Sperm‐egg interactions during fertilization in invertebrates

Cherchez la femme - impact of ocean acidification on the egg jelly coat and attractants for sperm

The impact of ocean acidification on marine invertebrate eggs and consequences for sperm chemotaxis are unknown. In the sea urchins Heliocidaris tuberculata and H. erythrogramma, with small (93µm) and large (393µm) eggs, respectively, we documented the effect of decreased pH on the egg jelly coat, an extracellular matrix that increases target size for sperm and contains sperm attracting molecules. In near future conditions (pH 7.8, 7.6) the jelly coat of H. tuberculata decreased by 11 and 21%, reducing egg target size by 9 and 17%, respectively. In contrast, the egg jelly coat of H. erythrogramma was not affected. The reduction in the jelly coat has implications for sperm chemotaxis in H. tuberculata. In the presence of decreased pH and egg chemicals, the sperm of this species increased their velocity, motility and linearity, behaviour that was opposite to that seen for sperm exposed to egg chemicals in ambient conditions. Egg chemistry appears to cause a reduction in sperm velocity where attractants guide them in the direction of the egg. Investigation of the effects of decreased pH on sperm isolated from egg chemistry does not provide an integrative assessment of the effects of ocean acidification on sperm function. Differences in the sensitivity of the jelly coat of the two species is likely associated with egg evolution in H. erythrogramma. We highlight important unappreciated impacts of ocean acidification on marine gamete functionality, and insights into potential winners and losers in a changing ocean, pointing to the advantage conveyed by evolution of large eggs.

[The dorsoventral inversion: An attempt of synthesis]

The invertebrates, with known exception of echinoderms, are hyponeurian and protostomian. By contrast, echinoderms, chordates and vertebrate are epineurian and deuterostomian. Convinced of the uniqueness origin of all species, Etienne Geoffroy Saint Hilaire (1772-1844), had postulated a complete inversion of body plan to explain this difference. He had to face up to the hostility of the fixist Georges Cuvier (1763-1832). Much later, famous embryologists such as Maurice Caullery still believed that this idea was erroneous. However, the progress of comparative embryology and of developmental biology gradually contributed to validate this idea. Based upon ancient and recent literature review, and re-examination of arthropods (Acanthoscelides obtectus Say), amphibians (Discoglossus), echinoderms (sea urchin) and mammals (rodents) embryos, we can raise up difference and common points of the gastrulation processes. The dorsoventral gradient is ensured by the couple Dpp (dorsal in arthropods)/SOG/chordin (ventral in arthropods), which appears as "inverted" in epineurians. Blastopore invagination occurs in arthopods in the ventral region, opposite to the vitellus mass (initially diffuse, then predominant on the dorsal side), whereas it occurs at the vegetative side in other hyponeurians and epineurians. It has been accepted that the BMP inhibits oral development in protostomian, whereas it activates it in Chordates. Therefore we assume, as Lowe does, that the oral cavity of deuterostomians might constitute a new structure related to the branchial system. The comparative analysis of the blastopore' orientation, the sperm penetration site, and the polarity axes of various embryos species allows to follow the different modifications and to hypothesize their relative chronology during evolution.

Factors affecting the toxicity of trace metals to fertilization success in broadcast spawning marine invertebrates: A review

