Electroporation of inositol 1,4,5-triphosphate induces repetitive calcium oscillations in murine oocytes

Modulators of calcium signalling at fertilization

Calcium (Ca2+) signals initiate egg activation across the animal kingdom and in at least some plants. These signals are crucial for the success of development and, in the case of mammals, health of the offspring. The mechanisms associated with fertilization that trigger these signals and the molecules that regulate their characteristic patterns vary widely. With few exceptions, a major contributor to fertilization-induced elevation in cytoplasmic Ca2+ is release from endoplasmic reticulum stores through the IP3 receptor. In some cases, Ca2+ influx from the extracellular space and/or release from alternative intracellular stores contribute to the rise in cytoplasmic Ca2+. Following the Ca2+ rise, the reuptake of Ca2+ into intracellular stores or efflux of Ca2+ out of the egg drive the return of cytoplasmic Ca2+ back to baseline levels. The molecular mediators of these Ca2+ fluxes in different organisms include Ca2+ release channels, uptake channels, exchangers and pumps. The functions of these mediators are regulated by their particular activating mechanisms but also by alterations in their expression and spatial organization. We discuss here the molecular basis for modulation of Ca2+ signalling at fertilization, highlighting differences across several animal phyla, and we mention key areas where questions remain.

Initiation and organization of events during the first cell cycle in mammals: applications in cloning

The technology of cloning involves transplanting a diploid nucleus into a mature oocyte cytoplast. The cytoplast is then activated to initiate the first cell cycle of development as a nuclear transplant embryo. Initiation and regulation of events during the first cell cycle are, therefore, critical for proper reprogramming of the donor nucleus and development as a cloned embryo. Activation is normally induced by the sperm and is mediated by a series of intracellular free calcium ([Ca(2+)](i)) oscillations that last for several hours. Although it is not known precisely how the sperm induces activation, current evidence favors the delivery, by the sperm, of a soluble protein factor that causes the production of IP3. IP3 acts to open a Ca(2+) channel in the endoplasmic reticulum and release Ca(2+) into the cytosol. A variety of methods have been used to duplicate or replace the sperm-induced [Ca(2+)](i) increase to cause activation in nuclear transplant embryos. It has been found that treatments that cause a single transient [Ca(2+)](i) activate some oocytes with the level of activation increasing as the oocyte ages. Attempts have been made to extend the period of time over which [Ca(2+)](i) oscillations occur. This has been successful in increasing activation rates of less mature oocytes but the techniques are still cumbersome. An alternative method, that has been very successful, is the combination of a treatment that elevates [Ca(2+)](i) and a treatment that maintains low levels of maturation promoting factor for several hours after the initial [Ca(2+)](i) elevation. The sperm also contributes the centrosome that organizes microtubules during the first cell cycle. One current hypothesis for regulation of sperm centrosomal activity consists of a dephosphorylation of sperm connecting piece proteins following sperm entry into the oocyte and activation of the oocyte. Dephosphorylation of these proteins results in the disassembly of the connecting piece and assembly of a functional centrosome. In nuclear transfer, centrosomal components are contributed by the donor cell. If the cell is fused to the cytoplast before centriole replication then a single aster forms. If the cell is fused after centriole replication then two asters form. In either case and even in parthenogenetic oocytes, which do not have centrioles, the first cell cycle progresses to metaphase. However, progress is slow and some defects are observed in the assembly of chromosomes into a metaphase plate.

Birth of normal mice following round spermatid injection without artificial oocyte activation

For fertilization using round spermatid injection (ROSI) in mice, oocytes need to be artificially preactivated because of the lack of oocyte-activating capacity in round spermatids of this species. However, when round spermatids were frozen-thawed before microinjection, 11-71% of injected oocytes developed into 2-cell embryos without any artificial activation. After being transferred into recipient females, 5-27% of these embryos reached term. At least some of the injected oocytes showed intracellular Ca(2+) oscillations, which normally occur after fertilization by mature spermatozoa. Thus, these round spermatids could transmit a sperm-borne oocyte-activating factor, which might have been released from spermatozoa and elongated spermatids in the same suspension by freezing and thawing. This possibility was further supported by activation of intact oocytes following transplantation of the pronuclei from ROSI-generated embryos. Thus, one-step ROSI can be achieved in mice simply by injecting frozen-thawed round spermatids into intact oocytes. Clearly, there is a need for careful interpretation of microinjection experiments when assessing the oocyte-activating capacity of spermatogenic cells, especially when they are derived from frozen-thawed stocks.

