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.

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.

Phospholipase C-zeta levels are not correlated with fertilisation rates in infertile couples

In mammals, 'oocyte activation' is triggered by certain proteins, one of which is phospholipase C-zeta. Recent evidence suggests that low expression of phospholipase C-zeta might be associated with male infertility, while a limited number of studies claimed the opposite. This study was designed to test whether quantity of phospholipase C-zeta and in vitro fertilisation rates are correlated or not, assessed by flow cytometry. Semen samples from 43 infertile couples were analysed for the percentage and mean fluorescent intensity (MFI) of phospholipase C-zeta protein. Results were confirmed by immunofluorescent labelling. Patients with a fertilisation rate of 40% or lower were involved in the low fertilisation group, while the high fertilization group consisted of patients with a fertilisation rate of 60% and higher. Quantitative analyses by flow cytometry showed no significant difference among the low fertilisation and high fertilisation groups when phospholipase C-zeta ratio or MFI was considered. No correlation was found between pregnancy rates and phospholipase C-zeta quantity. None of the total fertilisation failure cases were lack of phospholipase C-zeta. In fact, fertilisation was possible even when phospholipase C-zeta levels were very low. Thus, we concluded that phospholipase C-zeta quantity cannot be considered as a diagnostic tool for male infertility.

Localisation of phospholipase Cζ1 (PLCZ1) and postacrosomal WW-binding protein (WBP2 N-terminal like) on equine spermatozoa and flow cytometry quantification of PLCZ1 and association with cleavage in vitro

Oocyte activation is initiated when a fertilising spermatozoon delivers sperm-borne oocyte-activating factor(s) into the oocyte cytoplasm. Candidates for oocyte activation include two proteins, phospholipase Cζ1 (PLCZ1) and postacrosomal WW-binding protein (PAWP; also known as WBP2 N-terminal like (WBP2NL)). We localised PLCZ1 and WBP2NL/PAWP in stallion spermatozoa and investigated the PLCZ1 content and sperm parameters as well as cleavage after intracytoplasmic sperm injection (ICSI). PLCZ1 was identified as 71-kDa protein in the acrosomal and postacrosomal regions, midpiece and principal piece of the tail. Anti-WBP2NL antibody identified two WBP2NL bands (~28 and ~32kDa) in the postacrosomal region, midpiece and principal piece of the tail. PLCZ1 and WBP2NL expression was positively correlated (P=0.04) in sperm heads. Flow cytometry evaluation of PLCZ1 revealed large variations in fluorescence intensity and the percentage of positively labelled spermatozoa among stallions. PLCZ1 expression was significantly higher in viable than non-viable spermatozoa, and DNA fragmentation was negatively correlated with PLCZ1 expression and the percentage of positively labelled spermatozoa (P<0.05). The use of equine sperm populations considered to have high versus low PLCZ1 content resulted in significantly higher cleavage rates after ICSI of bovine and equine oocytes, supporting the importance of PLCZ1 for oocyte activation.

Phospholipase Cζ (PLCζ) versus postacrosomal sheath WW domain-binding protein (PAWP): Which molecule will survive as a sperm factor?

During mammalian fertilization, sperm is fused with the oocyte's membrane, triggering the resumption of meiosis from the metaphase II arrest, the extrusion of the second polar body, and the exocytosis of cortical granules; these events are collectively called 'oocyte activation.' In all species studied to date, the transient rise in the cytosolic level of calcium (in particular, the repeated calcium increases called 'calcium oscillations' in mammals) is required for these events. Researchers have focused on identifying the factor(s) that can induce calcium oscillations during fertilization. Sperm‐specific phospholipase C, i.e., PLC zeta (PLCζ), is a strong candidate of the factor(s), and several research groups using different species obtained evidence that PLCζ is a sperm factor that can induce calcium oscillations during fertilization. However, postacrosomal sheath Tryptophan‐Tryptophan (WW)—domain‐binding protein (PAWP) was recently shown to have a pivotal role in inducing calcium oscillations in some species. In this review, we focus on PLCζ and PAWP as sperm factors, and we discuss this controversy: Which of these two molecules survives as a sperm factor?