Significant amounts of trace metals have been released into both nearshore and deep sea environments in recent years, resulting in increased concentrations that can be toxic to marine organisms. Trace metals can negatively affect external fertilization processes in marine broadcast spawners and may cause a reduction in fertilization success at elevated concentrations. Due to its sensitivity and ecological importance, fertilization success has been widely used as a toxicity endpoint in ecotoxicological testing, which is an important method of evaluating the toxicity of contaminants for management planning. Ecotoxicological data regarding fertilization success are available across the major marine phyla, but there remain uncertainties that impair our ability to confidently interpret and analyse these data. At present, the cellular and biochemical events underlying trace metal toxicity in external fertilization are not known. Metal behavior and speciation play an important role in bioavailability and toxicity but are often overlooked, and disparities in experimental designs between studies limit the degree to which results can be synthesised and compared to those of other relevant species. We reviewed all available literature covering cellular toxicity mechanisms, metal toxicities and speciation, and differences in methodologies between studies. We conclude that the concept of metal toxicity should be approached in a more holistic manner that involves elucidating toxicity mechanisms, improving the understanding of metal behavior and speciation on bioavailability and toxicity, and standardizing the fertilization assay methods among different groups of organisms. We identify opportunities to improve the fertilization assay that will allow robust critical and comparative analysis between species and their sensitivities to trace metals during external fertilization, and enable data to be more readily extrapolated to field conditions.

Immunoglobulins against gp273, the ligand for sperm-egg interaction in the mollusc bivalve Unio elongatulus, are directed against charged O-linked oligosaccharide chains bearing a Lewis-like structure and interact with epitopes of the human zona pellucida

In oocytes of the mollusc bivalve Unio elongatulus, gp273 is the ligand molecule for sperm-egg interaction and binding is mediated by its O-glycans. A serum raised against this protein enabled its localization in the crater region, the area of the vitelline coat where sperm recognition occurs, and showed that after cyanogen bromide fragmentation, the anti-gp273 epitope(s) was retained by a peptide where the O-glycans are localized. In this article, we utilized purified anti-gp273 immunoglobulins to characterize the corresponding epitope by: (i) immunoblotting analysis of the protein after removal of O- and N-glycans; (ii) solid phase binding analysis of anti-gp273 IgG to gp273 N- and O-glycans; and (iii) binding analysis of the same antibody to commercially available oligosaccharides. The results showed that the epitope consists of O-glycans and contains a Lewis-like structure with fucose as determinant. Anti-gp273 IgG were then used to investigate human zona pellucida by immunoelectronmicroscopy and immunoblotting. Epitopes recognized by the antibody were demonstrated on the outer surface of the zona pellucida and shown to belong to a zona pellucida protein having electrophoretic mobility similar to human ZP3. Since human sperm specifically bind to gp273, and anti-gp273 interferes with this binding a functional role for these epitopes is suggested.

Synthesis of the Signal Molecule Acetylcholine during the Developmental Cycle of PARAMECIUM PRIMAURELIA (Protista, Ciliophora) and its Possible Function in Conjugation

We recently discovered, in mating-competent Paramecium primaurelia, the presence of functionally related molecules of the cholinergic system: the neurotransmitter acetylcholine (ACh), both its nicotinic and muscarinic receptors and its lytic enzyme acetylcholinesterase (AChE). Our results on the inhibition of mating-cell pairing in vivo in mating-competent cells treated with cholinomimetic drugs support the hypothesis that the cholinergic system plays a role in cell-to-cell adhesion. To investigate the possible function of the signal molecule ACh in conjugation in P. primaurelia, we attempted to detect the intracellular sites of ACh synthesis by localizing the ACh biosynthetic enzyme choline acetyltransferase (ChAT). Using immunocytochemical and histochemical methods, we have demonstrated the presence and activity of ChAT principally on the surface membrane of mating-competent cells and of mature but non-mating-competent cells. No evidence for ChAT activity was found in immature cells. Immunoblot analysis revealed the presence of immunoreactive bands, ranging in molecular mass from 42 to 133kDa, as reported for ChAT isolated from higher organisms. In vivo experiments showed that inhibition of ChAT activity by Congo Red, known to be a potent competitive inhibitor of acetyl coenzyme A, did not affect mating-cell pairing. Conversely, inhibition of AChE with BW284c51 or eserine, which block enzyme activity by reacting with a specific serine within the catalytic centre, significantly inhibited mating-cell pairing. Our results suggest that ACh has a negative modulating effect on conjugation in P. primaurelia.

