A cytosolic sperm factor triggers calcium oscillations in rat hepatocytes

Sperm centriolar factors and genetic defects that can predict pregnancy

The human sperm centrosome, comprising the two morphologically distinct centrioles and associated pericentriolar materials, plays a crucial role in fertilization and early embryonic development after fertilization. Once inside the oocyte, the sperm centrosome serves as a microtubule-organizing center, orchestrating mitotic spindle formation, chromosome segregation, and syngamy. Abnormalities of the sperm centrosome can lead to abnormal embryonic development and embryonic chromosomal instability, and are associated with pregnancy loss. Recent research has shed light on the molecular composition, regulation, and function of this vital organelle. Understanding the intricacies of the sperm centrosome is crucial for elucidating the mechanisms underlying successful fertilization and early embryonic development, as well as addressing infertility and developmental disorders associated with centrosomal defects.

The role and mechanism of action of sperm PLC-zeta in mammalian fertilisation

At mammalian fertilisation, the fundamental stimulus that triggers oocyte (egg) activation and initiation of early embryonic development is an acute rise of the intracellular-free calcium (Ca²⁺) concentration inside the egg cytoplasm. This essential Ca²⁺ increase comprises a characteristic series of repetitive Ca²⁺ oscillations, starting soon after sperm-egg fusion. Over the last 15 years, accumulating scientific and clinical evidence supports the notion that the physiological stimulus that precedes the cytosolic Ca²⁺ oscillations is a novel, testis-specific phospholipase C (PLC) isoform, known as PLC-zeta (PLCζ). Sperm PLCζ catalyses the hydrolysis of phosphatidylinositol 4,5-bisphosphate triggering cytosolic Ca²⁺ oscillations through the inositol 1,4,5-trisphosphate signalling pathway. PLCζ is the smallest known mammalian PLC isoform with the most elementary domain organisation. However, relative to somatic PLCs, the PLCζ isoform possesses a unique potency in stimulating Ca²⁺ oscillations in eggs that is attributed to its novel biochemical characteristics. In this review, we discuss the latest developments that have begun to unravel the vital role of PLCζ at mammalian fertilisation and decipher its unique mechanism of action within the fertilising egg. We also postulate the significant potential diagnostic and therapeutic capacity of PLCζ in alleviating certain types of male infertility.

Divergent effect of mammalian PLCζ in generating Ca²⁺ oscillations in somatic cells compared with eggs

Sperm PLCζ (phospholipase Cζ) is a distinct phosphoinositide-specific PLC isoform that is proposed to be the physiological trigger of egg activation and embryo development at mammalian fertilization. Recombinant PLCζ has the ability to trigger Ca²⁺ oscillations when expressed in eggs, but it is not known how PLCζ activity is regulated in sperm or eggs. In the present study, we have transfected CHO (Chinese-hamster ovary) cells with PLCζ fused with either YFP (yellow fluorescent protein) or luciferase and found that PLCζ-transfected cells did not display cytoplasmic Ca²⁺ oscillations any differently from control cells. PLCζ expression was not associated with changes in CHO cell resting Ca²⁺ levels, nor with a significantly changed Ca²⁺ response to extracellular ATP compared with control cells transfected with either YFP alone, a catalytically inactive PLCζ or luciferase alone. Sperm extracts containing PLCζ also failed to cause Ca²⁺ oscillations in CHO cells. Despite these findings, PLCζ-transfected CHO cell extracts exhibited high recombinant protein expression and PLC activity. Furthermore, either PLCζ-transfected CHO cells or derived cell extracts could specifically cause cytoplasmic Ca²⁺ oscillations when microinjected into mouse eggs. These data suggest that PLCζ-mediated Ca²⁺ oscillations may require specific factors that are only present within the egg cytoplasm or be inhibited by factors present only in somatic cell lines.