Participation of inositol trisphosphate and ryanodine receptors in Bufo arenarum oocyte activation

Calcium is considered the most important second messenger at fertilization. Transient release from intracellular stores is modulated through both agonist-gated channels, IP3Rs and RyRs, which can be found individually or together depending on the oocyte species. Using the four commonly used compounds (thimerosal, caffeine, heparin and ruthenium red), we investigated the existence and interdependence of both IP3Rs and RyRs in mature Bufo arenarum oocytes. We found that caffeine, a well known specific RyRs agonist, was able to trigger oocyte activation in a dose-dependent manner. Microinjection of 10 mM caffeine showed 100% of oocytes exhibiting characteristic morphological criteria of egg activation. Ruthenium red, the specific RyR blocker, was able to inhibit oocyte activation induced either by sperm or caffeine. Our present findings provide the first reported evidence of the existence of RyR in frogs. We further explored the relationship between IP3Rs and RyRs in B. arenarum oocytes by exposing them to the agonists of one class after injecting a blocker of the other class of receptor. We found that thimerosal overcame the inhibitory effect of RyR on oocyte activation, indicating that IP3Rs function as independent receptors. In contrast, previous injection of heparin delayed caffeine-induced calcium release, revealing a relative dependence of RyRs on functional IP3Rs, probably through a CICR mechanism. Both receptors play a role in Ca2+ release mechanisms although their relative contribution to the activation process is unclear.

Full-term mouse development by abolishing Zn2+-dependent metaphase II arrest without Ca2+ release

In vertebrates, a rise in intracellular free Ca2+ (Ca2+i) levels during fertilization initiates second metaphase (mII) exit and the developmental programme. The Ca2+ rise has long been considered to be crucial for development, but verifying this contribution would benefit from defining its role during fertilization. Here, we delineate the role of Ca2+ release during mII exit in wild-type mouse eggs and show that it is dispensable for full-term development. Exit from mII can be induced by Zn2+-specific sequestration without Ca2+ release, eliciting Cyclin B degradation in a manner dependent upon the proteasome pathway and intact microtubules, but not accompanied by degradation of the meiotic regulator Emi2. Parthenogenotes generated by Zn2+ sequestration developed in vitro with normal expression of Ca2+-sensitive genes. Meiotic exit induced by either Ca2+ oscillations or a single Ca2+ rise in oocytes containing a signaling-deficient sperm resulted in comparable developmental rates. In the absence of Ca2+ release, full-term development occurred ∼50% less efficiently, but at readily detectable rates, with the birth of 27 offspring. These results show in intact mouse oocytes that Zn2+ is essential for mII arrest and suggest that triggering meiotic exit is the sole indispensable developmental role of Ca2+ signaling in mammalian fertilization.

Improved postimplantation development of rabbit nuclear transfer embryos by activation with inositol 1,4,5-trisphosphate

Cloned rabbit embryos are characterized by their extremely poor postimplantation development, despite their high survivability until the blastocyst stage in vitro. This study examined whether the developmental failure of cloned rabbit embryos in vivo can be overcome by technical improvements to the activation protocol. Freshly collected cumulus cells were transferred into enucleated oocytes by intracytoplasmic injection. One to two hours later, the oocytes were activated by electroporation with Ca(2+) or inositol 1,4,5-trisphosphate (IP3), which is known to induce repeated rises in intracellular Ca(2+), as in normal fertilization. After transfer of embryos at the two- to four-cell stages, well-defined implantation sites with remnant fetal tissue were observed at term (day 28) only in the IP3-stimulation groups (0.9% and 5.8% per transferred embryo for single and triple stimulation groups, respectively). When some recipients in the same group were examined at days 16-20, a viable cloned fetus (day 19) with normal organogenesis was obtained. These findings clearly demonstrate that the oocyte activation protocol using IP3 enhances the postimplantation development of nuclear-transferred rabbit embryos.

