Primordial germ cell-mediated transgenesis and genome editing in birds

Abstract

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.

Keywords: Avian; Genome editing; Primordial germ cell; Transgenesis.

Fig. 1

Transgenic and genome editing system in mammals and birds. a In mammals, transgenic (TG) and genome edited (GE) offspring can be produced via direct introduction of genome editing tool into the zygote or microinjection of genome edited ESCs into the recipient blastocyst. b In birds, TG and GE offspring can be produced via injection of genome edited PGCs into the blood vessel of recipient

Fig. 2

Schematic representation of the development and migration of PGCs in mouse and chicken. a Mouse PGCs originated from epiblast, and migrate through dorsal mesentery to seettle in the genital ridge. b Chicken PGCs located at the center of area pellucida region, and they migrate through germinal crescent and vascular system to settle in the genital ridge

Fig. 3

Historical contributions to advancemnet of primordial germ cell-mediated production of germline chimeras and genetic modulation in birds. PGC, primordial germ cell; bPGC, embryonic blood-derived PGC; gPGC, embryonic gonad-derived PGC; HR, homologous recombination; TALEN, transcription activator-like effector nuclease; CRISPR/Cas9, clustered regularly interspaced short palindromic repeat (CRISPR)-CRISPR associated protein; HDR, homology-directed repair

Fig. 4

Strategies for the production of genome-edited birds. Avian PGCs can be isolated from embryonic blood (HH stages 14–16) and embryonic gonads (HH stage 26–28) by cell-surface antibody-mediated methods, density gradient centrifugation, and size-dependent isolation methods. Genome-edited birds can be produced by transplanting directly isolated or in vitro cultured PGCs into the blood vessels of recipient embryos after the introduction of genome editing tools. Avian genome editing systems can be applied to produce various avian models, such as avian disease resistance models, bioreactor models, and human disease models