Release of phospholipase C ζand [Ca2+]i oscillation-inducing activity during mammalian fertilization

During fertilization of mammalian eggs a factor from the sperm, the sperm factor (SF), is released into the ooplasm and induces persistent [Ca2+]ioscillations that are required for egg activation and embryo development. A sperm-specific phospholipase C (PLC), PLCz, is thought to be the SF. Here, we investigated whether the SF activity and PLCζare simultaneously and completely released into the ooplasm soon after sperm entry. To accomplish this, we enucleated sperm heads within 90 min of intracytoplasmic sperm injection (ICSI) and monitored the persistence of the [Ca2+]ioscillations in eggs in which the sperm had been withdrawn. We also stained the enucleatedsperm heads to ascertain the presence/absence of PLCζ. Our results show that by 90 min all the SF activity had been released from the sperm, as fertilized enucleated eggs oscillated as fertilized controls, even in cases in which oscillations were prolonged by arresting eggs at metaphase. In addition, we found that the released SF activity became associated with the pronucleus (PN), as induction of PN envelope breakdown evoked comparable [Ca2+]iresponses in enucleated and non-manipulated zygotes. Lastly, we found that PLCzlocalized to the equatorial area of bull sperm and to the post-acrosomal region of mouse sperm and that by 90 min after ICSI all the sperm’s PLCζimmunoreactivity was lost in both species. Altogether, our findings show that during fertilization the SF activity and PLCζimmunoreactivity are simultaneously released from the sperm, suggesting that PLCζmay be the only [Ca2+]ioscillation-inducing factor of mammalian sperm.

Calcium at fertilization and in early development

Fertilization calcium waves are introduced, and the evidence from which we can infer general mechanisms of these waves is presented. The two main classes of hypotheses put forward to explain the generation of the fertilization calcium wave are set out, and it is concluded that initiation of the fertilization calcium wave can be most generally explained in invertebrates by a mechanism in which an activating substance enters the egg from the sperm on sperm-egg fusion, activating the egg by stimulating phospholipase C activation through a src family kinase pathway and in mammals by the diffusion of a sperm-specific phospholipase C from sperm to egg on sperm-egg fusion. The fertilization calcium wave is then set into the context of cell cycle control, and the mechanism of repetitive calcium spiking in mammalian eggs is investigated. Evidence that calcium signals control cell division in early embryos is reviewed, and it is concluded that calcium signals are essential at all three stages of cell division in early embryos. Evidence that phosphoinositide signaling pathways control the resumption of meiosis during oocyte maturation is considered. It is concluded on balance that the evidence points to a need for phosphoinositide/calcium signaling during resumption of meiosis. Changes to the calcium signaling machinery occur during meiosis to enable the production of a calcium wave in the mature oocyte when it is fertilized; evidence that the shape and structure of the endoplasmic reticulum alters dynamically during maturation and after fertilization is reviewed, and the link between ER dynamics and the cytoskeleton is discussed. There is evidence that calcium signaling plays a key part in the development of patterning in early embryos. Morphogenesis in ascidian, frog, and zebrafish embryos is briefly described to provide the developmental context in which calcium signals act. Intracellular calcium waves that may play a role in axis formation in ascidian are discussed. Evidence that the Wingless/calcium signaling pathway is a strong ventralizing signal in Xenopus, mediated by phosphoinositide signaling, is adumbrated. The central role that calcium channels play in morphogenetic movements during gastrulation and in ectodermal and mesodermal gene expression during late gastrulation is demonstrated. Experiments in zebrafish provide a strong indication that calcium signals are essential for pattern formation and organogenesis.

PLCζ: a sperm-specific trigger of Ca2+ oscillations in eggs and embryo development

Upon fertilisation by sperm, mammalian eggs are activated by a series of intracellular Ca2+ oscillations that are essential for embryo development. The mechanism by which sperm induces this complex signalling phenomenon is unknown. One proposal is that the sperm introduces an exclusive cytosolic factor into the egg that elicits serial Ca2+ release. The ‘sperm factor’ hypothesis has not been ratified because a sperm-specific protein that generates repetitive Ca2+ transients and egg activation has not been found. We identify a novel, sperm-specific phospholipase C, PLCζ, that triggers Ca2+ oscillations in mouse eggs indistinguishable from those at fertilisation. PLCζ removal from sperm extracts abolishes Ca2+ release in eggs. Moreover, the PLCζ content of a single sperm was sufficient to produce Ca2+ oscillations as well as normal embryo development to blastocyst. Our results are consistent with sperm PLCζ as the molecular trigger for development of a fertilised egg into an embryo.

