Comparative Biology of Calcium Signaling during Fertilization and Egg Activation in Animals

Here we describe a procedure for cloning pigs by the use of in vitro culture systems. Four healthy male piglets from two litters were born following nuclear transfer of cultured somatic cells and subsequent embryo transfer. The initiation of five additional pregnancies demonstrates the reproducibility of this procedure. Its important features include extended in vitro culture of fetal cells preceding nuclear transfer, as well as in vitro maturation and activation of oocytes and in vitro embryo culture. The cell culture and nuclear transfer techniques described here should allow the use of genetic modification procedures to produce tissues and organs from cloned pigs with reduced immunogenicity for use in xenotransplantation.

Production of cloned pigs from in vitro systems.

Cyclic ADP-ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP) are two Ca(2+) messengers derived from NAD and NADP, respectively. Although NAADP is a linear molecule, structurally distinct from the cyclic cADPR, it is synthesized by similar enzymes, ADP-ribosyl cyclase and its homolog, CD38. The crystal structure of the cyclase has been solved and its active site identified. These two novel nucleotides have now been shown to be involved in a wide range of cellular functions including: cell cycle regulation in Euglena, a protist; gene expression in plants; and in animal systems, from fertilization to neurotransmitter release and long-term depression in brain. A battery of pharmacological reagents have been developed, providing valuable tools for elucidating the physiological functions of these two novel Ca(2+) messengers. This article reviews these recent results and explores the implications of the existence of multiple Ca(2+) messengers and Ca(2+) stores in cells.

Physiological Functions of Cyclic ADP-Ribose and NAADP as Calcium Messengers

Oocyte quality affects early embryonic survival, the establishment and maintenance of pregnancy, fetal development, and even adult disease. Quality, or developmental competence, is acquired during folliculogenesis as the oocyte grows, and during the period of oocyte maturation. Assisted reproductive technologies involving ovarian hyperstimulation, or collection of immature oocytes followed by maturation in vitro, perturb this process and result in oocytes with reduced quality. In domestic livestock species, offspring have been produced using in vitro oocyte maturation, although only a small percentage of the original pool of immature oocytes is capable of developing to the blastocyst stage and subsequently resulting in pregnancy. In vitro maturation, as it is currently undertaken, does not support the correct development of oocyte competence. Follicle size affects oocyte quality, potentially implicating messenger RNA or protein stores as factors involved in oocyte competence. Oocytes from preantral follicles grown in vitro are competent to resume meiosis, although development to the blastocyst stage is decreased. An offspring from oocytes produced using this technique was normal at birth but experienced delayed onset health issues, highlighting the importance of oocyte quality long after embryogenesis. Metabolism may play a critical role in oocyte quality because glycolytic activity in mature oocytes is correlated with increased embryonic development. Communication between the oocyte and its surrounding cumulus cells is also important for the development of a competent oocyte. Ovarian stimulation causes delayed embryonic development, increased abnormal blastocyst formation, fetal growth retardation, and increased fetal loss. Thus, although meiosis and even early development may be completed successfully, there are a variety of other processes occurring within the cytoplasm of the oocyte that are required for complete developmental competence. However, the cellular mechanisms that impart oocyte quality are unclear. Until the mechanisms involved in oocyte quality are elucidated, any effort to use assisted reproductive technologies in animals for production or biomedical purposes will be inefficient at best.

The effect of oocyte quality on development.

The importance of having high glutathione (GSH) level after bovine in vitro maturation on embryo development: effect of β-mercaptoethanol, cysteine and cystine.

The effect of stage of maturation of the germinal vesicle of porcine oocytes at the time of in vitro maturation on subsequent developmental competence was examined. A large variation exists in the germinal vesicle morphology of oocytes at the time of collection of cumulus-oocyte complexes (COCs) and after culture in the absence of dibutyryl cAMP (dbcAMP) for 20 h. However, the morphology of the germinal vesicle was synchronized to a specific stage after culture in the presence of 1 mM dbcAMP for 20 h. There was no difference in germinal vesicle breakdown rate (total mean, 75.0 +/- 5.4%) or in maturation rate (total mean, 82.1 +/- 2.1 %) at 28 and 44 h of culture, respectively. However, differences in meiotic progress of oocytes were observed (p < 0.05) at 36 h of culture when COCs were exposed to dbcAMP for the first 20 h of maturation, as compared to controls. The incidence of embryos that developed to the blastocyst stage after in vitro fertilization was higher (p < 0.05) when COCs were exposed to dbcAMP (21.5 +/- 2.5%) as compared to controls (9.2 +/- 1.6%). After transfer of experimental embryos to four recipient gilts, the three pregnant recipients delivered 19 live piglets. These results indicate that exposure of COCs to dbcAMP for the first 20 h of culture for maturation increases the homogeneity of oocyte nuclear maturation and improves the efficiency of in vitro production of swine embryos.

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Synchronization of meiosis in porcine oocytes by exposure to dibutyryl cyclic adenosine monophosphate improves developmental competence following in vitro fertilization.

