Intracytoplasmic sperm injection in the bovine induces abnormal [Ca2+]i responses and oocyte activation

Chlorogenic acid improves the development of porcine parthenogenetic embryos by regulating oxidative stress and ameliorating mitochondrial function

Chlorogenic acid (CGA) is an effective phenolic antioxidant that can scavenge hydroxyl radicals and superoxide anions. Herein, the protective effects and mechanisms leading to CGA-induced porcine parthenogenetic activation (PA) in early-stage embryos were investigated. Our results showed that 50 μM CGA treatment during the in vitro culture (IVC) period significantly increased the cleavage and blastocyst formation rates and improved the blastocyst quality of porcine early-stage embryos derived from PAs. Then, genes related to zygotic genome activation (ZGA) were identified and investigated, revealing that CGA can promote ZGA in porcine PA early-stage embryos. Further analysis revealed that CGA treatment during the IVC period decreased the abundance of reactive oxygen species (ROS), increased the abundance of glutathione and enhanced the activity of catalase and superoxide dismutase in porcine PA early-stage embryos. Mitochondrial function analysis revealed that CGA increased mitochondrial membrane potential and ATP levels and upregulated the mitochondrial homeostasis-related gene NRF-1 in porcine PA early-stage embryos. In summary, our results suggest that CGA treatment during the IVC period helps porcine PA early-stage embryos by regulating oxidative stress and improving mitochondrial function.

Phospholipase C Zeta 1 (PLCZ1): The Function and Potential for Fertility Assessment and In Vitro Embryo Production in Cattle and Horses

Phospholipase C Zeta 1 (PLCZ1) is considered a major sperm-borne oocyte activation factor. After gamete fusion, PLCZ1 triggers calcium oscillations in the oocyte, resulting in oocyte activation. In assisted fertilization, oocyte activation failure is a major cause of low fertility. Most cases of oocyte activation failures in humans related to male infertility are associated with gene mutations and/or altered PLCZ1. Consequently, PLCZ1 evaluation could be an effective diagnostic marker and predictor of sperm fertilizing potential for in vivo and in vitro embryo production. The characterization of PLCZ1 has been principally investigated in men and mice, with less known about the PLCZ1 impact on assisted reproduction in other species, such as cattle and horses. In horses, sperm PLCZ1 varies among stallions, and sperm populations with high PLCZ1 are associated with cleavage after intracytoplasmic sperm injection (ICSI). In contrast, bull sperm is less able to initiate calcium oscillations and undergo nuclear remodeling, resulting in poor cleavage after ICSI. Advantageously, injections of PLCZ1 are able to rescue oocyte failure in mouse oocytes after ICSI, promoting full development and birth. However, further research is needed to optimize PLCZ1 diagnostic tests for consistent association with fertility and to determine whether PLCZ1 as an oocyte-activating treatment is a physiological, efficient, and safe method for improving assisted fertilization in cattle and horses.

Effects of triton X-100 pretreatment of lyophilized and frozen-thawed ram sperm on preimplantation embryo developmental competence

In this study, it was aimed to determine the effect of destruction of lyophilized and frozen-thawed ram sperm plasma and acrosomal membrane on development of embryos produced by intracytoplasmic sperm injection (ICSI). Semen samples were divided into two groups for lyophilization (L) and freezing (F). For the removal of the plasma membrane, L and F groups were incubated with Triton X-100 (LTX-100 and FTX-100, respectively). Integrities of the plasma membrane, acrosome and chromatin structure were evaluated. Oocytes were injected with these sperm groups. Although no plasma membrane and acrosome integrities of the L (0.0%) group were detected, the plasma membrane integrity of the F group (69.4%) was significantly higher than the FTX-100 group (23.6%) (p < 0.05). The acrosome integrity of the FTX-100 group (3.80%) was significantly lower than the F group (55.6%) (p < 0.05). The chromatin integrities of L and F groups were higher than the Triton X-100 treated groups (p < 0.05). ICSIs with L, LTX-100, F and FTX-100 sperm were produced similar cleavage and blastocyst rates. In conclusion, data presented here confirm that ram spermatozoa can effectively be lyophilized and injected into oocytes for initiation of embryonic development and Triton X-100 pretreatment is not necessary while using lyophilized and frozen semen.

Bull spermatozoa selected by thermotaxis exhibit high DNA integrity, specific head morphometry, and improve ICSI outcome

BACKGROUND

Sperm migration by thermotaxis is a guidance mechanism that operates along the oviduct and it has proved to be a valid method for selecting spermatozoa with low DNA fragmentation (SDF) in mice, humans, and stallions. This study aimed to analyse if bull spermatozoa could be selected by thermotaxis and to assess their quality in terms of SDF as well as determine the presence of a specific sperm subpopulation based on sperm morphometry and assess their fertilizing capacity by ICSI.

METHODS

We used frozen-thawed sperm from 6 bulls and sperm selection by thermotaxis was performed with TALP medium supplemented with 25 mmol/L of HEPES and 5 mmol/L of caffeine. In these conditions, sperm selection was achieved, obtaining a net thermotaxis of 3.6%. Subsequently, we analysed the SDF of the migrated and not-migrated spermatozoa using the neutral COMET assay, and we evaluated the size of the sperm head using Hemacolor® staining with Motic Images Plus 3 software. Additionally, migrated and not-migrated spermatozoa by thermotaxis were used to fertilize bovine in vitro matured (IVM) oocytes by ICSI, a very inefficient procedure in cattle that is only successful when the oocyte is artificially activated.

RESULTS

The results showed lower SDF (χ², P < 0.001, 13.3% reduction, n = 8) and lower head size parameters (length and width, P < 0.01; and perimeter and area, P < 0.001; n = 4) in those spermatozoa migrated in comparison to those not-migrated. The distribution of sperm subpopulations structure varied between groups, highlighting cluster 2, characterized by spermatozoa with small head size, and high ellipticity and elongated heads, as the most abundant in the thermotaxis migrated group. When performed ICSI (without oocyte artificial activation) with the thermotactic sperm, the blastocyst rate was 32.2% ± 9.3% in the group microinjected with the thermotactic spermatozoa vs. 8.3% ± 7.8% in the group of not-migrated sperm (χ², P < 0.05).

CONCLUSION

Our results showed that bull sperm selection by thermotaxis has a much higher DNA integrity, small and elongated head size parameters, and different sperm subpopulation structure than the not-selected spermatozoa. Additionally, we evidenced that thermotactic spermatozoa improve ICSI success rates.