Increasing associations between defects in phospholipase C zeta and conditions of male infertility: not just ICSI failure?

PURPOSE

Oocyte activation is a fundamental event at mammalian fertilization. In mammals, this process is initiated by a series of characteristic calcium (Ca²⁺) oscillations, induced by a sperm-specific phospholipase C (PLC) termed PLCzeta (PLCζ). Dysfunction/reduction/deletion of PLCζ is associated with forms of male infertility where the sperm is unable to initiate Ca²⁺ oscillations and oocyte activation, specifically in cases of fertilization failure. This review article aims to systematically summarize recent advancements and controversies in the field to update expanding clinical associations between PLCζ and various male factor conditions. This article also discusses how such associations may potentially underlie defective embryogenesis and recurrent implantation failure following fertility treatments, alongside potential diagnostic and therapeutic PLCζ approaches, aiming to direct future research efforts to utilize such knowledge clinically.

METHODS

An extensive literature search was performed using literature databases (PubMed/MEDLINE/Web of Knowledge) focusing on phospholipase C zeta (PLCzeta; PLCζ), oocyte activation, and calcium oscillations, as well as specific male factor conditions.

RESULTS AND DISCUSSION

Defective PLCζ or PLCζ-induced Ca²⁺ release can be linked to multiple forms of male infertility including abnormal sperm parameters and morphology, sperm DNA fragmentation and oxidation, and abnormal embryogenesis/pregnancies. Such sperm exhibit absent/reduced levels, and abnormal localization patterns of PLCζ within the sperm head.

CONCLUSIONS

Defective PLCζ and abnormal patterns of Ca²⁺ release are increasingly suspected a significant causative factor underlying abnormalities or insufficiencies in Ca²⁺ oscillation-driven early embryogenic events. Such cases could potentially strongly benefit from relevant therapeutic and diagnostic applications of PLCζ, or even alternative mechanisms, following further focused research efforts.

Lower blastocyst quality after conventional vs. Piezo ICSI in the horse reflects delayed sperm component remodeling and oocyte activation

PURPOSE

The aim of this study was to evaluate the differential effects of conventional and Piezo-driven ICSI on blastocyst development, and on sperm component remodeling and oocyte activation, in an equine model.

METHODS

In vitro-matured equine oocytes underwent conventional (Conv) or Piezo ICSI, the latter utilizing fluorocarbon ballast. Blastocyst development was compared between treatments to validate the model. Then, oocytes were fixed at 0, 6, or 18 h after injection, and stained for the sperm tail, acrosome, oocyte cortical granules, and chromatin. These parameters were compared between injection techniques and between sham-injected and sperm-injected oocytes among time periods.

RESULTS

Blastocyst rates were 39 and 40%. The nucleus number was lower, and the nuclear fragmentation rate was higher, in blastocysts produced by Conv. Cortical granule loss started at 0H after both sperm and sham injection. The acrosome was present at 0H in both ICSI treatments, and persisted to 18H in significantly more Conv than Piezo oocytes (72 vs. 21%). Sperm head area was unchanged at 6H in Conv but significantly increased at this time in Piezo; correspondingly, at 6H significantly more Conv than Piezo oocytes remained at MII (80 vs. 9.5%). Sham injection did not induce significant meiotic resumption.

CONCLUSIONS

These data show that Piezo ICSI is associated with more rapid sperm component remodeling and oocyte meiotic resumption after sperm injection than is conventional ICSI, and with higher embryo quality at the blastocyst stage. This suggests that there is value in exploring the Piezo technique, utilized with a non-toxic fluorocarbon ballast, for use in clinical human ICSI.