The effect of hormone supplements on cytoplasmic maturation in vitro was examined by incubating oocyte-cumulus complexes, in a medium with PMSG (10 iu ml-1), hCG (10 iu ml-1) and oestradiol (1 microgram ml-1) for various periods and then transferring them to medium without added hormones for the remainder of the maturation period Exposure of oocyte-cumulus complexes to hormone supplements for 2 h improved only germinal vesicle breakdown and maturation rates compared with complexes not exposed to added hormones The removal of hormone supplements at 20 h after the start of culture enhanced the ability of oocytes to form male pronuclei 10 to 12 h after insemination Further, the effects of transfer of intact and oocytectomized oocyte-cumulus complexes to hormone-free medium at 20 h on cumulus expansion were examined The diameter and morphology of the intact oocyte-cumulus complexes were improved after the removal of oocyte-cumulus cell complexes from hormonal exposure The responses of oocytectomized oocyte-cumulus complexes to hormone were similar to those of intact oocyte-cumulus complexes with the exception of corona radiata expansion The results suggest that the removal of hormone supplements from maturation media at 20 h after culture enhanced cytoplasmic maturation and cumulus expansion Further, cumulus expansion does not appear to depend on intercellular communication between cumulus cells and oocytes Oocytectomy did influence expansion of the corona radiata during culture

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Effects of the duration of exposure to hormone supplements on cytoplasmic maturation of pig oocytes in vitro

The effects of sodium chloride (NaCl) in Whitten's medium on intracellular glutathione concentration and on cytoplasmic maturation, as determined by monospermic penetration and male pronuclear formation of porcine oocytes, were examined. Porcine cumulus-oocyte complexes were cultured for 20 h in BSA-free Whitten's medium containing different NaCl concentrations (44.50, 68.49, 92.40, 116.40, or 140.35 mM) and supplemented with 10% porcine follicular fluid and hormonal supplements; the complexes were then cultured without hormonal supplements for an additional 20-h period. The mean width of the perivitelline space of oocytes was increased with decreased concentration of NaCl in the culture medium. Intracellular glutathione concentration was elevated in oocytes cultured in medium with lower NaCl concentrations. After co-culture with spermatozoa for 6 h and culture in modified Whitten's medium for an additional 6 h, there were no differences in maturation and penetration rates among experimental groups. However, the rate of male pronuclear formation was higher in oocytes matured in media with the lower NaCl concentrations. In addition, the rates of monospermic penetration and male pronuclear formation were higher in oocytes matured in medium containing 44.50 mM NaCl (59.3 +/- 8.1 and 70.9 +/- 2.0%, respectively) than in medium containing 68.49 mM NaCl (39.4 +/- 5.5 and 57.1 +/- 4.5%, respectively). These data indicated that decreasing NaCl concentration in maturation medium for porcine oocytes below the customary level improved the quality of the matured oocytes as reflected in higher intracellular glutathione content, wider perivitelline space, higher monospermic penetration rate, and increased frequency of male pronuclear formation.

Use of low-salt culture medium for in vitro maturation of porcine oocytes is associated with elevated oocyte glutathione levels and enhanced male pronuclear formation after in vitro fertilization.

A series of integrated, effective techniques is required to produce pig embryos from follicular oocytes in vitro. The failure to form a male pronucleus and polyspermy have been serious problems in efforts to produce embryos efficiently in vitro from pig oocytes. The former problem is now considered to be due to oxidative stress and the latter has been partially solved by reducing the number of capacitated spermatozoa reaching the oocytes. By the use of new technology for in vitro production of embryos, an acceptable rate of blastocyst formation and the birth of live piglets has been achieved. However, even with the use of these improved in vitro maturation (IVM) and fertilization (IVF) conditions, the efficiency of production of in vitro blastocysts and offspring still remains relatively low. More recently the developmental competence of embryos matured and fertilized in vitro has been investigated through modification of culture conditions of oocytes during the germinal vesicle stage. Oocyte competence for early embryonic development appears to be achieved by active communication between the oocyte and follicular cells. Since the ovarian oocytes available for IVM are primarily those present in mid-size antral follicles of prepubertal gilts, more research is needed to gain an improved understanding of the culture conditions required to induce developmental competence in oocytes from both preantral and antral follicles as well as additional modifications in IVF systems to overcome the problem of polyspermic penetration.

Advances in in vitro production of pig embryos

Porcine cumulus-oocyte complexes were cultured in BSA-free Whitten's medium or modified Medium 199, each supplemented with porcine follicular fluid (PFF) and hormonal supplements (OMWM and OMM199, respectively) for 20 h; they then were cultured without hormonal supplements for an additional 20 (experiments 1 and 3) or 24 h (experiment 2). At the end of culture (experiment 1), the intracellular glutathione concentration was higher (p < 0.05) in oocytes matured in OMWM vs. OMM199. After activation by Ca2+ ionophore (experiment 2), the incidence of activation in the OMWM group was lower (p < 0.01) than in the OMM199 group. However, the incidence of pronuclear formation was higher (p < 0.01) in the OMWM group than in the OMM199 group at 8 h after activation. The percentage of embryos that developed to the morula stage was higher (p < 0.01) in the group matured in OMWM vs. OMM199 after 5 days of culture. After in vitro fertilization (experiment 3), the incidence of male pronuclear formation and the percentage of monospermic oocytes that formed one male and one female pronuclei were higher (p < 0.05) after maturation in OMWM vs. OMM199. The percentage of cleaved embryos that developed to the 8-cell and morula stages was higher (p < 0.05) in the OMWM group as compared to the OMM199 group. These results indicate that culture in modified Whitten's medium as compared with a standard medium (modified Medium 199) improves cytoplasmic maturation of porcine oocytes as evaluated by intracellular glutathione content, pronuclear formation, and development in vitro after artificial activation or fertilization in vitro.

H. Funahashi

Papers

Synchronization of meiosis in porcine oocytes by exposure to dibutyryl cyclic adenosine monophosphate improves developmental competence following in vitro fertilization.

Effects of the duration of exposure to hormone supplements on cytoplasmic maturation of pig oocytes in vitro

Use of low-salt culture medium for in vitro maturation of porcine oocytes is associated with elevated oocyte glutathione levels and enhanced male pronuclear formation after in vitro fertilization.

Advances in in vitro production of pig embryos

In vitro development of in vitro-matured porcine oocytes following chemical activation or in vitro fertilization.