Impact of intracytoplasmic sperm injection on the activation and fertilization process of oocytes

Intracytoplasmic sperm injection (ICSI) is used worldwide to treat preferentially severe cases of male factor infertility. In this review, data regarding the processes of oocyte activation and fertilization in non-assisted conception, conventional IVF and ICSI are discussed. The second messenger calcium shows a typical pattern after ICSI, which is different from that after subzonal insemination (SUZI), which is closer to the conditions of normal fertilization. The onset of calcium spikes is delayed. Sometimes a monophasic calcium pattern, in animals typical for parthenogenetic activation, is observed despite normal subsequent oocyte activation. Furthermore, the frequency of spikes is higher after SUZI, and only one phase instead of two is observed after ICSI for the second onset of calcium release. These alterations may be explained by the differences in oocyte activation after ICSI, since no oolemma-sperm contact is present. Sperm decondensation also follows another pattern after ICSI: as long as residuals of the acrosome are present on the sperm head, no sperm decondensation takes place at that site. Therefore, decondensation is delayed and pronucleus formation, especially that of the male pronucleus, takes longer after ICSI as compared with conventional IVF. Since studies have shown that gonosomes are located preferentially in the apical part of the sperm nucleus, this was proposed to be an explanation for a higher incidence of gonosomal aberrations in offspring after ICSI. However, other explanations, taking clinical data like the background risk of the parents into account, can also be offered for this phenomenon. These alternative theories are more likely to be associated with a slight instead of a frank increase in gonosomal aberrations. The inheritance of paternal mitochondrial DNA seems not to be a problem after ICSI, as shown by different studies. Mitochondrial DNA can be demonstrated in embryos after conventional IVF as after ICSI up to the blastocyst stage but not in children born after ICSI. Finally, lesion of the meiotic spindle by the ICSI procedure seems not to be a problem when data from different studies are taken into account. As assumed also at the beginning of the ICSI era, the meiotic spindle is almost always located in an area of < 90 degrees deviation from the polar body axis. Therefore, intrusion of the microinjection needle at the 90 degrees position might not endanger the spindle apparatus. To conclude, several studies using different approaches might show differences in the oocyte activation pattern, the 'choreography of fertilization' and pronucleus formation after ICSI. However, this different pattern does not necessarily mean that ICSI per se is a problem for embryonal development. The different pattern can be explained by the fact that ICSI uses another means of oocyte entry than the normal fertilization process. The clinical data of a high fertilization, cleavage and implantation rate, and especially the data from newborn babies, show that ICSI is a reliable procedure.

Ability of integrins to mediate fertilization, intracellular calcium release, and parthenogenetic development in bovine oocytes

The ability of arginine-glycine-aspartic acid (RGD; a sequence recognized by integrins) or non-RGD-containing peptides to block fertilization, induce intracellular Ca(2+) oscillations, and initiate parthenogenetic development in bovine oocytes was investigated. Addition of a soluble RGD peptide during fertilization at concentrations ranging from 10 to 1000 microg/ml significantly decreased (P<0.05) fertilization as compared to the in vitro-fertilized controls. The addition of non-RGD peptide had no effect on fertilization. Two intracellular Ca(2+) transients 21.5+/- 1.9 min apart were observed in 56 of 60 oocytes incubated in RGD peptide concentrations ranging from 20 to 1000 microg/ml. No intracellular Ca(2+) transients were observed in medium alone, non-RGD treatment groups or in the RGD peptide at 10 microg/ml. The percentage of oocytes activated with ionomycin and 6-dimethylaminopurine (63% cleavage and 34% blastocyst development) was significantly higher (P<0.05) than those activated with the RGD peptide and 6-dimethylaminopurine (35% cleavage and 19% blastocyst development). These groups were significantly higher (P<0.05) than either peptide alone, 6-dimethylaminopurine alone, or the non-RGD peptide and 6-dimethylaminopurine treatment groups. These data provide evidence that ligation of an integrin on bovine oocytes with a soluble RGD peptide is capable of blocking fertilization, inducing intracellular Ca(2+) transients, and initiating parthenogenetic development.

Targeted disruption of the mouse phospholipase C beta3 gene results in early embryonic lethality

In order to investigate the biological function of phosphatidylinositol-specific phospholipase C (PLC) we generated mutant mice by gene targeting. Homozygous inactivation of PLCbeta3 is lethal at embryonic day 2.5. These mutants show poor embryonic organization as well as reduced numbers of cells. Identical phenotypes were recorded in homozygous mutants generated from two independently targeted embryonic stem cell clones. Heterozygous mutant mice, however, are viable and fertile for at least two generations. We also showed that mouse PLCbeta3 is expressed in unfertilized eggs, 3-cell and egg cylinder stages of embryos. In conclusion, these results indicate that PLCbeta3 expression is essential for early mouse embryonic development.

