Establishment of Intracytoplasmic Sperm Injection Technique in Japanese Quail and its Possible Application for Poultry Resources and Transgenic Birds

Current Approaches to and the Application of Intracytoplasmic Sperm Injection (ICSI) for Avian Genome Editing

Poultry are one of the most valuable resources for human society. They are also recognized as a powerful experimental animal for basic research on embryogenesis. Demands for the supply of low-allergen eggs and bioreactors have increased with the development of programmable genome editing technology. The CRISPR/Cas9 system has recently been used to produce transgenic animals and various animals in the agricultural industry and has also been successfully adopted for the modification of chicken and quail genomes. In this review, we describe the successful establishment of genome-edited lines combined with germline chimera production systems mediated by primordial germ cells and by viral infection in poultry. The avian intracytoplasmic sperm injection (ICSI) system that we previously established and recent advances in ICSI for genome editing are also summarized.

The birth of quail chicks after intracytoplasmic sperm injection

Intracytoplasmic sperm injection (ICSI) has been successfully used to produce offspring in several mammalian species including humans. However, ICSI has not been successful in birds because of the size of the egg and difficulty in mimicking the physiological polyspermy that takes place during normal fertilization. Microsurgical injection of 20 or more spermatozoa into an egg is detrimental to its survival. Here, we report that injection of a single spermatozoon with a small volume of sperm extract (SE) or its components led to the development and birth of healthy quail chicks. SE contains three factors – phospholipase Cζ (PLCZ), aconitate hydratase (AH) and citrate synthase (CS) – all of which are essential for full egg activation and subsequent embryonic development. PLCZ induces an immediate, transient Ca2+ rise required for the resumption of meiosis. AH and CS are required for long-lasting, spiral-like Ca2+ oscillations within the activated egg, which are essential for cell cycle progression in early embryos. We also found that co-injection of cRNAs encoding PLCZ, AH and CS support the full development of ICSI-generated zygotes without the use of SE. These findings will aid our understanding of the mechanism of avian fertilization and embryo development, as well as assisting in the manipulation of the avian genome and the production of transgenic and cloned birds.

Application of intracytoplasmic sperm injection (ICSI) for fertilization and development in birds

Intracytoplasmic sperm injection (ICSI) technology in birds has been hampered due to opacity of oocyte. We developed ICSI-assisted fertilization and gene transfer in quail. This paper reviews recent advances of our ICSI experiments. The oocyte retrieved from the oviduct and a quail sperm was injected into the oocyte under a stereomicroscope. The oocyte was cultured for 24h at 41°C under 5% CO2 in air. The fertilization and development was assessed by microscopic observation. The fertility rate ranged 12-18% and development varied from stage II to V in trials. To improve the fertility rate, phospholipase C (PLC) zeta was injected with a sperm. It was increased to 37-50%. Furthermore, injection of inositol trisphosphate increased to over 85%. Quail oocyte can be fertilized with chicken sperm and so can testicular elongated spermatid. To extend embryonic development, chicken eggshell was used as a surrogate culture at 37°C after the 24h incubation at 41°C under 5% CO2 in air. It survived up to 2days thereafter. Finally, gene transfer was attempted in quail egg. The sperm membrane was disrupted with Triton X-100 (TX-100) and was injected with PLCzeta cRNA and enhanced green fluorescent protein (EGFP) gene in oocyte. The GFP expression was evaluated at 24h incubation at 41°C under 5% CO2 in air in the embryos. While the expression was not detected in the control oocytes, the experimental treatment induced blastoderm development (44%) of the oocytes and 86% of blastoderm showed fluorescent emission. In addition, PCR analysis detected EGFP fragments in 50% of GFP-expressing blastoderm. Our ICSI method may be the first step toward the production of transgenic birds.

Comparative biology of sperm factors and fertilization-induced calcium signals across the animal kingdom

Fertilization causes mature oocytes or eggs to increase their concentrations of intracellular calcium ions (Ca²⁺) in all animals that have been examined, and such Ca²⁺ elevations, in turn, provide key activating signals that are required for non-parthenogenetic development. Several lines of evidence indicate that the Ca²⁺ transients produced during fertilization in mammals and other taxa are triggered by soluble factors that sperm deliver into oocytes after gamete fusion. Thus, for a broad-based analysis of Ca²⁺ dynamics during fertilization in animals, this article begins by summarizing data on soluble sperm factors in non-mammalian species, and subsequently reviews various topics related to a sperm-specific phospholipase C, called PLCζ, which is believed to be the predominant activator of mammalian oocytes. After characterizing initiation processes that involve sperm factors or alternative triggering mechanisms, the spatiotemporal patterns of Ca²⁺ signals in fertilized oocytes or eggs are compared in a taxon-by-taxon manner, and broadly classified as either a single major transient or a series of repetitive oscillations. Both solitary and oscillatory types of fertilization-induced Ca²⁺ signals are typically propagated as global waves that depend on Ca²⁺ release from the endoplasmic reticulum in response to increased concentrations of inositol 1,4,5-trisphosphate (IP₃). Thus, for taxa where relevant data are available, upstream pathways that elevate intraoocytic IP3 levels during fertilization are described, while other less-common modes of producing Ca²⁺ transients are also examined. In addition, the importance of fertilization-induced Ca²⁺ signals for activating development is underscored by noting some major downstream effects of these signals in various animals.