PLCζ Induced Ca2+ Oscillations in Mouse Eggs Involve a Positive Feedback Cycle of Ca2+ Induced InsP3 Formation From Cytoplasmic PIP2

Egg activation at fertilization in mammalian eggs is caused by a series of transient increases in the cytosolic free Ca²⁺ concentration, referred to as Ca²⁺ oscillations. It is widely accepted that these Ca²⁺ oscillations are initiated by a sperm derived phospholipase C isoform, PLCζ that hydrolyses its substrate PIP₂ to produce the Ca²⁺ releasing messenger InsP₃. However, it is not clear whether PLCζ induced InsP₃ formation is periodic or monotonic, and whether the PIP₂ source for generating InsP₃ from PLCζ is in the plasma membrane or the cytoplasm. In this study we have uncaged InsP₃ at different points of the Ca²⁺ oscillation cycle to show that PLCζ causes Ca²⁺ oscillations by a mechanism which requires Ca²⁺ induced InsP₃ formation. In contrast, incubation in Sr²⁺ media, which also induces Ca²⁺ oscillations in mouse eggs, sensitizes InsP₃-induced Ca²⁺ release. We also show that the cytosolic level Ca²⁺ is a key factor in setting the frequency of Ca²⁺ oscillations since low concentrations of the Ca²⁺ pump inhibitor, thapsigargin, accelerates the frequency of PLCζ induced Ca²⁺ oscillations in eggs, even in Ca²⁺ free media. Given that Ca²⁺ induced InsP₃ formation causes a rapid wave during each Ca²⁺ rise, we use a mathematical model to show that InsP₃ generation, and hence PLCζ's substate PIP₂, has to be finely distributed throughout the egg cytoplasm. Evidence for PIP₂ distribution in vesicles throughout the egg cytoplasm is provided with a rhodamine-peptide probe, PBP10. The apparent level of PIP₂ in such vesicles could be reduced by incubating eggs in the drug propranolol which also reversibly inhibited PLCζ induced, but not Sr²⁺ induced, Ca²⁺ oscillations. These data suggest that the cytosolic Ca²⁺ level, rather than Ca²⁺ store content, is a key variable in setting the pace of PLCζ induced Ca²⁺ oscillations in eggs, and they imply that InsP₃ oscillates in synchrony with Ca²⁺ oscillations. Furthermore, they support the hypothesis that PLCζ and sperm induced Ca²⁺ oscillations in eggs requires the hydrolysis of PIP₂ from finely spaced cytoplasmic vesicles.

Phospholipase C zeta and calcium oscillations at fertilisation: The evidence, applications, and further questions

Oocyte activation is a fundamental event at mammalian fertilisation, initiated by a series of characteristic calcium (Ca²⁺) oscillations in mammals. This characteristic pattern of Ca²⁺ release is induced in a species-specific manner by a sperm-specific enzyme termed phospholipase C zeta (PLCζ). Reduction or absence of functional PLCζ within sperm underlies male factor infertility in humans, due to mutational inactivation or abrogation of PLCζ protein expression. Underlying such clinical implications, a significant body of evidence has now been accumulated that has characterised the unique biochemical and biophysical properties of this enzyme, further aiding the unique clinical opportunities presented. Herein, we present and discuss evidence accrued over the past decade and a half that serves to support the identity of PLCζ as the mammalian sperm factor. Furthermore, we also discuss the potential novel avenues that have yet to be examined regarding PLCζ mechanism of action in both the oocyte, and the sperm. Finally, we discuss the advances that have been made regarding the clinical therapeutic and diagnostic applications of PLCζ in potentially treating male infertility as a result of oocyte activation deficiency (OAD), and also possibly more general cases of male subfertility.

Antigen unmasking enhances visualization efficacy of the oocyte activation factor, phospholipase C zeta, in mammalian sperm

STUDY QUESTION

Is it possible to improve clinical visualization of phospholipase C zeta (PLCζ) as a diagnostic marker of sperm oocyte activation capacity and male fertility?

SUMMARY ANSWER

Poor PLCζ visualization efficacy using current protocols may be due to steric or conformational occlusion of native PLCζ, hindering antibody access, and is significantly enhanced using antigen unmasking/retrieval (AUM) protocols.

WHAT IS KNOWN ALREADY

Mammalian oocyte activation is mediated via a series of intracellular calcium (Ca²⁺) oscillations induced by sperm-specific PLCζ. PLCζ represents not only a potential clinical therapeutic in cases of oocyte activation deficiency but also a diagnostic marker of sperm fertility. However, there are significant concerns surrounding PLCζ antibody specificity and detection protocols.