Factors affecting electrical activation of porcine oocyte matured in vitro

This work was undertaken to improve conditions for in vitro maturation and activation of porcine oocytes. Experiments were designed to compare: (i) electrical pulse frequency, (ii) methods of oocyte preparation, (iii) maturation conditions, and (iv) electrical poration medium on development. Oocytes were harvested by follicle dissection or aspiration, co-cultured with follicle shells in M199 based medium with or without media changes at 38.5 degrees C in 5% CO2 under non-static conditions for 48 h and electroactivated using single or multiple pulses (current strength 1.0 kV/cm for 50 microseconds in 0.28 M inositol or mannitol based media with 10 mM histidine) at different time intervals. The results showed: (i) neither the pulse frequency nor the pulse interval influenced rates of pronuclear formation but multiple pulse activation (3 pulses at 5 min intervals) induced a higher incidence of development and progression through the 4-cell block in contrast to one pulse activation; (ii) both the rate of nuclear maturation (88.6% vs. 77.6%) and post-activation cleavage (89.8% vs. 67.4%) were higher (P < 0.05) when oocytes were collected by follicle dissection rather than by aspiration; (iii) while changing to a hormone-free medium at 24 h was without effect on maturation (91.9% vs. 91.7%), rate of cleavage (81.6% vs. 72.3%, P < 0.05) at 24 h was enhanced by the medium change; and (iv) oocytes activated with 3 pulses 5 min apart in mannitol based medium at 48-49 h and at 53-54 h formed pronuclei at a comparable rate but subsequent parthenogenetic development was higher in the older eggs. By contrast, inositol-based medium supported development of young and old eggs equally well. Calcium and magnesium ions are, however, necessary in both mannitol and inositol media for activation of porcine oocytes matured in vitro. The present results suggest that optimal parthenogenetic activation and early development of IVM pig oocytes could be obtained if oocytes are harvested by dissection, cultured for 24 h in hormone-containing medium before being placed in hormone free medium and activated at 48 h in inositol based medium using a three pulse activation system.

Phospholipase C in mouse oocytes: characterization of beta and gamma isoforms and their possible involvement in sperm-induced Ca2+ spiking

This study involved an investigation of the role of phospholipase C (PLC) in generating repetitive Ca2+ spikes at fertilization. Using a PCR-based strategy we have demonstrated that mouse oocytes have mRNA coding for PLC beta 1, PLC beta 3 and PLC gamma isoenzymes. Furthermore, immunodetection of PLC gamma 1 using monoclonal antibodies reveals that PLC gamma 1 protein is present in mature mouse oocytes, ruling out the possibility that mRNA was being transcribed but not expressed. We were unsuccessful at detecting the presence of PLC beta protein, but the presence of this isoform can be inferred from functional studies. The PLC inhibitor, U73122, exerted an inhibitory effect on oocytes activated by spermatozoa or acetylcholine at concentrations of 10 and 30 microM respectively, while its inactive analogue had no effect. The soluble tyrosine kinase inhibitors, genistein (100 microM), herbimycin (10 microM) and geldanamycin (0.6 microM) which could affect signalling through PLC gamma hindered but never completely inhibited Ca2+ spiking in response to fertilization. We conclude that the activation of PLC to generate InsP3 may play a critical role in fertilization.

The existence of inositol 1,4,5-trisphosphate and ryanodine receptors in mature bovine oocytes

Intracellular Ca2+ (Ca2+i) transients during fertilization are critical to the activation of eggs in all species studied. Activation of both the inositol 1,4,5-trisphosphate (IP3) receptor (IP3R) and ryanodine receptor (RYR) are responsible for the calcium oscillations during fertilization in sea urchin eggs. Using in vitro matured bovine oocytes loaded with Fura-2 AM ester as Ca2+i indicator, we addressed whether IP3Rs and RYRs coexist in mammalian eggs. Our results indicate that microinjection of 50–250 nM IP3 or 10–20 mM caffeine, 100–200 microM ryanodine and 4–8 microM cyclic ADP-ribose all induced Ca2+i release. The Ca2+i release induced by 250 nM IP3 could only be inhibited by prior injection of 1 mg/ml heparin which was overcome by continuous injection of IP3 to 1 microM. Prior injection of either 50 microM ruthenium red, 50 microM procaine or 1 % vehicle medium (VM) did not affect the Ca2+i release induced by IP3. Prior injection of heparin or VM did not affect the Ca2+i release induced by 10–20 mM caffeine or 200 microM ryanodine, but prior injection of 50 microM ruthenium red or procaine completely inhibited the effect of 10–20 mM caffeine. In addition, continuous injection of caffeine up to 40 mM overcame the inhibitory effect of ruthenium red or procaine. The same 50 microM concentration of ruthenium red or procaine only partially blocked the effect of 200 microM ryanodine, but 200 microM ruthenium red or procaine completely blocked the effect of 200 microM ryanodine.(ABSTRACT TRUNCATED AT 250 WORDS)

