Isolation and Characterization of Chicken Primordial Germ Cells and Their Application in Transgenesis

Comparison of genes involved in brain development: insights into the organization and evolution of the telencephalic pallium

The mechanisms underlying the organization and evolution of the telencephalic pallium are not yet clear.. To address this issue, we first performed comparative analysis of genes critical for the development of the pallium (Emx1/2 and Pax6) and subpallium (Dlx2 and Nkx1/2) among 500 vertebrate species. We found that these genes have no obvious variations in chromosomal duplication/loss, gene locus synteny or Darwinian selection. However, there is an additional fragment of approximately 20 amino acids in mammalian Emx1 and a poly-(Ala)6-7 in Emx2. Lentiviruses expressing mouse or chick Emx2 (m-Emx2 or c-Emx2 Lv) were injected into the ventricle of the chick telencephalon at embryonic Day 3 (E3), and the embryos were allowed to develop to E12-14 or to posthatchling. After transfection with m-Emx2 Lv, the cells expressing Reelin, Vimentin or GABA increased, and neurogenesis of calbindin cells changed towards the mammalian inside-out pattern in the dorsal pallium and mesopallium. In addition, a behavior test for posthatched chicks indicated that the passive avoidance ratio increased significantly. The study suggests that the acquisition of an additional fragment in mammalian Emx2 is associated with the organization and evolution of the mammalian pallium.

Single-cell RNA sequencing of mitotic-arrested prospermatogonia with DAZL::GFP chickens and revealing unique epigenetic reprogramming of chickens

Background

Germ cell mitotic arrest is conserved in many vertebrates, including birds, although the time of entry or exit into quiescence phase differs. Mitotic arrest is essential for the normal differentiation of male germ cells into spermatogonia and accompanies epigenetic reprogramming and meiosis inhibition from embryonic development to post-hatch. However, mitotic arrest was not well studied in chickens because of the difficulty in obtaining pure germ cells from relevant developmental stage.

Results

We performed single-cell RNA sequencing to investigate transcriptional dynamics of male germ cells during mitotic arrest in DAZL::GFP chickens. Using differentially expressed gene analysis and K-means clustering to analyze cells at different developmental stages (E12, E16, and hatch), we found that metabolic and signaling pathways were regulated, and that the epigenome was reprogrammed during mitotic arrest. In particular, we found that histone H3K9 and H3K14 acetylation (by HDAC2) and DNA demethylation (by DNMT3B and HELLS) led to a transcriptionally permissive chromatin state. Furthermore, we found that global DNA demethylation occurred gradually after the onset of mitotic arrest, indicating that the epigenetic-reprogramming schedule of the chicken genome differs from that of the mammalian genome. DNA hypomethylation persisted after hatching, and methylation was slowly re-established 3 weeks later.

Conclusions

We found a unique epigenetic-reprogramming schedule of mitotic-arrested chicken prospermatogonia and prolonged hypomethylation after hatching. This will provide a foundation for understanding the process of germ-cell epigenetic regulation in several species for which this process is not clearly described. Our findings on the biological processes related to sex-specific differentiation of prospermatogonia could help studying germline development in vitro more elaborately.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40104-022-00712-4.

Oct4 dependent chromatin activation is required for chicken primordial germ cell migration

Primordial germ cells (PGCs) are the undifferentiated progenitors of the gametes. Unlike the poor maintenance of cultured mammalian PGCs, the avian PGCs can be expanded in vitro indefinitely while preserving pluripotency and germline competence. In mammals, the Oct4 is the master transcription factor that ensures the stemness of pluripotent cells such as PGCs, but the specific function of Oct4 in chicken PGCs remains unclear. As expected, the loss of Oct4 in chicken PGCs reduced the expression of key pluripotency factors and promoted the genes involved in endoderm and ectoderm differentiation. Furthermore, the global active chromatin was reduced as shown by the depletion of the H3K27ac upon Oct4 suppression. Interestingly, the de-activated chromatin caused the down-regulation of adjacent genes which are mostly known regulators of cell junction, chemotaxis and cell migration. Consequently, the Oct4-deficient PGCs show impaired cell migration and could not colonize the gonads when re-introduced into the bloodstream of the embryo. We propose that, in addition to maintaining pluripotency, the Oct4 mediated chromatin activation is dictating chicken PGC migration.

Current Approaches and Applications in Avian Genome Editing

Advances in genome-editing technologies and sequencing of animal genomes enable researchers to generate genome-edited (GE) livestock as valuable animal models that benefit biological researches and biomedical and agricultural industries. As birds are an important species in biology and agriculture, their genome editing has gained significant interest and is mainly performed by using a primordial germ cell (PGC)-mediated method because pronuclear injection is not practical in the avian species. In this method, PGCs can be isolated, cultured, genetically edited in vitro, and injected into a recipient embryo to produce GE offspring. Recently, a couple of GE quail have been generated by using the newly developed adenovirus-mediated method. Without technically required in vitro procedures of the PGC-mediated method, direct injection of adenovirus into the avian blastoderm in the freshly laid eggs resulted in the production of germ-line chimera and GE offspring. As more approaches are available in avian genome editing, avian research in various fields will progress rapidly. In this review, we describe the development of avian genome editing and scientific and industrial applications of GE avian species.

A novel F-box domain containing cyclin F like gene is required for maintaining the genome stability and survival of chicken primordial germ cells

The stability and survival of germ cells are controlled by the germline-specific genes, however, such genes are less known in the avian species. Using a microarray-based the National Center for Biotechnology Information Gene Expression Omnibus dataset, we found an unigene (Gga.9721) that upregulated in the chicken primordial germ cells (PGCs). The unigene showed 97% identities with an uncharacterized chicken cyclin F like gene. The predicted chicken cyclin F like gene was further characterized through expression and regulation in the chicken PGCs. The sequence analysis revealed that the gene shows identities with cyclin F gene and contains an F-box domain. The expression of chicken cyclin F like was detected specifically in the gonads, PGCs, and germline cells. The knockdown of cyclin F like gene resulted in DNA damage and apoptosis in the PGCs. The genes related to stemness and germness were downregulated, whereas, genes related to apoptosis and DNA damage response were increased in the PGCs after the knockdown of chicken cyclin F like. We further observed that the Nanog homeobox controlled the transcriptional activity of chicken cyclin F like gene in PGCs. Collectively, the chicken cyclin F like gene, which is not reported in any other species, is required for maintaining the genome stability of germ cells.