Primary structure and developmental expression of Dp ZP2, a vitelline envelope glycoprotein homolog of mouse ZP2, in Discoglossus pictus, one of the oldest living Anuran species

A glycoprotein of the Xenopus vitelline envelope, gp 69/64, which mediates sperm binding, is closely related to the components of ZPA family, such as the mouse zona pellucida ZP2. To test the generality of these findings, we studied Discoglossus pictus, a species evolutionary distant from Xenopus and identified as a protein of 63 kDa in the vitelline envelope. Preliminary studies suggest that this protein may bind sperm at fertilization. We found that the 63-kDa protein is glycosylated and contains both N- and O-linked chains. We have cloned the cDNA encoding the Discoglossus protein of 63 kDa (Dp ZP2) by screening a Discoglossus cDNA library using Xenopus gp 69/64 cDNA as a probe. Analysis of the deduced sequence of Discoglossus protein revealed 48% identity with Xenopus gp 69/64 and 37-40% identity with mouse ZP2. The sequence conservation included a ZP domain, a potential furin cleavage site and a putative transmembrane domain. The N-terminus region of Dp ZP2 was 40% identical to the corresponding region of Xenopus gp 69/64 which has been shown to be essential for sperm binding to the VE. Although, as of yet, there is no evidence for sperm binding at the Dp ZP2 N-terminus, it is interesting that in this region three potential O-glycosylation sites are conserved in both species, in contrast to N-glycosylation sites. It was found that the Dp ZP2 mRNA is expressed in stage 1 oocytes and in the follicle cells surrounding the oocyte. Similarly, in Xenopus oocytes, the gp 69/64m RNA, was found in the oocytes, as well as in the somatic cells. Mol. Reprod. Dev. 59:133-143, 2001.

A 140-kDa glycopeptide from the sperm ligand of the vitelline coat of the freshwater bivalve Unio elongatulus, only contains O-linked oligosaccharide chains and mediates sperm-egg interaction

In previous studies we found that sperm binding activity in the vitelline coat of the freshwater bivalve Unio elongatulus is located on the O-linked oligosaccharide chains of gp273, one of the two major components of the extracellular coat, and that fucose plays a key role in this interaction. In this paper we report the partial characterization of a large glycopeptide (about 140 kDa) obtained by cyanogen bromide fragmentation of gp273, that maintains sperm binding activity. Lectin blotting revealed that the glycopeptide reacted with lectins from Arachis hypogaea (PNA) and Lotus tetragonolobus (LTA) but not Canavalia ensiformis (ConA). No other PNA-positive fragments could be detected in the electrophoretic pattern of fragmented gp273 but several ConA-positive fragments of lower molecular weight were present indicating that all the O-linked chains are clustered together in this fragment. Two-dimensional gel electrophoresis of the fragment revealed it to be acidic in nature in contrast with the neutral character of the whole gp273 molecule. Competition binding assay showed that this fragment is a strong inhibitor of the interaction, whereas no effect was detected using the ConA-positive peptides. This confirms that the sperm receptor activity of gp273 is related to its O-linked chains. The immunodominance of this fragment is also discussed.

Specific glycoconjugates are present at the oolemma of the fertilization site in the egg of Discoglossus pictus (Anurans) and bind spermatozoa in an in vitro assay