Changes in cortical endoplasmic reticulum clusters in the fertilized mouse oocyte†

Oocytes from many invertebrate and vertebrate species exhibit unique endoplasmic reticulum (ER) specializations (cortical ER clusters), which are thought to be essential for egg activation. In examination of cortical ER clusters, we observed that they were tethered to previously unreported fenestrae within the cortical actin layer. Furthermore, studies demonstrated that sperm preferentially bind to the plasma membrane overlying the fenestrae, establishing close proximity to underlying ER clusters. Moreover, following sperm-oocyte fusion, cortical ER clusters undergo a previously unrecognized global change in volume and shape that persists through sperm incorporation, before dispersing at the pronuclear stage. These changes did not occur in oocytes from females mated with Izumo1 -/- males. In addition to these global changes, highly localized ER modifications were noted at the sperm binding site as cortical ER clusters surround the sperm head during incorporation, then form a diffuse cloud surrounding the decondensing sperm nucleus. This study provides the first evidence that cortical ER clusters interact with the fertilizing sperm, indirectly through a previous unknown lattice work of actin fenestrae, and then directly during sperm incorporation. These observations raise the possibility that oocyte ER cluster-sperm interactions provide a competitive advantage to the oocyte, which may not occur during assisted reproductive technologies such as intracytoplasmic sperm injection.

Past, present and future of ICSI in livestock species

During the past 2 decades, intracytoplasmic sperm injection (ICSI) has become a routine technique for clinical applications in humans. The widespread use among domestic species, however, has been limited to horses. In horses, ICSI is used to reproduce elite individuals and, as well as in humans, to mitigate or even circumvent reproductive barriers. Failures in superovulation and conventional in vitro fertilization (IVF) have been the main reason for the use of this technology in horses. In pigs, ICSI has been successfully used to produce transgenic animals. A series of factors have resulted in implementation of ICSI in pigs: need to use zygotes for numerous technologies, complexity of collecting zygotes surgically, and problems of polyspermy when there is utilization of IVF procedures. Nevertheless, there have been very few additional reports confirming positive results with the use of ICSI in pigs. The ICSI procedure could be important for use in cattle of high genetic value by maximizing semen utilization, as well as for utilization of spermatozoa from prepubertal bulls, by providing the opportunity to shorten the generation interval. When attempting to utilize ICSI in ruminants, there are some biological limitations that need to be overcome if this procedure is going to be efficacious for making genetic improvements in livestock in the future. In this review article, there is an overview and projection of the methodologies and applications that are envisioned for ICSI utilization in these species in the future.

Bovine ICSI: limiting factors, strategies to improve its efficiency and alternative approaches

Intracytoplasmic sperm injection (ICSI) is an assisted reproductive technique mainly used to overcome severe infertility problems associated with the male factor, but in cattle its efficiency is far from optimal. Artificial activation treatments combining ionomycin (Io) with 6-dimethylaminopurine after piezo-ICSI or anisomycin after conventional ICSI have recently increased the blastocyst rate obtained. Compounds to capacitate bovine spermatozoa, such as heparin and methyl-β-cyclodextrin and compounds to destabilize sperm membranes such as NaOH, lysolecithin and Triton X-100, have been assessed, although they have failed to substantially improve post-ICSI embryonic development. Disulfide bond reducing agents, such as dithiothreitol (DTT), dithiobutylamine and reduced glutathione, have been assessed to decondense the hypercondensed head of bovine spermatozoa, the two latter being more efficient than DTT and less harmful. Although piezo-directed ICSI without external activation has generated high fertilization rates and modest rates of early embryo development, other studies have required exogenous activation to improve the results. This manuscript thoroughly reviews the different strategies used in bovine ICSI to improve its efficiency and proposes some alternative approaches, such as the use of extracellular vesicles (EVs) as 'biological methods of oocyte activation' or the incorporation of EVs in the in vitro maturation and/or culture medium as antioxidant defence agents to improve the competence of the ooplasm, as well as a preincubation of the spermatozoa in estrous oviductal fluid to induce physiological capacitation and acrosome reaction before ICSI, and the use of hyaluronate in the sperm immobilization medium.

Mysteries and unsolved problems of mammalian fertilization and related topics

Mammalian fertilization is a fascinating process that leads to the formation of a new individual. Eggs and sperm are complex cells that must meet at the appropriate time and position within the female reproductive tract for successful fertilization. I have been studying various aspects of mammalian fertilization over 60 years. In this review, I discuss many different aspects of mammalian fertilization, some of my laboratory's contribution to the field, and discuss enigmas and mysteries that remain to be solved.

Effects of intracytoplasmic sperm injection timing and fertilization methods on the development of bovine spindle transferred embryos

Cytoplasmic replacement by spindle transfer (ST) technique can be applied to improve the developmental competence of poor qualitied or aged oocytes. In cattle, ST technology has not been well established for producing embryos and the calves successfully using intracytoplasmic sperm injection (ICSI) or in vitro fertilization (IVF). The objective of this study was to develop a novel procedure for producing bovine ST embryos, which could be fundamental to applying ST technology in other mammals. In the present study, the efficacies of performing ICSI before (ICSI-ST) or after (ST-ICSI) and IVF on the development of ST bovine embryos were investigated. Results indicated that the blastocyst rate of ST embryos produced by ICSI-ST (24.7%) was higher (P < 0.05) than that produced by ST-ICSI (5.9%). On the other hand, ST-IVF had the highest fertilization rate (97.3%), polyspermy rate (24.7%), and lowest blastocyst rate (22.7%) when compared to denuded oocytes (DO), zona cut oocytes (ZC), and cumulus-oocyte complexes (COCs)-IVF groups. Finally, the in vitro development rates of ICSI-ST (24.5%) and ST-IVF (25.2%) were not significantly different (P > 0.05). However, the pregnancy rate (46.7%) and birth rate (33.3%) of ST-IVF were higher (P < 0.05) than those of ICSI-ST (6.3% and 0%, respectively). The percentage of mitochondrial DNA (mtDNA) heteroplasmy derived from donor karyoplasts of the 5 claves was between 2% and 18%. Taken together, performing ICSI prior to ST can improve the embryonic development of ST bovine embryos. Moreover, using IVF, instead of ICSI, for ST oocyte fertilization dramatically increased the pregnancy rate and birth rate of ST calves, in which mtDNA heteroplasmy derived from donor karyoplasts exists.

An Update on Semen Physiology, Technologies, and Selection Techniques for the Advancement of In Vitro Equine Embryo Production: Section II

Simple Summary

In order to improve fertilization and pregnancy rates within artificial insemination or in vitro fertilization techniques in horses, producers may choose to select the best sperm within an ejaculate. In this paper, we review conventional and novel methods of sperm selection.

Abstract

As the use of assisted reproductive technologies (ART) and in vitro embryo production (IVP) expand in the equine industry, it has become necessary to further our understanding of available semen selection techniques. This segment of our two-section review will focus on the selection of spermatozoa based on quality and sex for equine intracytoplasmic sperm injection (ICSI), as well as current and future developments in sperm sorting technologies. Ultimately, novel methods of semen selection will be assessed based on their efficacy in other species and their relevance and future application towards ARTs in the horse.

