The human glucosamine-6-phosphate deaminase gene: cDNA cloning and expression, genomic organization and chromosomal localization

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.

Relationship between Potential Sperm Factors Involved in Oocyte Activation and Sperm DNA Fragmentation with Intra-Cytoplasmic Sperm Injection Clinical Outcomes

Objective

The present study aimed to simultaneously evaluate the association between expression of three potential factors [post-acrosomal sheath WW domain-binding protein (PAWP), phospholipase Cζ (PLCζ), and truncated form of the kit receptor (TR-KIT)] as candidates of oocyte activation with fertilization rate and early embryonic development.

Materials and Methods

In this experimental study, semen samples were collected from 35 intra-cytoplasmic sperm injection (ICSI) candidates and analyzed according to World Health Organization criteria (2010). Each sample was divided into two parts. The first part was processed for insemination by density-gradient centrifugation (DGC) and the second part was prepared for assessment of sperm morphology (Papanicolaou staining), DNA fragmentation [transferase dUTP nick end labeling (TUNEL)], and three Sperm-borne oocyte-activating factor (s) (SOAFs)-PLCζ, PAWP, and TR-KIT.

Results

Significant positive correlations existed between the percentages of PLCζ, PAWP, and TR-KIT with fertilization rate. In addition, significant negative correlations existed between the percentage of DNA fragmentation with the percentages of PLCζ and PAWP. We did not find a relationship between percentages of PLCζ, PAWP, and TR-KIT with embryo quality and pregnancy rate (P>0.05). There was a significant negative correlation between percentage of DNA fragmentation with fertilization and embryo quality.

Conclusion

Oocyte activation was associated with the studied sperm factors (PAWP, PLCζ, and TR-KIT). These factors might hold the potential to be considered as diagnostic factors in the assessment of semen samples to evaluate their potential to induce oocyte activation. In addition, we observed a significant association between DNA fragmentation with fertilization, as well as embryo quality and expression of PAWP and PLCζ, which indicated that men with high degrees of DNA fragmentation might require artificial oocyte activation. Whether such action should take place, and its cost and benefits should be evaluated in the future.

A QM/MM MD study of the pH-dependent ring-opening catalysis and lid motif flexibility in glucosamine 6-phosphate deaminase

QM/MM MD and MM MD simulations reveal pH-dependent proton-shuttle ring-opening mechanisms of GlcN6P and dynamical behavior of the lid motif inSmuNagB.

Calcium signalling in early embryos

The onset of development in most species studied is triggered by one of the largest and longest calcium transients known to us. It is the most studied and best understood aspect of the calcium signals that accompany and control development. Its properties and mechanisms demonstrate what embryos are capable of and thus how the less-understood calcium signals later in development may be generated. The downstream targets of the fertilization calcium signal have also been identified, providing some pointers to the probable targets of calcium signals further on in the process of development. In one species or another, the fertilization calcium signal involves all the known calcium-releasing second messengers and many of the known calcium-signalling mechanisms. These calcium signals also usually take the form of a propagating calcium wave or waves. Fertilization causes the cell cycle to resume, and therefore fertilization signals are cell-cycle signals. In some early embryonic cell cycles, calcium signals also control the progress through each cell cycle, controlling mitosis. Studies of these early embryonic calcium-signalling mechanisms provide a background to the calcium-signalling events discussed in the articles in this issue.

Calcium and fertilization: the beginning of life

The explosive increase in Ca2+ that occurs in the cytosol at fertilization is brought about by the activation of Ca2+-release channels in the intracellular stores. Inositol 1,4,5-trisphosphate (InsP3) is traditionally considered to be the messenger that initiates the increase and spreading of the activating Ca2+ wave. In line with this hypothesis, recent evidence suggests that the penetrating sperm delivers into mammalian eggs a novel isoform of phospholipase C (PLC), which promotes the formation of InsP3. By contrast, data from echinoderms studies indicate that the newly discovered second messenger nicotinic adenine dinucleotide phosphate (NAADP) promotes an initial, localized increase in Ca2+, which is then followed by the InsP3-mediated globalization of the Ca2+ wave. The mechanism by which the interacting sperm triggers the production of NAADP and subsequently that of InsP3 remains obscure.