Cell cycle-dependent repetitive Ca(2+)waves induced by a cytosolic sperm extract in mature ascidian eggs mimic those observed at fertilization

Sperm-triggered Ca(2+) oscillations occur throughout the animal kingdom. The mechanism sperm use to trigger Ca(2+) oscillations at fertilization has not been resolved in any egg. The temporal, spatial and regulatory characteristics of the Ca(2+) oscillations during fertilization in ascidians offer a unique advantage over other systems for determining the mechanism of fertilization. For example, sperm trigger two phases of Ca(2+) oscillations that are all waves in ascidians. The first of these Ca(2+) waves begins at the point of sperm-egg fusion while a second phase of Ca(2+) waves originates at a vegetal protrusion termed the contraction pole. In addition, cyclin B1-dependent kinase activity provides a form of positive feedback, maintaining the second phase of Ca(2+) waves during meiosis and thereby ensuring meiotic exit. We therefore prepared cytosolic ascidian sperm extracts or MonoQ-fractionated ascidian sperm extracts from this urochordate to investigate if a Ca(2+)-releasing sperm-borne factor was responsible for egg activation. Spatially, ascidian sperm extract induced repetitive Ca(2+) waves that mimicked the spatial pattern displayed during fertilization: all the second-phase Ca(2+) waves originated at a vegetal protrusion termed the contraction pole (thus mimicking fertilisation). We also demonstrated that ascidian sperm extract-induced Ca(2+) oscillations were maintained when CDK activity was elevated and MAP kinase activity was low, as found previously for sperm-triggered Ca(2+) oscillations. As would be predicted, large doses of ascidian sperm extract injected into prophase-stage oocytes, lacking CDK activity, failed to induce any Ca(2+) release even though they responded to microinjection of the Ca(2+)-releasing second messenger inositol 1,4,5-trisphosphate. Finally, since the Ca(2+)-releasing activity from Mono-Q fractionated ascidian sperm extract eluted predominantly as one fraction, this may imply that one factor is responsible for the Ca(2+)-releasing activity. These data support a model of egg activation whereby the sperm introduces a Ca(2+)-releasing cytosolic factor into the egg. We demonstrated that ascidian sperm contain a protein factor(s) that is regulated by the egg CDK activity and can trigger all the Ca(2+)waves observed at fertilization with a spatial pattern that mimics those initiated by sperm.

Cloning, sequencing, and expression analysis of mouse glucosamine-6-phosphate deaminase (GNPDA/oscillin)

It was reported that a hamster protein, called "oscillin," with a sequence related to that of an Escherichia coli GNPDA triggered Ca(2+) oscillations in mammalian oocytes when introduced into their cytoplasm upon fertilization. Recently, it was shown that GNPDA/oscillin is ubiquitously expressed in rat tissues and that a recombinant hamster GNPDA/oscillin protein does not exhibit oscillin activity when injected into oocytes. In the mouse, the nature and role of such a GNPDA/oscillin is not known, but another candidate protein, tr-kit, has been proposed as a sperm factor causing oocyte activation. In order to clarify this issue, we have characterized the mouse homolog of hamster and human GNPDA/oscillin, and examined its expression along with that of tr-kit, in parallel. We report here the molecular cloning and sequencing of mouse GNPDA/oscillin, which shows over 96% identity with the hamster and human homologs. Using specific primers, we performed an RT-PCR analysis to determine the tissue distribution of mouse GNPDA/oscillin mRNA. Unlike tr-kit mRNA which is expressed solely in mouse testis, GNPDA/oscillin mRNA is detected in unfertilized oocytes and in all tissues examined including testis, heart, thymus, liver, ovary, uterus, kidney, spleen, and lung. The protein itself is also detected in all tissues examined by Western blots. Indirect immunofluorescence studies, using an antibody raised against hamster GNPDA, demonstrate that GNPDA is lost with the acrosome reaction of mouse spermatozoa, is localized in the equatorial and neck regions of the human spermatozoa and the post-acrosomal region of the hamster spermatozoa. Our results thus indicate that mouse GNPDA/oscillin, the homolog of hamster oscillin, unlike tr-kit, does not exhibit some of the required characteristics expected from a putative sperm-derived oocyte-activating factor.