In the egg of the anuran Discoglossus pictus, the site of fertilization is restricted to the central portion of an animal hemisphere indentation (the dimple). Previous studies showed that the acrosome reaction of D. pictus sperm is triggered in the jelly, and yet sperm arrive at the dimple surface with the plasma membrane at an early stage of vesiculation. Reactivity of the dimple surface with specific lectins suggests that fucose might be utilized as a marker of glycoproteins located at the dimple surface. In this paper, proteins of the egg surface were labeled with the membrane impermeable sulfo-NHS-biotin. Four main bands of 200, 230, 260, and 270 kDa labeled only at the dimple surface, although they were detected in the cortex of the whole egg. The 270-kDa band reacted with Galanthus nivalis agglutinin only in the cortex of the dimple, suggesting that this band is differently glycosylated according to its localization. The alpha-l-fucose-specific lectin Ulex europaeus agglutinin I was utilized both in lectin blotting and in affinity chromatography and cross-reacted with the 200- and 270/260-kDa bands. Furthermore, two polypeptides were obtained by exposure of intact eggs to lysylendoproteinase C. They were also reactive to Ulex europaeus agglutinin I. The 200- and 270/260-kDa bands were eluted from the acrylamide gels and adsorbed to polystyrene beads. An assay for sperm binding to 200-kDa glycoprotein-bound beads was developed. Sperm stuck to the beads before but not after Ca-ionophore treatment. When the beads were coated with the 270/260-kDa glycoproteins, binding occurred after ionophore treatment. In these assays, the 200- and 270/260-kDa glycoproteins competitively inhibited sperm binding to the beads coated with the corresponding glycoprotein. These results indicate that the assayed glycoproteins, located either in the glycocalyx or in the plasma membrane of the fertilization site, are involved in sperm binding.

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and electron microscopy for the characterization of the vitelline coat glycoproteins of the polarized egg of Unio elongatulus

The vitelline coat (VC) glycoproteins of the Unio elongatulus egg, purified as previously described (Focarelli and Rosati, 1993: Mol Reprod Dev 35:44-51) and indicated as gp220 and gp180 by virtue of their apparent molecular weights in sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE), were analyzed by matrix-assisted laser desorption/ionization mass spectrometry (MALDI MS). The analysis confirmed the purity of our preparations and the mass of gp180, but gave a mass of 273,000 for gp220. Intact VCs and purified VC components were then visualized in stereo images of platinum replicas produced by the quick-freeze, deep-etch, and rotary shadowing techniques: gp180 revealed a c-like shape and gp273 a rosette-like shape. The intact VCs were found to consist of two layers, the internal one clearly fibrous and the external one compact. Since purified preparations of gp180 spontaneously formed fibrils of similar width to those present in the inner VC layer, this layer presumably consists mainly of this component. The prevalence of gp273 in the outer layer is also suggested and discussed.

Identification of sulfated oligosialic acid units in the O-linked glycan of the sea urchin egg receptor for sperm

The Strongylocentrotus purpuratus sea urchin egg receptor for sperm is a cell surface glycoprotein with a molecular mass of 350 kDa. Recent studies indicate that the sulfated O-linked glycans isolated from the receptor bind to acrosome-reacted sperm. The purified receptor was analyzed with respect to amino acid and carbohydrate content and shown to be composed of 70% carbohydrate by weight. Compositional analysis indicated that both N- and O-linked oligosaccharide chains were present. After peptide:N-glycanase treatment of the receptor to remove most of the N-linked glycan chains, the majority of the sialic acid residues remained associated with the receptor and were shown by several types of experiments to be composed of sulfated oligosialic acid units attached to the O-linked glycan chains of the receptor. Chemical and physical studies on oligosialic chains discovered earlier in the Pronase-generated glycopeptide fraction isolated from the egg cell surface complex of another species of sea urchin, Hemicentrotus pulcherrimus, established that these molecules had the structure: (SO(4)-)-9Neu5Gc alpha2(-->5-O(glycolyl)Neu5Gc alpha2-->)n. Based on comparative and analytical studies, it was concluded that this sulfated oligosaccharide is a component of a GalNAc-containing chain that is O-linked to the polypeptide chain of the sea urchin egg receptor for sperm. Using a competitive inhibition of fertilization bioassay it was shown that the sulfated oligosialic acid chains derived from the S. purpuratus egg cell surface complex inhibited fertilization; the nonsulfated form of this oligosialic chain had little inhibitory activity.