Two mammalian glucosamine-6-phosphate deaminases: a structural and genetic study

Glucosamine-6-phosphate deaminase (EC 3.5.99.6) is an allosteric enzyme that catalyzes the reversible conversion of D-glucosamine-6-phosphate into D-fructose-6-phosphate and ammonium. Here we describe the existence of two mammalian glucosamine-6-phosphate deaminase enzymes. We present the crystallographic structure of one of them, the long human glucosamine-6-phosphate deaminase, at 1.75 A resolution. Crystals belong to the space group P2(1)2(1)2(1) and present a whole hexamer in the asymmetric unit. The active-site lid (residues 162-182) presented significant structural differences among monomers. Interestingly the region with the largest differences, when compared with the Escherichia coli homologue, was found to be close to the active site. These structural differences can be related to the kinetic and allosteric properties of both mammalian enzymes.

Cloning and functional characterization of GNPI2, a novel human homolog of glucosamine-6-phosphate isomerase/oscillin

The enzyme, glucosamine-6-phosphate isomerase (GNPI) or deaminase (GNPDA) (EC 5.3.1.10), catalyzes the conversion of GNP to fructose-6-phosphate and ammonia, with an aldo/keto isomerization and an amination/deamination. A hamster sperm-derived protein (Oscillin) with high similarity to bacterial GNPI has been proved to be capable of inducing calcium oscillation in eggs at fertilization. GNPI/Oscillin was supposed to be an important factor in starting embryonic development. From the cDNA library of human dendritic cells (DC), we isolated a novel full-length cDNA encoding a 276-amino acid-residue protein that shares high homology with human GNPI/Oscillin. So, the novel molecule is named as GNPI2. The GNPI2 gene consists of seven exons and six introns. It is mapped to chromosome 4. Northern blot analysis indicated that the tissue distribution of GNPI2 mRNA is different from that of human GNPI or Oscillin mRNA. GNPI2 is ubiquitously expressed in most of human tissues with high expression in testis, ovary, placenta, and heart. Like GNPI, the recombinant GNPI2 has been proved to have the enzymatic activity to catalyze the conversion of GNP to fructose-6-phosphate. Our results indicated that GNPI2 is a novel protein with definite function as a GNPI.

Egg activation at fertilization: where it all begins

A centrally important factor in initiating egg activation at fertilization is a rise in free Ca(2+) in the egg cytosol. In echinoderm, ascidian, and vertebrate eggs, the Ca(2+) rise occurs as a result of inositol trisphosphate-mediated release of Ca(2+) from the endoplasmic reticulum. The release of Ca(2+) at fertilization in echinoderm and ascidian eggs requires SH2 domain-mediated activation of a Src family kinase (SFK) and phospholipase C (PLC)gamma. Though some evidence indicates that a SFK and PLC may also function at fertilization in vertebrate eggs, SH2 domain-mediated activation of PLC gamma appears not to be required. Much work has focused on identifying factors from sperm that initiate egg activation at fertilization, either as a result of sperm-egg contact or sperm-egg fusion. Current evidence from studies of ascidian and mammalian fertilization favors a fusion-mediated mechanism; this is supported by experiments indicating that injection of sperm extracts into eggs causes Ca(2+) release by the same pathway as fertilization.

Cytoplasmic inheritance and its implications for animal biotechnology

At fertilization, the mammalian sperm transmits the haploid paternal genome. However, it also carries a variety of other factors into the oocyte that have the potential to affect embryo development. These include mRNAs left over from spermatogenesis, mitochondria with their own DNA, cytoskeletal and contractile elements, remnants of the sperm plasma membrane and, in many species, the sperm centriole. While most of these elements are eliminated, some play essential roles in early embryogenesis. In this review, I summarize the latest information on these phenomena and indicate some of the implications for animal biotechnology and, in particular, cloning.

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.