Injections of porcine sperm extracts trigger fertilization-like calcium oscillations in oocytes of a marine worm

The precise mechanisms by which sperm trigger calcium transients in eggs or oocytes during fertilization remain unknown. Based on time-lapse confocal microscopy, we show that intracellular injections of porcine sperm extracts cause the oocytes of a marine nemertean worm to undergo repetitive calcium oscillations resembling those obtained during normal fertilizations. Such findings are consistent with the view that fertilization involves a soluble sperm factor (SF) which is capable of eliciting calcium transients without binding to externally situated receptors on the oocyte plasmalemma. This study also describes for the first time the wave-like propagation patterns of SF-induced calcium transients that are generated in a heterologous combination of gametes obtained from different phyla of animals. Such cross-reactivity between distantly related taxa suggests that the intracellular signaling pathways triggered by sperm factors can be well conserved.

Spatiotemporal analysis of [Ca2+]i rises in mouse eggs after intracytoplasmic sperm injection (ICSI)

Intracytoplasmic sperm injection (ICSI) into mammalian eggs induces repetitive rises in intracellular Ca2+ concentration ([Ca2+]i) which are the pivotal signal in fertilization. Spatiotemporal aspects of [Ca2+]i rises following ICSI into the periphery of mouse eggs were investigated with high-speed confocal microscopy. The first Ca2+ response was generated 25-30 min after ICSI, when [Ca2+]i increased slowly and reached a certain level. The [Ca2+]i rise occurred synchronously over the ooplasm, attained the peak in 40-70 s, and lasted for 5-7 min. Succeeding Ca2+ responses occurred at intervals of 20-30 min, associated with the faster rate of [Ca2+]i rise and the shorter duration as Ca2+ oscillations progressed. The [Ca2+]i rises took the form of a wave that started from an arbitrary cortical region, but not from the vicinity of the injected sperm head. The Ca2+ wave became more pronounced and propagated across the egg faster in the later Ca2+ responses. An artifactual [Ca2+]i rise was inevitably produced during the ICSI procedure. The larger artifact affected the subsequent first Ca2+ response, resulting in the faster [Ca2+]i rise (time to peak, 10-20 s), slight spatial heterogeneity of [Ca2+]i rise in the ooplasm (but not a wave) and the shorter duration (3-4 min). The artifact slightly affected the amplitude of the second Ca2+ response, but little affected the later Ca2+ responses. It is suggested that the factor(s) that leaked out of the injected spermatozoon diffuses to a wide area and sensitizes Ca2+ channels of the endoplasmic reticulum to induce Ca2+ release synchronously over the ooplasm. The enhanced sensitization leads to propagating Ca2+ release initiated from the cortex that is more sensitive to the sperm factor.

A novel protein for Ca2+ signaling at fertilization

At fertilization in all species studied the sperm activates the egg by causing an increase in the level of cytoplasmic free Ca2+ concentration. It is still not established how the sperm causes the changes in Ca2+ in the egg, which in the majority of eggs is due to release from internal stores. Current hypotheses about the signaling molecules involved in fertilization are confounded by the fact that for many eggs the fertilization-associated Ca2+ increase is readily mimicked by parthenogenetic activating agents. One exception to this is found for mammalian eggs where there are a series of Ca2+ oscillations observed at fertilization that have distinct characteristics. In this context we discuss three different theories of how sperm trigger Ca2+ release in eggs. We present the case that the sperm mediates its Ca2+ mobilization effects after gamete membrane fusion by introducing a specific protein into the egg cytoplasm. Our argument is based upon the fact that only the mammalian sperm protein factor can trigger a pattern of Ca2+ oscillations that is similar to that induced by the sperm in mammalian eggs. The sperm factor activity is correlated with a novel signaling protein that we have called oscillin and which may mediate Ca2+ release via a novel mechanism.