Ouabain-induced activation of phospholipase C zeta and its contributions to bovine sperm capacitation

The sperm-derived oocyte activating factor, phospholipase C zeta (PLC ζ), is the only PLC isoform reported in cattle. The objectives were to (1) localize PLC ζ in fresh and capacitated bovine sperm and (2) investigate the activation of PLC ζ during bull sperm capacitation and contributions of PLC activity to this process. We confirmed interaction of testis-specific isoform of Na/K-ATPase (ATP1A4) with PLC ζ (immunolocalization and immunoprecipitation) and tyrosine phosphorylation (immunoprecipitation) of PLC ζ (a post-translational protein modification commonly involved in activation of PLC in somatic cells) during capacitation. Furthermore, incubation of sperm under capacitating conditions upregulated PLC-mediated hyperactivated motility, tyrosine phosphoprotein content, acrosome reaction, and F-actin formation (flow cytometry), implying that PLC activity is enhanced during capacitation and contributing to these capacitation processes. In conclusion, we inferred that PLC ζ is activated during capacitation by tyrosine phosphorylation through a mechanism involving ATP1A4, contributing to capacitation-associated biochemical events.

Dysregulated Gene Expression of Imprinted and X-Linked Genes: A Link to Poor Development of Bovine Haploid Androgenetic Embryos

Mammalian uniparental embryos are efficient models for genome imprinting research and allow studies on the contribution of the paternal and maternal genomes to early embryonic development. In this study, we analyzed different methods for production of bovine haploid androgenetic embryos (hAE) to elucidate the causes behind their poor developmental potential. Results indicate that hAE can be efficiently generated by using intracytoplasmic sperm injection and oocyte enucleation at telophase II. Although androgenetic haploidy does not disturb early development up to around the 8-cell stage, androgenetic development is disturbed after the time of zygote genome activation and hAE that reach the morula stage are less capable to reach the blastocyst stage of development. Karyotypic comparisons to parthenogenetic- and ICSI-derived embryos excluded chromosomal segregation errors as causes of the developmental constraints of hAE. However, analysis of gene expression indicated abnormal levels of transcripts for key long non-coding RNAs involved in X chromosome inactivation and genomic imprinting of the KCNQ1 locus, suggesting an association with X chromosome and some imprinted loci. Moreover, transcript levels of methyltransferase 3B were significantly downregulated, suggesting potential anomalies in hAE establishing de novo methylation. Finally, the methylation status of imprinted control regions for XIST and KCNQ1OT1 genes remained hypomethylated in hAE at the morula and blastocyst stages, confirming their origin from spermatozoa. Thus, our results exclude micromanipulation and chromosomal abnormalities as major factors disturbing the normal development of bovine haploid androgenotes. In addition, although the cause of the arrest remains unclear, we have shown that the inefficient development of haploid androgenetic bovine embryos to develop to the blastocyst stage is associated with abnormal expression of key factors involved in X chromosome activity and genomic imprinting.

Intracytoplasmic Sperm Injection in Cattle

Intracytoplasmic sperm injection (ICSI) involves the microinjection of sperm into a matured oocyte. Although this reproductive technology is successfully used in humans and many animal species, the efficiency of this procedure is low in the bovine species mainly due to failed oocyte activation following sperm microinjection. This review discusses various reasons for the low efficiency of ICSI in cattle, potential solutions, and future directions for research in this area, emphasizing the contributions of testis-specific isoforms of Na/K-ATPase (ATP1A4) and phospholipase C zeta (PLC ζ). Improving the efficiency of bovine ICSI would benefit the cattle breeding industries by effectively utilizing semen from elite sires at their earliest possible age.

Preventing polyspermy in mammalian eggs-Contributions of the membrane block and other mechanisms

The egg's blocks to polyspermy (fertilization of an egg by more than one sperm) were originally identified in marine and aquatic species with external fertilization, but polyspermy matters in mammalian reproduction too. Embryonic triploidy is a noteworthy event associated with pregnancy complications and loss. Polyspermy is a major cause of triploidy with up to 80% of triploid conceptuses being the result of dispermic fertilization. The mammalian female reproductive tract regulates the number of sperm that reach the site of fertilization, but mammals also utilize egg-based blocks to polyspermy. The egg-based blocks occur on the mammalian egg coat (the zona pellucida) and the egg plasma membrane, with apparent variation between different mammalian species regarding the extent to which one or both are used. The zona pellucida block to polyspermy has some similarities to the slow block in water-dwelling species, but the mammalian membrane block to polyspermy differs substantially from the fast electrical block that has been characterized in marine and aquatic species. This review discusses what is known about the incidence of polyspermy in mammals and about the mammalian membrane block to polyspermy, as well as notes some lesser-characterized potential mechanisms contributing to polyspermy prevention in mammals.

Transient Sperm Starvation Improves the Outcome of Assisted Reproductive Technologies

To become fertile, mammalian sperm must undergo a series of biochemical and physiological changes known as capacitation. These changes involve crosstalk between metabolic and signaling pathways and can be recapitulated in vitro. In this work, sperm were incubated in the absence of exogenous nutrients (starved) until they were no longer able to move. Once immotile, energy substrates were added back to the media and sperm motility was rescued. Following rescue, a significantly higher percentage of starved sperm attained hyperactivated motility and displayed increased ability to fertilize in vitro when compared with sperm persistently incubated in standard capacitation media. Remarkably, the effects of this treatment continue beyond fertilization as starved and rescued sperm promoted higher rates of embryo development, and once transferred to pseudo-pregnant females, blastocysts derived from treated sperm produced significantly more pups. In addition, the starvation and rescue protocol increased fertilization and embryo development rates in sperm from a severely sub-fertile mouse model, and when combined with temporal increase in Ca²⁺ ion levels, this methodology significantly improved fertilization and embryo development rates in sperm of sterile CatSper1 KO mice model. Intracytoplasmic sperm injection (ICSI) does not work in the agriculturally relevant bovine system. Here, we show that transient nutrient starvation of bovine sperm significantly enhanced ICSI success in this species. These data reveal that the conditions under which sperm are treated impact post-fertilization development and suggest that this “starvation and rescue method” can be used to improve assisted reproductive technologies (ARTs) in other mammalian species, including humans.

Bovine eggs release zinc in response to parthenogenetic and sperm-induced egg activation

Upon fertilization or parthenogenesis, zinc is released into the extracellular space through a series of exocytic events termed zinc sparks, which are tightly coordinated with intracellular calcium transients. The zinc spark reduces the total amount of intracellular zinc, and this reduction is necessary and sufficient to induce egg activation even in the absence of calcium transients. In addition, this zinc release contributes to the block to polyspermy through modification of the zona pellucida. The zinc spark has been documented in all organisms examined to date including the mouse, two species of nonhuman primates, and human. Here we determined whether zinc sparks occur in the bovine, an important model of gamete development in mono-ovulatory mammalian species. We obtained metaphase II-arrested (MII) bovine eggs following in vitro maturation. Total zinc, assessed in single cells using X-Ray Fluorescence Microscopy, was significantly more abundant in the bovine egg compared to iron and copper. Studies with intracellular fluorescent probes revealed that labile zinc pools are localized to discrete cytoplasmic punctae enriched at the cortex. To determine whether zinc undergoes dynamic fluxes during egg activation, we parthenogenetically activated bovine eggs using two approaches: ionomycin or bovine phospholipase C zeta (bPlcζ). Both these methods induced zinc sparks coordinately with intracellular calcium transients. The zinc spark was also observed in bovine eggs following intracytoplasmic sperm injection. These results establish that zinc is the most abundant transition metal in the bovine egg, and zinc flux during egg activation - induced by chemical activation or sperm - is a highly conserved event across mammalian species.