STUDY DESIGN, SIZE DURATION

Two PLCζ polyclonal antibodies, with confirmed PLCζ specificity, were employed in mouse, porcine and human sperm. Experiments evaluated PLCζ visualization efficacy, and whether AUM improved this. Antibodies against two sperm-specific proteins [post-acrosomal WW-binding protein (PAWP) and acrosin] were used as controls.

PARTICIPANTS/MATERIALS, SETTING, METHODS

Aldehyde- and methanol-fixed sperm were subject to immunofluorescence analysis following HCl exposure (pH = 0.1-0.5), acid Tyrode's solution exposure (pH = 2.5) or heating in 10 mM sodium citrate solution (pH = 6.0). Fluorescence intensity of at least 300 cells was recorded for each treatment, with three independent repeats.

MAIN RESULTS AND THE ROLE OF CHANCE

Despite high specificity for native PLCζ following immunoblotting using epitope-specific polyclonal PLCζ antibodies in mouse, porcine and human sperm, immunofluorescent visualization efficacy was poor. In contrast, sperm markers PAWP and acrosin exhibited relatively impressive results. All methods of AUM on aldehyde-fixed sperm enhanced visualization efficacy for PLCζ compared to visualization efficacy before AUM (P < 0.05 for all AUM interventions), but exerted no significant change upon PAWP or acrosin immunofluorescence following AUM. All methods of AUM enhanced PLCζ visualization efficacy in mouse and human methanol-fixed sperm compared to without AUM (P < 0.05 for all AUM interventions), while no significant change was observed in methanol-fixed porcine sperm before and after. In the absence of aldehyde-induced cross-linkages, such results suggest that poor PLCζ visualization efficacy may be due to steric or conformational occlusion of native PLCζ, hindering antibody access. Importantly, examination of sperm from individual donors revealed that AUM differentially affects observable PLCζ fluorescence, and the proportion of sperm exhibiting detectable PLCζ fluorescence in sperm from different males.

LIMITATIONS, REASONS FOR CAUTION

Direct correlation of fertility outcomes with the level of PLCζ in the sperm samples studied was not available. Such analyses would be required in future to determine whether the improved methodology for PLCζ visualization we propose would indeed reflect fertility status.

WIDER IMPLICATIONS OF THE FINDINGS

We propose that AUM alters conformational interactions to enhance PLCζ epitope availability and visualization efficacy, supporting prospective application of AUM to reduce misinterpretation in clinical diagnosis of PLCζ-linked male infertility. Our current results suggest that it is perhaps prudent that previous studies investigating links between PLCζ and fertility parameters are re-examined in the context of AUM, and may pave the way for future work to answer significant questions such as how PLCζ appears to be kept in an inactive form in the sperm.

LARGE SCALE DATA

Not applicable.

STUDY FUNDING/COMPETING INTERESTS

J.K. is supported by a Health Fellowship award from the National Institute for Social Care and Health Research (NISCHR). M.N. is supported by a Marie Curie Intra-European Research Fellowship award. This work was also partly funded by a research grant from Cook Medical Technologies LLC. There are no competing financial interests to declare.

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.

Sperm content of postacrosomal WW binding protein is related to fertilization outcomes in patients undergoing assisted reproductive technology

OBJECTIVE

To determine the levels of postacrosomal WW binding protein (PAWP) in the spermatozoa of men that were used clinically for intracytoplasmic sperm injection (ICSI) and to correlate them with infertility treatment outcomes.

DESIGN

Prospective clinical and laboratory study.

SETTING

University-based laboratory and infertility clinic.

PATIENT(S)

Men undergoing ICSI for the treatment of couples' infertility (n=110).

INTERVENTION(S)

Quantitative analysis of sperm PAWP levels by flow cytometry and developmental analysis of PAWP expression by immunoblotting, immunofluorescence, and immunohistochemistry.

MAIN OUTCOME MEASURE(S)

PAWP flow-cytometric levels and immunolocalization in spermatozoa.