Stimulation of repetitive calcium transients in mouse eggs

1. We have combined cell membrane electroporation by electrical field (EF) stimulation with a rapid perfusion system in order to stimulate repetitive increases in cytoplasmic free [Ca2+] ([Ca2+]i) in mouse eggs. [Ca2+]i was monitored by ratio fluorescent measurements of intracellular indo-1 on individual eggs. The conditions required to cause different types of [Ca2+]i increases were established and the effects of these [Ca2+]i changes upon egg activation examined. 2. The rapid perfusion of non-ionic medium caused a single [Ca2+]i increase. However, to generate repetitive [Ca2+]i increases, eggs were exposed to EF pulses in the presence of Ca2+ and then washed rapidly with culture medium. Sequential EF pulse application led to prolonged elevation of [Ca2+]i levels and eventual cell lysis unless rapid reperfusion with culture medium was achieved. Transient increases in [Ca2+]i in eggs could also be generated by EF pulses in the presence of inositol 1,4,5-trisphosphate (InsP3). 3. In response to EF stimulation fertilized eggs showed [Ca2+]i increases that were enhanced relative to unfertilized eggs. The responses in these fertilized eggs were often followed by repetitive [Ca2+]i oscillations, despite the fact that the [Ca2+]i oscillations associated with sperm penetration had ceased by this stage. 4. In unfertilized mouse eggs the [Ca2+]i increases appeared to be due to direct cation influx since repeated EF pulses caused repeated influx of Mn2+ as monitored by quenching of fluorescence of fura-2 loaded eggs. 5. Under conditions that stimulated reproducible patterns of [Ca2+]i transients we found that a single large [Ca2+]i transient did not cause significant egg activation, but that inducing repetitive [Ca2+]i transients was effective in activating eggs. The speed of activation as judged by the rate of pronuclear formation was also dependent upon the frequency of pulse application. 6. These data show that combining EF pulses with a rapid and precise sequential perfusion system can be used to manipulate [Ca2+]i levels in mammalian eggs. This provides a means of artificial mimicry of the [Ca2+]i transients seen after fertilization. It appears that Ca2+ influx during EF pulses does not cause significant Ca2+ release from internal stores in unfertilized eggs, but after fertilization Ca2+ influx does induce Ca2+ release. It is also apparent that mouse eggs are more successfully activated by repetitive [Ca2+]i increases than by single large [Ca2+]i rises. We suggest that our data provide direct evidence for the hypothesis that a cellular response to oscillations of intracellular [Ca2+]i can be distinct from that to monotonic rises in [Ca2+]i.

Parthenogenetic development of bovine oocytes matured in vitro for 24 hr and activated by ethanol and cycloheximide

This research was undertaken to improve development of parthenogenetic embryos following various combined treatments of ethanol and cycloheximide. In Experiment 1 in vitro matured oocytes (IVM, 24 hr) were treated with 7% ethanol for 5 min followed by incubation in 10 micrograms/ml cycloheximide in Medium 199 for 0 (control), 5, 10, and 20 hr. Development to 2-8 cells following culture for 3 days was similar among treated groups (32-41%; P > 0.05), which was higher than that of controls (6%; P < 0.05). Experiment 2 compared pre-ethanol exposures for 0, 1, 2.5, and 5 min, followed by 5 hr cycloheximide treatment on activation development. One- to 5-min groups resulted in 42-44% cleavage contrasted to 1-12% for controls (P < 0.05). Experiment 3 examined the effect on oocyte development of ethanol and different concentrations of cycloheximide (0, 1, 5, and 10 micrograms/ml). Cleavage to 2-8 cells was similar among the 5 and 10 micrograms/ml cycloheximide groups (36% and 42%, P > 0.05) but lower (P < 0.05) for the 1 micrograms/ml group (24%) and the controls (2-13%). When 5 micrograms/ml cycloheximide was used (Experiment 4), pre-exposure to ethanol (1, 2.5, and 5 min) resulted in more oocytes cleaved (38-41%) than in the cycloheximide alone group (0%) or the control (0%, P < 0.05). Experiment 5 tested blastocyst development of the activated oocytes with or without cytochalasin B treatment.(ABSTRACT TRUNCATED AT 250 WORDS)