Improved embryo development using high cysteamine concentration during IVM and sperm co-culture with COCs previous to ICSI in bovine

In contrast to other species, intracytoplasmic sperm injection (ICSI) in bovine remains inefficient, resulting in low embryo developmental rates. It is unclear whether such inefficiency is due to the poor response of bovine ooplasms to the injection stimulus, or to the inability of bull sperm to induce oocyte activation. In order to facilitate these events, two strategies were assessed: the use of high concentration of cysteamine [Cys] during IVM; and the selection of sperm attached to cumulus cells after incubation with COCs for ICSI. First, COCs were IVM with increasing [Cys] and subjected to IVF. Zygotes from all groups were cultured under different O₂ tensions and development to blastocyst was evaluated. In a second experiment, sperm were co-cultured for 3 h with COCs and acrosome reaction was studied. Afterwards, the best IVM and IVC conditions determined on Experiment 1 were used for ICSI assay. COCs were matured for 21 h with 1 (Cys 1) or 0.1 mM Cys (Cys 0.1 groups, standard condition). In addition, COCs were incubated for ≥3 h with 16 × 10⁶ sperm/ml and only sperm attached to cumulus cells were selected for ICSI (ICSI + Co-cult groups). After chemical activation, embryos were cultured in SOF medium under low O₂ tension. Cleavage and blastocyst rates were evaluated at days 2 and 7 of IVC, respectively. Finally, the relative expression of eight genes indicators of embryo quality was compared between ICSI and IVF control blastocysts by qPCR. Cleavage rates were higher for Cys 0.1 ICSI + Co-cult and Cys 1 ICSI + Co-cult groups (n = 117, 92% and n = 116, 79%, respectively) compared to their controls (n = 132, 60% for Cys 0.1 ICSI and n = 108, 52% for Cys 1 ICSI) (p ≤ 0.05). Interestingly, the combined treatment (Cys 1 ICSI + Co-cult) showed higher blastocyst rates than all other ICSI groups (23 vs. 11, 18 and 14% for Cys 0.1 ICSI + Co-cult, Cys 1 ICSI, and Cys 0.1 ICSI, respectively) (p ≤ 0.05). Moreover, incubation with COCs increased the rates of live acrosome reacted sperm (p ≤ 0.05). The relative abundance of mRNAs coding for INFτ, CAT, DNMT1, OCT4, and HDAC3 did not differ between treatments (p ≤ 0.05). SOD2, HADC1 and HADC2 expression was higher for Cys 0.1 ICSI than for IVF embryos (p ≤ 0.05). Group Cys 1 ICSI did not differ from IVF for those three genes, neither did Cys 1 ICSI + Co-cult, except for HDAC1 (p ≤ 0.05). In conclusion, the use of 1 mM Cys during IVM and of sperm incubated with mature COCs might be a good strategy to improve ICSI outcomes in cattle.

Defective sperm head decondensation undermines the success of ICSI in the bovine

The efficiency of intracytoplasmic sperm injection (ICSI) in the bovine is low compared to other species. It is unknown whether defective oocyte activation and/or sperm head decondensation limit the success of this technique in this species. To elucidate where the main obstacle lies, we used homologous and heterologous ICSI and parthenogenetic activation procedures. We also evaluated whether in vitro maturation negatively impacted the early stages of activation after ICSI. Here we showed that injected bovine sperm are resistant to nuclear decondensation by bovine oocytes and this is only partly overcome by exogenous activation. Remarkably, when we used heterologous ICSI, in vivo-matured mouse eggs were capable of mounting calcium oscillations and displaying normal PN formation following injection of bovine sperm, although in vitro-matured mouse oocytes were unable to do so. Together, our data demonstrate that bovine sperm are especially resistant to nuclear decondensation by in vitro-matured oocytes and this deficiency cannot be simply overcome by exogenous activation protocols, even by inducing physiological calcium oscillations. Therefore, the inability of a suboptimal ooplasmic environment to induce sperm head decondensation limits the success of ICSI in the bovine. Studies aimed to improve the cytoplasmic milieu of in vitro-matured oocytes and to replicate the molecular changes associated with in vivo capacitation and acrosome reaction will deepen our understanding of the mechanism of fertilization and improve the success of ICSI in this species.

Intracytoplasmic sperm injection in domestic and wild mammals

Intracytoplasmic sperm injection (ICSI) has become a useful technique for clinical applications in the horse-breeding industry. However, both ICSI blastocyst and offspring production continues to be limited for most farm and wild species. This article reviews technical differences of ICSI performance among species, possible biological and methodological reasons for the variable efficiency and potential strategies to improve the outcomes. One of the major applications of ICSI in animal production is the reproduction of high-value specimens. Unfortunately, some domestic species like the bovine show low rates of pronuclei formation after sperm injection, which led to the development of various artificial activation protocols and sperm pre-treatments that are discussed in this article. The impact of ICSI technique on equine breeding programs is considered in detail, since in contrast to other species, its use for elite horse reproduction has increased in recent years. ICSI has also been used to produce genetically modified animals; however, despite numerous attempts in several domestic species, only transgenic pigs have been consistently produced. Finally, the ICSI is a promising tool for genetic rescue of endangered and wild species. In conclusion, while ICSI has become a consistent ART for some species, it needs further development for others. The low results obtained for some domestic species, the high training needed and the equipment required have limited this technique to the production of elite specimens or for research purposes.

Reproduction biotechnologies in germplasm banking of livestock species: a review

Many biotechnologies are currently used in livestock breeding with the aim of improving reproductive efficiency and increasing the rate of genetic progress in production animals. Semen cryopreservation is the most widely used cryobiotechnology, although vitrification techniques now allow embryos and oocytes to be banked in ever-increasing numbers. Cryopreservation of other types of germplasm (reproductive tissue in general) is also possible, although the techniques are still in the early stages of development for use in livestock species. Although still in their infancy, these techniques are increasingly being used in aquaculture. Germplasm conservation enables reproductive tissues from both animals and fish to be preserved to generate offspring in the future without having to maintain large numbers of living populations of these species. However, such measures need careful planning and coordination. This review explains why the preservation of genetic diversity is needed for livestock and fish, and describes some of the issues involved in germplasm banking. Furthermore, some recent developments in semen handling leading to improved semen cryopreservation and biosecurity measures are also discussed.