Mechanism of Ca2+ release at fertilization in mammals

At fertilization in mammals the sperm triggers a series of oscillations in intracellular Ca2+ within the egg. These Ca2+ oscillations activate the development of the egg into an embryo. It is not known how the sperm triggers these Ca2+ oscillations. There are currently three different theories for Ca2+ signaling in eggs at fertilization. One idea is that the sperm acts as a conduit for Ca2+ entry into the egg after membrane fusion. Another idea is that the sperm acts upon plasma membrane receptors to stimulate a phospholipase C (PLC) within the egg which generates inositol 1,4, 5-trisphosphate (InsP(3)). We present a third idea that the sperm causes Ca2+ release by introducing a soluble protein factor into the egg after gamete membrane fusion. In mammals this sperm factor is also referred to as an oscillogen because, after microinjection, the factor causes sustained Ca2+ oscillations in eggs. Our recent data in sea urchin egg homogenates and intact eggs suggests that this sperm factor has phospholipase C activity that leads to the generation of InsP(3). We then present a new version of the soluble sperm factor theory of signaling at fertilization. J. Exp. Zool. (Mol. Dev. Evol.) 285:267-275, 1999.

Transmembrane regulation of intracellular calcium by a plasma membrane sodium/calcium exchanger in mouse ova

Regulation of cytoplasmic free calcium concentration ([Ca2+)]i) is a key factor for maintenance of viability of cells, including oocytes. Indeed, during fertilization of an ovum, [Ca2+]i is known to undergo oscillations, but it is unknown how basal [Ca2+]i or calcium oscillations are regulated. In the present study we investigated the role of the plasma membrane in regulating [Ca2+]i of metaphase II-arrested mouse oocytes (ova). Ova were collected from B6C3F1 mice treated with eCG (10 IU) and hCG (5 IU), and intracellular calcium was determined by means of fura-2. Extracellular calcium flux across the zona pellucida was detected noninvasively by a calcium ion-selective, self-referencing microelectrode that was positioned by a computer-controlled micromanipulator. Under basal conditions ova exhibited a calcium net efflux of 20.6 +/- 5.2 fmol/cm2 per sec (n = 69). Treatment of ova with ethanol (7%) or thapsigargin (25 nM-2.5 microM) transiently increased intracellular calcium and stimulated calcium efflux that paralleled levels of [Ca2+]i. The presence of a Na+/Ca2+ exchanger was indicated by experiments employing both bepridil, an inhibitor of Na+/Ca2+ exchange, and sodium-depleted media. In the presence of bepridil, a net influx of calcium was revealed across the zona pellucida, which was reflected by an increase in the [Ca2+]i. In addition, replenishment of extracellular sodium to ova that had been incubated in sodium-depleted media induced a large calcium efflux, consistent with the actions of Na+/Ca2+ exchange. Sodium/calcium exchange in mouse ova may be an important mechanism that regulates [Ca2+]i.

Spatiotemporal dynamics of the [Ca2+]i rise induced by microinjection of sperm extract into mouse eggs: preferential induction of a Ca2+ wave from the cortex mediated by the inositol 1,4,5-trisphosphate receptor

Hamster sperm extract (SE) possessing Ca2+ oscillation-inducing activity was microinjected into the peripheral or central region of mouse eggs, and the first increase in intracellular Ca2+ concentration ([Ca2+]i), together with the spread of fluorescence-labeled SE in the ooplasm, was investigated by imaging with confocal microscopy. Injection into the periphery always induced a Ca2+ wave that started from the injection site after a delay of 5 to 30 s depending on the concentration of SE. The diluted SE caused a wave of two-step [Ca2+]i rises, which was always observed at fertilization. Injection into the center could induce a radial Ca2+ wave with relatively high dose of SE, but lower dose of SE caused a [Ca2+]i rise after a longer delay which was initiated synchronously over the ooplasm or was preceded in a peripheral area. Injection of diluted SE remarkably prolonged the delay time and reduced the rate of [Ca2+]i rise. The critical concentration of SE needed to induce [Ca2+]i rise was significantly lower in the periphery. These results indicate that the sensitivity to SE is higher in the cortex. SE-induced [Ca2+]i rises were blocked by an antibody against the type 1 inositol 1,4,5-trisphosphate receptor (InsP3R). The cortex was substantially more sensitive to injected InsP3 induction of Ca2+ release than the center. It is suggested that the cortex of mouse eggs may involve a functionally specialized organization of InsP3Rs and Ca2+ pools in which a cytosolic sperm factor(s) could act upon sperm-egg fusion to cause Ca2+ release, leading to the Ca2+ wave at fertilization.