Effect on fertilization of antiserum against sperm-binding protein from homo- and heterologous sea urchin egg surfaces

Exploiting common targets in human fertilization and HIV infection: development of novel contraceptive microbicides

The continued high rates of unintended pregnancies and the unrelentless expansion of the acquired immune deficiency syndrome (AIDS) epidemic, especially in less developed countries, warrant the development of novel strategies to help individuals avoid these risks. Dually active compounds displaying contraceptive and microbicidal anti-human immunodeficiency virus (anti-HIV) properties constitute one such strategy. Sharing the same anatomical and functional context, sperm fertilization and genital infection by HIV offer an opportunity for simultaneous intervention. Some of the molecules and mechanisms used by sperm to fertilize the oocyte are similar, if not identical, to those used by HIV while infecting host cells. An example of common structures is the lipid membrane surrounding the spermatozoon and the HIV core. Disruption of its architecture by surface-active compounds exerts both spermicidal and virucidal activity. A more specific alteration of lipid rafts [membrane microdomains enriched in cholesterol and glycosylphosphatidylinositol (GPI)-anchored proteins] by beta-cyclodextrins also results in similar effects. During fertilization and infection, both sperm and HIV interact with their target cell receptors through chemical charges, hydrophobic forces and carbohydrate recognition. Anionic polymers such as cellulose sulphate and polystyrene sulphonate (PSS) inhibit sperm and HIV cell binding. Because some of the molecules involved in this interaction, e.g. heparin sulphate proteoglycan, are also used by other pathogens to infect their target tissues, polyanions exert broad antimicrobial activity as well. During fertilization and infection, sperm and HIV, as well as other microbes, use signal transduction molecules and mechanisms such as adenyl cyclase/cyclic adenosine monophosphate (cAMP)-dependent kinase, calcium and tyrosine phosphorylation, whose inhibition has been shown to impair sperm function and HIV replication. These commonalities at the level of sperm and HIV structure, cell binding and fusion processes, and signalling pathways therefore provide the biological framework to develop bifunctional inhibitors with both antimicrobial and contraceptive properties.

Sperm-binding proteins

Gamete recognition and binding are mediated by specific proteins on the surface of the sperm and egg. Identification and characterization of some of these proteins from several model systems, particularly mouse and sea urchin, have focused interest on the general properties and functions of gamete recognition proteins. Sperm-binding proteins located in egg extracellular coats as well as sperm-binding proteins that are localized to the egg plasma membrane are presented in the context of their structure and function in gamete binding. Unifying and disparate characteristics are discussed in light of the diverse biology of fertilization among species. Outstanding questions, alternative mechanisms and models, and strategies for future work are presented.

Purification of the sperm-binding factor and identification of a sperm attack molecule from the egg of the sea urchin, Hemicentrotus pulcherrimus

A sperm-binding factor, which seems to have a primary role in binding sperm to the egg, was isolated from the egg surface of Hemicentrotus pulcherrimus and purified by monitoring the neutralization of the fertilization inhibition exerted by Fab fragments against crude sperm-binding factor. An improved purification for this sperm-binding factor is described in the present paper. The preparation of purified sperm-binding factor revealed one major protein band with an apparent molecular weight of 61,000 after sodium dodecylsulfate-polyacrylamide gel electrophoresis. A substance with the fertilization inhibitory effect on sperm, was isolated by diethylaminoethyl (DEAE)-Sephadex column chromatography in the course of purification of the sperm-binding factor and termed "sperm attack molecule." One precipitin line was formed between the sperm attack molecule and anti-crude sperm-binding factor serum in a double-immunodiffusion test. Fab fragments were prepared against partially purified sperm-binding factor or sperm attack molecule, and the effect of these Fab fragments on eggs was investigated. Anti-sperm-binding factor Fab fragments inhibited the fertilizability of eggs, whereas anti-sperm attack molecule Fab fragments did not. However, anti-sperm attack molecule Fab fragments impaired elevation of the vitelline layer. It is possible that the sperm attack molecule prevents polyspermy. Sperm attack molecule contains 3.7% neutral sugars. Its inhibitory effect was cancelled by trypsin or heat.

A 225 K dalton glycoprotein is the active core structure of the sperm-binding factor of the sea urchin, Anthocidaris crassispina

Fab fragments against 225 000 D glycoprotein (225 K), 87 000 D protein (87K) 80 000 D glycoprotein (80 K) of partially purified sperm-binding factor of Anthocidaris crassispina were prepared, and their effects upon fertilizability of dejellied homologous and heterologous eggs examined. Only the 225 K Fab impaired the fertilizability of homologous, not heterologous eggs by decreasing their sperm-binding capacity. It was concluded that 225 K glycoprotein is the active core structure of the sperm-binding factor of this species. The possible participation of the other two proteins as residual ingredients of the sperm-binding factor was also discussed. Immunofluorescence studies showed that 225 K core protein is localized on the whole surface of unfertilized eggs and of fully-grown oocytes. The fluorescence disappeared following fertilization.

Ionic regulation of egg activation

Developing cells have constantly to make decisions: when to proliferate and divide, when and how to differentiate. It is an increasingly attractive idea that these decisions involve changes in intracellular cation concentrations. Our ideas about the mechanisms of changes in intracellular cations come largely from the application of biophysical techniques in the study of excitable tissues. These ideas are proving very valuable to the investigation of the control of proliferation and cell development and it is evident that the ionic mechanisms which pertain in nerve and muscle have their counterparts in other cells. Just as alterations in intracellular ion concentrations serve a signalling function in excitable tissue, so too they act as signals during development. Since almost all the quantitative data on the ionic mechanisms of fertilization come from work on sea urchins we have confined our review to sea urchin eggs.

The immunobiology of placentation

A review of the current theories on the nature of the placental allograft and the evolution of placentation in the presence of a cellular immune response highlights the inability of current hypotheses to explain the success of both invertebrate and vertebrate placentation. Only one theory -- "the anatomical separation of the fetus from the mother" -- appears relevant. How this separation might develop during implantation is discussed.

Identification of a sperm receptor on the surface of the eggs of the sea urchin Arbacia punctulata

The possibility that the surface of the egg of the sea urchin Arbacia punctulata contains a species-specific receptor for sperm has been investigated. The extent of fertilization of eggs of A. punctulata, which is proportional to the number of sperm, is unaffected by the presence of either eggs or membranes prepared from eggs of Strongylocentrotus purpuratus. In marked contrast, membranes prepared from eggs of A. punctulata quantitatively inhibit fertilization of A. punctulata eggs by A. punctulata sperm. Several lines of evidence indicate that this inhibition is due to the presence of a membrane-associated glycoprotein that binds to the sperm, thus preventing them from interacting with receptor on the surface of the eggs. First, eggs treated with trypsin are incapable of being fertilized, although they can be activated with the Ca2+ ionophore A23187. Moreover, membranes prepared from eggs pretreated with trypsin do not inhibit fertilization of eggs. Second, receptor isolated in soluble form from surface membranes binds to sperm and thus prevents them from fertilizing eggs; the inhibition by soluble receptor is species-specific. Third, the soluble receptor binds to concanavalin A-Sepharose. Fourth, eggs are incapable of being fertilized if they are pretreated with concanavalin A. The specificity of inhibition, and the affect of trypsin and concanavalin A on intact eggs, suggest that the receptor is a species-specific macromolecule located on the surface of the eggs. The sensitivity of the receptor to trypsin, and its ability to bind to concanavalin A, indicate that it is a glycoprotein.