Sperm capacitation pretreatment positively impacts bovine intracytoplasmic sperm injection

The efficiency of intracytoplasmic sperm injection (ICSI) in bovines is low compared to other species due in part to inadequate egg activation and sperm nucleus decondensation after injection. We hypothesized that this low efficiency is due to the lack of complete sperm capacitation, so we evaluated the effects of isobutylmethylxanthine (IBMX) and methyl-β-cyclodextrin (MβCD) on bovine sperm capacitation and on the preimplantation developmental potential of bovine embryos generated by ICSI. Treatment with IBMX and MβCD decreased sperm viability (between 13-30%); nevertheless, 0.4 mM IBMX and 1 mM MβCD increased (p < 0.05) capacitation metrics-that is, acrosome exocytosis, intracellular calcium level, plasma membrane fluidity, and tyrosine phosphorylation-compared to the control. After ICSI, embryos injected with IBMX- and MβCD-treated sperm showed similar cleavage to the untreated group (range 82-88%). Pronucleus formation rate was higher with MβCD-pretreatment (54%) compared to the control group (25%), and blastocyst rate was significantly improved with MβCD-pretreatment (24%) compared to the IBMX (18%) and control (17%) groups. Importantly, embryo quality-as assessed by the total number of cells, cell allocation, and apoptotic cell index-was not affected by the sperm treatments. In conclusion, MβCD pretreatment of sperm improved the efficiency of blastocyst production in bovine ICSI.

Efficient pig ICSI using Percoll-selected spermatozoa; evidence for the essential role of phospholipase C-ζ in ICSI success

In pigs, the damaged sperm membrane leads to leakage of phospholipase C-ζ (PLCζ), which has been identified as a sperm factor, and a reduction of oocyte-activating ability. In this study, we investigated whether sperm selected by Percoll gradient centrifugation (Percoll) have sufficient PLCζ, and whether the efficiency of fertilization and blastocyst formation after intracytoplasmic sperm injection (ICSI) using Percoll-selected sperm can be improved. Percoll-selected sperm (Percoll group) or sperm without Percoll selection (Control group) were used. A proportion of the oocytes injected with control sperm were subjected to electrical stimulation at 1 h after ICSI (Cont + ES group). It was found that the Percoll group showed a large amount of PLCζ in comparison with the Control group. Furthermore, application of Percoll-selected sperm for ICSI increased the efficiency of fertilization and embryo development. Thus, these results indicate the Percoll-selected sperm have sufficient PLCζ and high oocyte-activating ability after ICSI in pigs.

Improved preimplantation development of bovine ICSI embryos generated with spermatozoa pretreated with membrane-destabilizing agents lysolecithin and Triton X-100

In cattle, intracytoplasmic sperm injection (ICSI) has a low efficiency. The acrosome content may be responsible for this effect because of the large amount of hydrolytic enzymes that are released within the oocyte. With the aim of removing the acrosome and destabilize the membranes, cryopreserved bovine spermatozoa were treated with lysolecithin (LL) and Triton X-100 (TX) at different concentrations. We evaluated the membrane integrity, the acrosome integrity, DNA integrity, and the variation of phospholipase C zeta. The rates of development (cleavage and blastocysts) were also evaluated along with pronuclear formation and the embryo quality. Spermatozoa incubated with LL and TX (0.01%, 0.02%, 0.03%, and 0.04%) decreased (P < 0.0001) sperm viability in a dose-dependent manner. The acrosome reaction was also increased (P < 0.0001) in all tested concentrations of LL and TX achieving 100% at 0.05% concentration in both treatments. Terminal deoxynucleotidyl transferase dUTP nick-end labeling assay reported an increase (P < 0.05) in DNA fragmentation only with the highest concentration of LL (0.06%), whereas all concentrations assessed of TX reported an increased respect to the control. Phospholipase C zeta expression decreased (P < 0.05) in spermatozoa treated with LL and TX at all concentrations tested. A higher cleavage rate was observed in ICSI-TX (66%) and ICSI-LL (65%) groups compared with the untreated control group (51%) and the blastocyst formation rate significantly increased in the ICSI-LL group (29%) compared with the control (21%). No differences were observed in the pronuclear formation and quality of the embryos. In conclusion, the destabilization of the plasma membrane and the release of the acrosomal content with LL and TX before ICSI improve the rate of embryonic development, without affecting the quality of the embryos produced by this technique.

Microtubule assembly crucial to bovine embryonic development in assisted reproductive technologies

Centrosome integrity and microtubule network are crucial to the events around fertilization, including pronuclear development, migration and fusion, and the first mitotic division. The present review highlights the importance of bull spermatozoal centrosomes to function as a microtubule‐organizing center for successful fertilization and the subsequent embryonic development. Spermatozoal centrosomes need to be blended with ooplasmic pericentriolar materials accurately to nucleate and organize the sperm aster. Dysfunction of the spermatozoal centrosomes is associated with fertilization failure, which has been overcome with supplemental stimuli for oocyte activation following intracytoplasmic sperm injection in humans. Even though the spermatozoal centrosomes are functionally intact, abnormal sperm aster formation was frequently observed in vitrified‐warmed bovine oocytes, with delayed pronuclear development and migration. Treatment of the post‐warm oocytes with Rho‐associated coiled‐coil kinase inhibitor or α‐tocopherol inhibited the incidence of the abnormal aster formation, resulting in higher blastocyst yields following in vitro fertilization and culture. Thus, understanding of centrosomal function made it possible to improve the performance of advanced reproductive technologies.

Effect of anisomycin, a protein synthesis inhibitor, on the in vitro developmental potential, ploidy and embryo quality of bovine ICSI embryos

Increasing the efficiency of intracytoplasmic sperm injection (ICSI) in domestic animals has been attempted by many researchers, however embryonic development to the blastocyst stage remains low compared with that of in vitro fertilization (IVF) embryos. One of the main problems observed in cattle is inadequate oocyte activation after ICSI. The present study compared the effect of cycloheximide (CHX), 6-dimethylaminopurine (DMAP), and anisomycin (ANY) on the fertilization rate, development, ploidy and quality of bovine embryos generated by ICSI. Although no differences were observed between treatments in terms of cleavage, higher blastocyst rates were observed for ANY (37.3%) compared with CHX (21.8%, P 0.05) treatments. No differences were observed in the quality of embryos as assessed by the total number of cells, their distribution to the different embryo compartments [inner cell mass (ICM) and trophectoderm (TE)], the proportion of ICM cells to the total cell numbers and terminal deoxynucleotidyl transferase dUTP nick end labelling (TUNEL)-positive cells. Similarly, no differences were observed in the normal ploidy of embryos (56, 67, and 55%) for ANY, CHX and DMAP, respectively. However, higher fertilization rates were observed for ANY (75%) and CHX (87%) treatments compared with DMAP (35%). In conclusion, ANY showed a superior developmental rate compared with CHX treatment. Although no significant differences were observed compared with an improved protocol of DMAP (2Io-DMAP), the lower fertilization rate recorded with DMAP strongly suggests that ANY could be a better alternative for oocyte activation than traditional chemical compounds used currently in ICSI.