Identification of genes differentially expressed by hypoxia in hepatocellular carcinoma cells

In order to identify genes differentially expressed under hypoxia (1% O2, 5% CO2, balance N2), we performed mRNA differential display analysis using total RNA extracted from hypoxic and normoxic HepG2, human hepatocellular carcinoma (HCC) cells. Of the differentially expressed genes by hypoxia, some of cDNA fragments were cloned and sequenced. The expression patterns of these clones by hypoxia were confirmed by Northern blot analysis and the quantitative RT-PCR. Down-regulated genes by hypoxia have homology to cDNA sequences encoding cytochrome oxidase subunit II and ADP/ATP translocase, respectively. Up-regulated gene by hypoxia was identified as Homo sapiens oscillin. Moreover, novel genes induced by hypoxia represent partial sequences of cDNAs that have not been reported or functionally identified. Up- or down-regulated expression of these genes in response to hypoxia may contribute to human hepatocarcinogenesis.

A cytosolic sperm protein factor mobilizes Ca2+ from intracellular stores by activating multiple Ca2+ release mechanisms independently of low molecular weight messengers

Ca2+ oscillations can be induced in mammalian eggs and somatic cells by microinjection of a cytosolic sperm protein factor. The nature of the sperm factor-induced Ca2+ signaling was investigated by adding sperm protein extracts to homogenates of sea urchin eggs, which contain multiple classes of Ca2+ release mechanisms. We show that the sperm factor mobilizes Ca2+ from non-mitochondrial Ca2+ stores in egg homogenates after a distinct latency. This latency is abolished by preincubation of sperm extracts with egg cytosol. The preincubation step is highly temperature-dependent and generates a high molecular weight, protein-based Ca2+-releasing agent that can also mobilize Ca2+ from purified egg microsomes. This Ca2+ release appears to be mediated via both inositol 1,4,5-trisphosphate and ryanodine receptors, since homologous desensitization of these two release mechanisms by their respective agonists inhibits further release by the sperm factor. However, sperm factor-induced Ca2+ release by these channels is independent of inositol 1,4, 5-trisphosphate or cADPR since antagonists of either of these two messengers did not block the Ca2+ release effected by the sperm factor. The sperm protein factor may cause Ca2+ release via an enzymatic step that generates a protein-based Ca2+-releasing agent.

A novel signalling mechanism for generating Ca2+ oscillations at fertilization in mammals

At fertilization in mammals the sperm activates the egg by triggering a series of oscillations in the intracellular free Ca2+ concentration. The precise sequence of events that occur between sperm-egg contact and the increases in intracellular Ca2+ remains unknown. Here, we discuss recent evidence supporting the hypothesis that a cytosolic sperm protein enters the egg after gamete membrane fusion and triggers Ca2+ oscillations from within the egg cytoplasm. Biochemical studies suggest that there exists a novel sperm protein, named oscillin, that specifically comigrates with Ca2+ oscillation-inducing activity. Oscillin has been immunolocalised to the region of the sperm that first fuses with the egg. The concept of a specific protein that triggers Ca2+ oscillations may have wider physiological significance since sperm oscillin can induce Ca2+ oscillations in somatic cells, such as neurons and hepatocytes. Unravelling the novel signalling system involved in mammalian fertilization may help reveal some fundamental molecular mechanisms responsible for triggering cytoplasmic Ca2+ oscillations.

Calcium oscillations in mammalian eggs triggered by a soluble sperm protein

At fertilization in mammals, the sperm induces a characteristic series of Ca2+ oscillations in the egg which serve as the essential trigger for egg activation and early development of the embryo. It is not known how the sperm initiates this fundamental process, however, nor has any pathway linking sperm-egg membrane-receptor binding with intracellular Ca2+ release been demonstrated. Microinjection of sperm extracts into mammalian eggs elicits Ca2+ oscillations identical to those occurring at fertilization, which suggests that sperm may introduce a Ca2+ oscillation-inducing factor into the egg on gamete membrane fusion. Here we identify a soluble sperm protein that exhibits Ca2+ oscillation-inducing ('oscillogen') activity in eggs. Sperm oscillogen exists as an oligomer with a subunit of M(r) 33K and a specific intracellular localization at the equatorial segment of the sperm head. Cloning of the 33K oscillogen complementary DNA indicates similarity with a hexose phosphate isomerase found in prokaryotes. This sperm-derived oscillogen, termed oscillin, may represent the physiological trigger for development in mammals.