RESULT(S)

A strong positive correlation was found between PAWP expression levels and fertilization rates after ICSI, with high levels of PAWP being associated with higher fertilization rates; the positive correlation was independent of age, DNA fragmentation index, and other sperm parameters. PAWP expression levels were correlated with embryonic development, with high levels of PAWP being associated with a lower number of arrested embryos within 3-5 days post-ICSI. PAWP expression was detected during the late stages of human spermiogenesis in elongating spermatids, confirming previous findings in various animal models.

CONCLUSION(S)

Our clinical data from infertile couples demonstrate significant correlations between sperm PAWP levels and both fertilization rates and normal embryonic development after ICSI. Considering its proposed role in the initiation of oocyte activation, we suggest that PAWP could have potential applications in the diagnosis and treatment of infertility.

Sperm PLCζ: from structure to Ca2+ oscillations, egg activation and therapeutic potential

Significant evidence now supports the assertion that cytosolic calcium oscillations during fertilization in mammalian eggs are mediated by a testis-specific phospholipase C (PLC), termed PLC-zeta (PLCζ) that is released into the egg following gamete fusion. Herein, we describe the current paradigm of PLCζ in this fundamental biological process, summarizing recent important advances in our knowledge of the biochemical and physiological properties of this enzyme. We describe the data suggesting that PLCζ has distinct features amongst PLCs enabling the hydrolysis of its substrate, phosphatidylinositol 4,5-bisphosphate (PIP2) at low Ca(2+) levels. PLCζ appears to be unique in its ability to target PIP2 that is present on intracellular vesicles. We also discuss evidence that PLCζ may be a significant factor in human fertility with potential therapeutic capacity.

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

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

Molecular characteristics of horse phospholipase C zeta (PLCζ)

A sperm-specific phospholipase C (PLC), PLCzeta (PLCζ), is thought to underlie the initiation of calcium ([Ca(2+) ]i ) oscillations that induce egg activation in mammals. In large domestic species, only bovine, porcine and recently equine PLCζ have been cloned, and the physiological functions of these molecules have not been fully characterized. Here, we evaluated the physiological functions of equine PLCζ (ePLCζ) in mouse oocytes. ePLCζ was cloned from testis using RT-PCR. The expression of ePLCζ messenger RNA was confirmed in testis but not in other tissues. Microinjection of ePLCζ complementary RNA (cRNA) into mouse oocytes induced long-lasting [Ca(2+) ]i oscillations, and most of the injected oocytes formed pronuclei (PN). The injection of cRNAs encoding horse, mouse, human and cow PLCζ into mouse oocytes showed that ePLCζ had the highest [Ca(2+) ]i oscillation-inducing activity among the species tested. Mutation of D202R, which renders the protein inactive, abrogated the activity of ePLCζ. The nuclear translocation ability of ePLCζ was defective when expressed in mouse oocytes. Taken together, our findings show for the first time that ePLCζ has highest activity of the mammalian species studied to date. Our findings will be useful for the improvement of reproductive technologies in the horse.

Variance in total levels of phospholipase C zeta (PLC-ζ) in human sperm may limit the applicability of quantitative immunofluorescent analysis as a diagnostic indicator of oocyte activation capability

OBJECTIVE

To examine whether similar levels of phospholipase C zeta (PLC-ζ) protein are present in sperm from men whose ejaculates resulted in normal oocyte activation, and to examine whether a predominant pattern of PLC-ζ localization is linked to normal oocyte activation ability.

DESIGN

Laboratory study.

SETTING

University laboratory.

PATIENT(S)

Control subjects (men with proven oocyte activation capacity; n = 16) and men whose sperm resulted in recurrent intracytoplasmic sperm injection failure (oocyte activation deficient [OAD]; n = 5).

INTERVENTION(S)

Quantitative immunofluorescent analysis of PLC-ζ protein in human sperm.

MAIN OUTCOME MEASURE(S)

Total levels of PLC-ζ fluorescence, proportions of sperm exhibiting PLC-ζ immunoreactivity, and proportions of PLC-ζ localization patterns in sperm from control and OAD men.