A spontaneous increase in intracellular Ca2+ in metaphase II human oocytes in vitro can be prevented by drugs targeting ATP-sensitive K+ channels

STUDY QUESTION

Could drugs targeting ATP-sensitive K(+) (K(ATP)) channels prevent any spontaneous increase in intracellular Ca(2+) that may occur in human metaphase II (MII) oocytes under in vitro conditions?

SUMMARY ANSWER

Pinacidil, a K(ATP) channel opener, and glibenclamide, a K(ATP) channel blocker, prevent a spontaneous increase in intracellular Ca(2+) in human MII oocytes.

WHAT IS KNOWN ALREADY

The quality of the oocyte and maintenance of this quality during in vitro processing in the assisted reproductive technology (ART) laboratory is of critical importance to successful embryo development and a healthy live birth. Maintenance of Ca(2+) homeostasis is crucial for cell wellbeing and increased intracellular Ca(2+) levels is a well-established indicator of cell stress.

STUDY DESIGN, SIZE, DURATION

Supernumerary human oocytes (n = 102) collected during IVF/ICSI treatment that failed to fertilize were used from October 2013 to July 2015. All experiments were performed on mature (MII) oocytes. Dynamics of intracellular Ca(2+) levels were monitored in oocytes in the following experimental groups: (i) Control, (ii) Dimethyl sulfoxide (DMSO; used to dissolve pinacidil, glibenclamide and 2,4-Dinitrophenol (DNP)), (iii) Pinacidil, (iv) Glibenclamide, (v) DNP: an inhibitor of oxidative phosphorylation, (vi) Pinacidil and DNP and (vii) Glibenclamide and DNP.

PARTICIPANTS/MATERIALS/SETTINGS/METHODS

Oocytes were collected under sedation as part of routine treatment at an assisted conception unit from healthy women (mean ± SD) age 34.1 ± 0.6 years, n = 41. Those surplus to clinical use were donated for research. Oocytes were loaded with Fluo-3 Ca(2+)-sensitive dye, and monitored by laser confocal microscopy for 2 h at 10 min intervals. Time between oocyte collection and start of Ca(2+) monitoring was 80.4 ± 2.1 h.

MAIN RESULTS AND THE ROLE OF CHANCE

Intracellular levels of Ca(2+) increased under in vitro conditions with no deliberate challenge, as shown by Fluo-3 fluorescence increasing from 61.0 ± 11.8 AU (AU = arbitrary units; n = 23) to 91.8 ± 14.0 AU (n = 19; P < 0.001) after 2 h of monitoring. Pinacidil (100 µM) inhibited this increase in Ca(2+) (85.3 ± 12.3 AU at the beginning of the experiment, 81.7 ± 11.0 AU at the end of the experiment; n = 13; P = 0.616). Glibenclamide (100 µM) also inhibited the increase in Ca(2+) (74.7 ± 10.6 AU at the beginning and 71.8 ± 10.9 AU at the end of the experiment; n = 13; P = 0.851. DNP (100 mM) induced an increase in intracellular Ca(2+) that was inhibited by glibenclamide (100 µM; n = 9) but not by pinacidil (100 µM; n = 5).

LIMITATIONS, REASONS FOR CAUTION

Owing to clinical and ethical considerations, it was not possible to monitor Ca(2+) in MII oocytes immediately after retrieval. MII oocytes were available for our experimentation only after unsuccessful IVF or ICSI, which was, on average, 80.4 ± 2.1 h (n = 102 oocytes) after the moment of retrieval. As the MII oocytes used here were those that were not successfully fertilized, it is possible that they may have been abnormal with impaired Ca(2+) homeostasis and, furthermore, the altered Ca(2+) homeostasis might have been associated solely with the protracted incubation.

WIDER IMPLICATIONS OF THE FINDINGS

These results show that maintenance of oocytes under in vitro conditions is associated with intracellular increase in Ca(2+), which can be counteracted by drugs targeting K(ATP) channels. As Ca(2+) homeostasis is crucial for contributing to a successful outcome of ART, these results suggest that K(ATP) channel openers and blockers should be tested as drugs for improving success rates of ART.

STUDY FUNDING/COMPETING INTERESTS

University of Dundee, MRC (MR/K013343/1, MR/012492/1), NHS Tayside. Funding NHS fellowship (Dr Sarah Martins da Silva), NHS Scotland. The authors declare no conflicts of interest.

Egg Activation at Fertilization by a Soluble Sperm Protein

The most fundamental unresolved issue of fertilization is to define how the sperm activates the egg to begin embryo development. Egg activation at fertilization in all species thus far examined is caused by some form of transient increase in the cytoplasmic free Ca2+ concentration. What has not been clear, however, is precisely how the sperm triggers the large changes in Ca2+ observed within the egg cytoplasm. Here, we review the studies indicating that the fertilizing sperm stimulates a cytosolic Ca2+ increase in the egg specifically by delivering a soluble factor that diffuses into the cytosolic space of the egg upon gamete membrane fusion. Evidence is primarily considered in species of eggs where the sperm has been shown to elicit a cytosolic Ca2+ increase by initiating Ca2+ release from intracellular Ca2+ stores. We suggest that our best understanding of these signaling events is in mammals, where the sperm triggers a prolonged series of intracellular Ca2+ oscillations. The strongest empirical studies to date suggest that mammalian sperm-triggered Ca2+ oscillations are caused by the introduction of a sperm-specific protein, called phospholipase C-zeta (PLCζ) that generates inositol trisphosphate within the egg. We will discuss the role and mechanism of action of PLCζ in detail at a molecular and cellular level. We will also consider some of the evidence that a soluble sperm protein might be involved in egg activation in nonmammalian species.

Vertebrate Reproduction

Vertebrate reproduction requires a myriad of precisely orchestrated events-in particular, the maternal production of oocytes, the paternal production of sperm, successful fertilization, and initiation of early embryonic cell divisions. These processes are governed by a host of signaling pathways. Protein kinase and phosphatase signaling pathways involving Mos, CDK1, RSK, and PP2A regulate meiosis during maturation of the oocyte. Steroid signals-specifically testosterone-regulate spermatogenesis, as does signaling by G-protein-coupled hormone receptors. Finally, calcium signaling is essential for both sperm motility and fertilization. Altogether, this signaling symphony ensures the production of viable offspring, offering a chance of genetic immortality.

