards the culture of avian pluripotent stem cells has been significant. In the future, the answers to fundamental questions regarding segregation of the avian germ line and the molecular basis of pluripotency should foster the full use of avian pluripotent stem cells.

Avian transgenesis: progress towards the promise

The hen has long held promise as a low cost, high-yield bioreactor for the production of human biopharmaceuticals in egg whites. A typical egg white contains 3.5-4.0 grams of protein, more than half of which comes from a single gene (ovalbumin). Harnessing the power of the gene to express a recombinant protein could yield up to a gram or more of the protein in the naturally sterile egg. Accordingly, a major effort has been underway for more than a decade to develop robust methods for modification of the chicken genome. This effort intensified in the mid-1990s when several avian transgenic companies entered the scene. Progress has been made in that time but much remains to be done.

Production of germline chimeras by transfer of chicken gonadal primordial germ cells maintained in vitro for an extended period

We previously reported that germline chimeras could be produced by transfer of chicken gonadal primordial germ cells (gPGCs) cultured for a short term (5 days). This study was subsequently undertaken to examine whether gPGCs maintained in vitro for an extended period could retain their specific characteristics to induce germline transmission. Chicken (White Leghorn, WL) gPGCs were retrieved from embryos at stage 28 (5.5 days of incubation) and continuously cultured for 2 months in modified Dulbecco's minimal essential medium without subpassage and changing of the feeder cell layer. After the identification of gPGC characteristics using Periodic acid-Shiff's (PAS) reaction and anti stage-specific embryonic antigen-1 (SSEA-1) antibody staining at the end of the culture, cultured gPGCs were injected into the dorsal aorta of Korean Ogol Chicken (KOC) recipient embryos at stage 17 (2.5 days of incubation). Nineteen chickens (13 males and 6 females) were hatched, grown to sexual maturity, and subsequently subjected to testcross analysis employing artificial insemination with adult KOC. Of these, four (three males and one female) hatched chickens with white coat color. The percentage of germline chimerism was 21% (4/19). The results of this study demonstrated that gPGCs could maintain their specific characteristics for up to 2 months in vitro, resulting in the birth of germline chimeras following transfer to recipient embryos.

Production of duck-chicken chimeras by transferring early blastodermal cells

Duck blastodermal cells isolated from Stage X embryos of Maya ducks were injected into subgerminal cavity of recipient Stage X chicken embryos treated with gamma-irradiation or untreated. Eleven somatic chimeras were obtained based on plumage color and were raised to sexual maturity. To test for germline chimerism, progeny tests were performed by mating the chimeras with Maya ducks. A total of 622 eggs was collected and incubated. Fertility rate and hatchability were 2.9% (18/622) and 1.0% (6/622), respectively. The six duck hatchlings were from Chimera 9801 and were considered to be derived from the germ cells developed from the donor Maya blastodermal cells, indicating that Chimera 9801 is a germline chimera.

Derivation and characterization of pluripotent embryonic germ cells in chicken

Embryonic germ (EG) cell lines established from primordial germ cells (PGCs) are undifferentiated and pluripotent stem cells. To date, EG cells with proven germ-line transmission have been completely established only in the mouse with embryonic stem (ES) cells. We isolated PGCs from 5.5-day-old (stage 28) chicken embryonic gonads and established a putative chicken EG cell line with EG culture medium supplemented with stem cell factor (SCF), leukemia inhibitory factor (LIF), basic fibroblast growth factor (bFGF), interleukin-11 (IL-11), and insulin-like growth factor-I (IGF-I). These cells grew continuously for ten passages (4 months) on a feeder layer of mitotically active chicken embryonic fibroblasts. After several passages, these cells were characterized by screening with the periodic acid-Schiff reaction, anti-SSEA-1 antibody, and a proliferation assay. The chicken EG cells maintained characteristics of gonadal PGCs and undifferentiated stem cells. When cultured in suspension, the chicken EG cells successfully formed an embryoid body and differentiated into a variety of cell types. The chicken EG cells were injected into stage X blastodermal layer and produced chimeric chickens with various differentiated tissues derived from the EG cells. Chicken EG cells will be useful for the production of transgenic chickens and for studies of germ cell differentiation and genomic imprinting.

Improved transfection efficiency of chicken gonadal primordial germ cells for the production of transgenic poultry

Electroporation is a common method of DNA transfection for many types of eukaryotic cells, but has not been attempted in avian primordial germ cells (PGCs). DNA uptake in chicken primordial germ cells (PGCs) was tested using electroporation with and without dimethyl sulfoxide (DMSO). Gonadal tissue and chicken embryonic fibroblasts (CEFs) were isolated from 6-day-old embryos (stage 29), transfected with pCMV beta carrying the bacterial lacZ gene, and cultured for 24 h. Gonadal primordial germ cells (gPGCs) were purified from culture using a Ficoll gradient. The addition of DMSO significantly increased the transfection efficiency of gPGCs but had no effect on chicken embryonic fibroblasts. Electroporation of gPGCs resulted in an 80% transfection efficiency compared with about 17% observed with liposomes. Approximately 200 transfected gPGCs were injected into 2.5-day-old (stage 17) recipient embryos and the eggs were incubated for an additional 3.5 days, 7.5 days or to hatching. The exogenous gene was detectable in 100%, 67% and 41% of the 6-day-old (stage 29), 10-day-old (stage 36) recipient embryos and hatched chicks gonads, respectively. PCR analysis of DNA from the hatched chicks showed that exogenous lacZ DNA was detected only in the gonad and not the liver and heart. These results indicated that electroporation was a suitable means of transfecting avian gPCGs for the goal of producing transgenic poultry.