RESULT(S)

Sperm from control subjects presented a significantly higher proportion of sperm exhibiting PLC-ζ immunofluorescence compared with infertile men diagnosed with OAD (82.6% and 27.4%, respectively). Total levels of PLC-ζ in sperm from individual control and OAD patients exhibited significant variance, with sperm from 10 out of 16 (62.5%) exhibiting levels similar to OAD samples. Predominant PLC-ζ localization patterns varied between control and OAD samples with no predictable or consistent pattern.

CONCLUSION(S)

The results indicate that sperm from control men exhibited significant variance in total levels of PLC-ζ protein, as well as significant variance in the predominant localization pattern. Such variance may hinder the diagnostic application of quantitative PLC-ζ immunofluorescent analysis.

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.

Molecular cloning and characterization of phospholipase C zeta in equine sperm and testis reveals species-specific differences in expression of catalytically active protein

Oocyte activation at fertilization is brought about by the testis-specific phospholipase C zeta (PLCZ), owing to its ability to induce oscillations in intracellular Ca(2+) concentration ([Ca(2+)](i)). Whereas this is a highly conserved mechanism among mammals, important species-specific differences in PLCZ sequence, activity, and expression have been reported. Thus, the objectives of this research were to clone and characterize the intracellular Ca(2+)-releasing activity and expression of equine PLCZ in sperm and testis. Molecular cloning of equine PLCZ yielded a 1914-bp sequence that translated into a protein of the appropriate size (~73 kDa), as detected with an anti-PLCZ-specific antibody. Microinjection of 1 μg/μl of equine PLCZ cRNA supported [Ca(2+)](i) oscillations in murine oocytes that were of a higher relative frequency than those generated by an equivalent concentration of murine Plcz cRNA. Immunofluorescence revealed expression of PLCZ over the acrosome, equatorial segment, and head-midpiece junction; unexpectedly, PLCZ also localized to the principal piece of the flagellum in all epididymal, uncapacitated, and capacitated sperm. Immunostaining over the acrosome was abrogated after induction of acrosomal exocytosis. Moreover, injection of either sperm heads or tails into mouse oocytes showed that PLCZ in both fractions is catalytically active. Immunohistochemistry on equine testis revealed expression as early as the round spermatid stage, and injection of these cells supported [Ca(2+)](i) oscillations in oocytes. In summary, we report that equine PLCZ displays higher intrinsic intracellular Ca(2+)-releasing activity than murine PLCZ and that catalytically active protein is expressed in round spermatids as well as the sperm flagellum, emphasizing important species-specific differences. Moreover, some of these results may suggest potential novel roles for PLCZ in sperm physiology.

Effect of sperm extract injection volume, injection of PLCzeta cRNA, and tissue cell line on efficiency of equine nuclear transfer

We evaluated the effect of different activation methods on blastocyst development after equine nuclear transfer. All activation treatments were followed by incubation in 2 mM 6-dimethylaminopurine for 4 h. In Experiment 1, reconstructed oocytes were injected with sperm extract for 0.1, 0.2, 0.4, 0.8, or 1.6 sec using a FemtoJet injection device, then treated with ionomycin. The blastocyst rate (9.8%) for 0.1-sec injection was significantly higher than that for 0.2 sec (0%) or 0.8 sec (1.4%). In Experiment 2, injection of murine PLCzeta cRNA before or after ionomycin treatment did not increase blastocyst development (0 and 4.5%) over a control treatment (injection of sperm extract after ionomycin exposure; 5.6%). Transfer of 10 blastocysts produced in Experiments 1 and 2 resulted in five pregnancies, all lost before 70 days of gestation. In Experiment 3, cells from a second biopsy sample from the same horse produced significantly more blastocysts than did the original sample (4/44 vs. 0/58; p < 0.05). Transfer of these four blastocysts produced two viable foals. In Experiment 4, blastocyst development rates did not differ between oocytes in metaphase I or II at the time of nuclear transfer (16.7 and 3.0%, respectively). A healthy foal was produced from a blastocyst originating from a metaphase I oocyte.