Subcellular localization of phospholipase Cζ in human sperm and its absence in DPY19L2-deficient sperm are consistent with its role in oocyte activation

We recently identified the DPY19L2 gene as the main genetic cause of human globozoospermia (70%) and described that Dpy19l2 knockout (KO) mice faithfully reproduce the human phenotype of globozoospermia making it an excellent model to characterize the molecular physiopathology of globozoospermia. Recent case studies on non-genetically characterized men with globozoospermia showed that phospholipase C, zeta (PLCζ), the sperm factor thought to induce the Ca(2+) oscillations at fertilization, was absent from their sperm, explaining the poor fertilization potential of these spermatozoa. Since 30% of globozoospermic men remain genetically uncharacterized, the absence of PLCζ in DPY19L2 globozoospermic men remains to be formally established. Moreover, the precise localization of PLCζ and the reasons underlying its loss during spermatogenesis in globozoospermic patients are still not understood. Herein, we show that PLCζ is absent, or its presence highly reduced, in human and mouse sperm with DPY19L2-associated globozoospermia. As a consequence, fertilization with sperm from Dpy19l2 KO mice failed to initiate Ca(2+) oscillations and injected oocytes remained arrested at the metaphase II stage, although a few human oocytes injected with DPY19L2-defective sperm showed formation of 2-pronuclei embryos. We report for the first time the subcellular localization of PLCζ in control human sperm, which is along the inner acrosomal membrane and in the perinuclear theca, in the area corresponding to the equatorial region. Because these cellular components are absent in globozoospermic sperm, the loss of PLCζ in globozoospermic sperm is thus consistent and reinforces the role of PLCζ as an oocyte activation factor necessary for oocyte activation. In our companion article, we showed that chromatin compaction during spermiogenesis in Dpy19l2 KO mouse is defective and leads to sperm DNA damage. Together, these defects explain the poor fertilization potential of DPY19L2-globozoospermic sperm and the compromised developmental potential of embryos obtained using sperm from patients with a deletion of the DPY19L2 gene.

Ovum pick up, intracytoplasmic sperm injection and somatic cell nuclear transfer in cattle, buffalo and horses: from the research laboratory to clinical practice

Assisted reproductive techniques developed for cattle in the last 25 years, like ovum pick up (OPU), intracytoplasmic sperm injection (ICSI), and somatic cell nuclear transfer, have been transferred and adapted to buffalo and horses. The successful clinical applications of these techniques require both the clinical skills specific to each animal species and an experienced laboratory team to support the in vitro phase of the work. In cattle, OPU can be considered a consolidated technology that is rapidly outpacing conventional superovulation for embryo transfer. In buffalo, OPU represents the only possibility for embryo production to advance the implementation of embryo-based biotechnologies in that industry, although it is still mainly in the developmental phase. In the horse, OPU is now an established procedure for breeding from infertile and sporting mares throughout the year. It requires ICSI that in the horse, contrary to what happens in cattle and buffalo, is very efficient and the only option because conventional IVF does not work. Somatic cell nuclear transfer is destined to fill a very small niche for generating animals of extremely high commercial value. The efficiency is low, but because normal animals can be generated it is likely that advancing our knowledge in that field might improve the technology and reduce its cost.

Preimplantation embryo metabolism and culture systems: experience from domestic animals and clinical implications

Despite advantages of in vitro embryo production in many species, widespread use of this technology is limited by generally lower developmental competence of in vitro derived embryos compared to in vivo counterparts. Regardless, in vivo or in vitro gametes and embryos face and must adjust to multiple microenvironments especially at preimplantation stages. Moreover, the embryo has to be able to further adapt to environmental cues in utero to result in the birth of live and healthy offspring. Enormous strides have been made in understanding and meeting stage-specific requirements of preimplantation embryos, but interpretation of the data is made difficult due to the complexity of the wide array of culture systems and the remarkable plasticity of developing embryos that seem able to develop under a variety of conditions. Nevertheless, a primary objective remains meeting, as closely as possible, the preimplantation embryo requirements as provided in vivo. In general, oocytes and embryos develop more satisfactorily when cultured in groups. However, optimization of individual culture of oocytes and embryos is an important goal and area of intensive current research for both animal and human clinical application. Successful culture of individual embryos is of primary importance in order to avoid ovarian superstimulation and the associated physiological and psychological disadvantages for patients. This review emphasizes stage specific shifts in embryo metabolism and requirements and research to optimize in vitro embryo culture conditions and supplementation, with a view to optimizing embryo culture in general, and culture of single embryos in particular.

Sperm capacitation combined with removal of the sperm acrosome and plasma membrane enhances paternal nucleus remodelling and early development of bovine androgenetic embryos

The androgenetic embryo is a useful model for functional analysis of the paternal genome during embryogenesis. However, few studies have focused on the factors involved in the suppressed developmental competence of such embryos or why sperm cloning-derived androgenetic embryos fail to develop beyond the morula stage in large domestic animals. To overcome this developmental failure, we tried to improve sperm decondensation, as well as to enhance embryonic development by sperm capacitation and removal of the acrosome and plasma membrane before injection of the spermatozoa. Before injection of the spermatozoa, we quantified the effects of sperm capacitation combined with sperm pretreatment on the acrosome and plasma membrane status. We also evaluated sperm decondensation potential, sperm viability and chromatin integrity. Immunostaining data showed that the sperm acrosome and plasma membrane could be more efficiently removed after capacitation. Dithiothreitol-induced sperm decondensation potential was improved with capacitation and removal of the acrosome and plasma membrane. Although most spermatozoa lost viability after pretreatment, their chromatin remained integrated. The patterns of paternal chromatin remodelling within uncleaved androgenetic embryos and the nucleus morphology of cleaved embryos indicated that capacitation combined with membrane disruption could make injected spermatozoa decondense synchronously not only with each other, but also with the developmental pace of the ooplasm. We successfully produced androgenetic blastocysts, and efficiency increased with sperm pretreatment. In conclusion, sperm decondensation and the early development of androgenetic embryos were enhanced with sperm capacitation and removal of the acrosome and plasma membrane prior to sperm injection.

Improved bovine ICSI outcomes by sperm selected after combined heparin-glutathione treatment

Despite widespread application of intracytoplasmic sperm injection (ICSI) in human-assisted reproductive techniques (ART), the efficiency of this method is still far from satisfactory in livestock, particularly in the bovine species with its unique sperm condensation. On the basis of the natural chemical structure of chromatin in condensed sperm, we developed a novel combined heparin-reduced glutathione (GSH) sperm pretreatment that improves the efficiency of bovine ICSI via selection of the most appropriate sperm at the time of ICSI. Assessment of sperm DNA integrity revealed that this pretreatment can be considered as a safe and efficient approach for in vitro sperm decondensation when compared to conventional sperm pretreatments with dithiothreitol (DTT). Injection of completely decondensed bull sperm derived from this pretreatment significantly improved fertilization and blastocyst formation rates compared to untreated or intact sperm injection (34.8 ± 2.7 and 29.1 ± 1.5 vs. 12.0 ± 3.2 and 15.9 ± 1.2%, respectively; p<0.05). Real-time PCR analysis revealed that expression of pluripotent and anti-apoptosis markers in blastocysts derived by injection of completely decondensed sperm from heparin-GSH pretreatment were comparable to IVF when compared to the DTT pretreatment and control ICSI groups (p<0.05). The results of this study suggested that the degree of sperm decondensation derived from heparin-GSH pretreatment may affect ICSI efficiency in bovine.

Alterations to the bull sperm surface proteins that bind sperm to oviductal epithelium

Three Binder of SPerm proteins (BSP1, BSP3, BSP5) are secreted by bovine seminal vesicles into seminal plasma and adsorbed onto sperm. When sperm inseminated into the female reach the oviduct, the BSP proteins bind them to its epithelial lining, forming a sperm storage reservoir. Previously, we reported that binding of capacitated sperm to oviductal epithelium in vitro is lower than that of uncapacitated sperm and we proposed that reduced binding was due to loss of BSP proteins during capacitation. Because of differences in amino acid sequences, we predicted that each BSP would respond differently to capacitating conditions. To test whether all three BSP proteins were lost from sperm during capacitation and whether the kinetics of loss differed among the three BSP proteins, ejaculated bull sperm were incubated under various capacitating conditions, and then the amounts of BSP proteins remaining on the sperm were assayed by Western blotting. Capacitation was assayed by analysis of protein tyrosine phosphorylation. While loss of BSP1 was not detected, most of the BSP5 was lost from sperm during incubation in TALP medium, even without addition of the capacitation enhancers heparin and dbcAMP-IBMX. Surprisingly, a smaller molecular mass was detected by anti-BSP3 antibodies in extracts of incubated sperm. Its identity was confirmed as BSP3 by mass spectrometry, indicating that BSP3 undergoes modification on the sperm surface. These changes in the composition of BSP proteins on sperm could play a role in releasing sperm from the storage reservoir by modifying sperm interactions with the oviductal epithelium.

Ca2+ signaling during mammalian fertilization: requirements, players, and adaptations

Changes in the intracellular concentration of calcium ([Ca(2+)](i)) represent a vital signaling mechanism enabling communication among cells and between cells and the environment. The initiation of embryo development depends on a [Ca(2+)](i) increase(s) in the egg, which is generally induced during fertilization. The [Ca(2+)](i) increase signals egg activation, which is the first stage in embryo development, and that consist of biochemical and structural changes that transform eggs into zygotes. The spatiotemporal patterns of [Ca(2+)](i) at fertilization show variability, most likely reflecting adaptations to fertilizing conditions and to the duration of embryonic cell cycles. In mammals, the focus of this review, the fertilization [Ca(2+)](i) signal displays unique properties in that it is initiated after gamete fusion by release of a sperm-derived factor and by periodic and extended [Ca(2+)](i) responses. Here, we will discuss the events of egg activation regulated by increases in [Ca(2+)](i), the possible downstream targets that effect these egg activation events, and the property and identity of molecules both in sperm and eggs that underpin the initiation and persistence of the [Ca(2+)](i) responses in these species.

Effect of ICSI on gene expression and development of mouse preimplantation embryos

BACKGROUND

In vitro culture (IVC) and IVF of preimplantation mouse embryos are associated with changes in gene expression. It is however not known whether ICSI has additional effects on the transcriptome of mouse blastocysts.

METHODS

We compared gene expression and development of mouse blastocysts produced by ICSI and cultured in Whitten's medium (ICSI(WM)) or KSOM medium with amino acids (ICSI(KSOMaa)) with control blastocysts flushed out of the uterus on post coital Day 3.5 (in vivo). In addition, we compared gene expression in embryos generated by IVF or ICSI using WM. Global pattern of gene expression was assessed using the Affymetrix 430 2.0 chip.

RESULTS

Blastocysts from ICSI fertilization have a reduction in the number of trophoblastic and inner cell mass cells compared with embryos generated in vivo. Approximately 1000 genes are differentially expressed between ICSI blastocyst and in vivo blastocysts; proliferation, apoptosis and morphogenetic pathways are the most common pathways altered after IVC. Unexpectedly, expression of only 41 genes was significantly different between embryo cultured in suboptimal conditions (WM) or optimal conditions (KSOM(aa)).

CONCLUSIONS

Our results suggest that fertilization by ICSI may play a more important role in shaping the transcriptome of the developing mouse embryo than the culture media used.

Species-specific differences in the activity and nuclear localization of murine and bovine phospholipase C zeta 1

Injection of mammalian sperm extracts or cRNA of the sperm-specific phospholipase C zeta 1 (PLCZ1) has been shown to trigger repetitive oscillations in the concentration of free calcium ([Ca(2+)](i)), leading to oocyte activation and embryo development in all mammals studied to date. While PLCZ1 has cross-species activity, it has also been observed that species-specific differences may exist in the frequency and pattern of the resulting [Ca(2+)](i) oscillations following PLCZ1 cRNA injection into oocytes of different species. Accordingly, we used a crossover design strategy to directly investigate the activity of murine and bovine PLCZ1 in both murine and bovine oocytes. In murine oocytes, injection of murine Plcz1 cRNA induced [Ca(2+)](i) oscillations at 10-fold lower concentrations than bovine PLCZ1, although in bovine oocytes bovine PLCZ1 was more effective than murine Plcz1 at inducing [Ca(2+)](i) oscillations. Investigation of ITPR1 (IP(3)R1) down-regulation in bovine oocytes by PLCZ1 cRNA also showed that bovine PLCZ1 was more active in homologous oocytes. To determine whether these PLCZs exhibited similar cellular distribution, Venus-tagged PLCZ1 cRNA was injected into oocytes, and PLCZ1 was overexpressed. Bovine PLCZ1 failed to accumulate in the pronucleus (PN) of bovine or murine zygotes, despite possessing a putative nuclear localization signal. Conversely, murine PLCZ1 accumulated in the PN of both murine and bovine zygotes. These results demonstrate that murine PLCZ1 and bovine PLCZ1 possess species-specific differences in activity and suggest potential differences in the mode of action of the protein between the two species. Variation in sperm PLCZ1 protein content among species, along with oocyte-specific differences in the localization and availability of PLCZ1 substrates, may further contribute to optimize the activation stimulus to enhance embryo development.

Activation of bovine somatic cell nuclear transfer embryos by PLCZ cRNA injection

The production of cloned animals by the transfer of a differentiated somatic cell into an enucleated oocyte circumvents fertilization. During fertilization, the sperm delivers a sperm-specific phospholipase C (PLCZ) that is responsible for triggering Ca(2)(+) oscillations and oocyte activation. During bovine somatic cell nuclear transfer (SCNT), oocyte activation is artificially achieved by combined chemical treatments that induce a monotonic rise in intracellular Ca(2)(+) and inhibit either phosphorylation or protein synthesis. In this study, we tested the hypothesis that activation of bovine nuclear transfer embryos by PLCZ improves nuclear reprogramming. Injection of PLCZ cRNA into bovine SCNT units induced Ca(2)(+) oscillations similar to those observed after fertilization and supported high rates of blastocyst development similar to that seen in embryos produced by IVF. Furthermore, gene expression analysis at the eight-cell and blastocyst stages revealed a similar expression pattern for a number of genes in both groups of embryos. Lastly, levels of trimethylated lysine 27 at histone H3 in blastocysts were higher in bovine nuclear transfer embryos activated using cycloheximide and 6-dimethylaminopurine (DMAP) than in those activated using PLCZ or derived from IVF. These results demonstrate that exogenous PLCZ can be used to activate bovine SCNT-derived embryos and support the hypothesis that a fertilization-like activation response can enhance some aspects of nuclear reprogramming.