Mammalian sperm-egg recognition: does fertilin beta have a major role to play?

Porcine model for the study of sperm capacitation, fertilization and male fertility

Mammalian fertilization remains a poorly understood event with the vast majority of studies done in the mouse model. The purpose of this review is to revise the current knowledge about semen deposition, sperm transport, sperm capacitation, gamete interactions and early embryonic development with a focus on the porcine model as a relevant, alternative model organism to humans. The review provides a thorough overview of post-ejaculation events inside the sow's reproductive tract including comparisons with humans and implications for human fertilization and assisted reproductive therapy (ART). Porcine methodology for sperm handling, preservation, in vitro capacitation, oocyte in vitro maturation, in vitro fertilization and intra-cytoplasmic sperm injection that are routinely used in pig research laboratories can be successfully translated into ART to treat human infertility. Last, but not least, new knowledge about mitochondrial inheritance in the pig can provide an insight into human mitochondrial diseases and new knowledge on polyspermy defense mechanisms could contribute to the development of new male contraceptives.

Adhesion molecules in gamete transport, fertilization, early embryonic development, and implantation-role in establishing a pregnancy in cattle: A review

Cell-cell adhesion molecules have critically important roles in the early events of reproduction including gamete transport, sperm-oocyte interaction, embryonic development, and implantation. Major adhesion molecules involved in reproduction include cadherins, integrins, and disintegrin and metalloprotease domain-containing (ADAM) proteins. ADAMs on the surface of sperm adhere to integrins on the oocyte in the initial stages of sperm-oocyte interaction and fusion. Cadherins act in early embryos to organize the inner cell mass and trophectoderm. The trophoblast and uterine endometrial epithelium variously express cadherins, integrins, trophinin, and selectin, which achieve apposition and attachment between the elongating conceptus and uterine epithelium before implantation. An overview of the major cell-cell adhesion molecules is presented and this is followed by examples of how adhesion molecules help shape early reproductive events. The argument is made that a deeper understanding of adhesion molecules and reproduction will inform new strategies that improve embryo survival and increase the efficiency of natural mating and assisted breeding in cattle.

Deficiency in Sperm-Egg Protein Interaction as a Major Cause of Fertilization Failure

Complete elucidation of fertilization process at molecular level is one of the unresolved challenges in sexual reproduction studies, and understanding the molecular mechanism is crucial in overcoming difficulties in infertility and unsuccessful in vitro fertilization. Sperm-oocyte interaction is one of the most remarkable events in fertilization process, and deficiency in protein-protein interactions which mediate this interaction is a major cause of unexplained infertility. Due to detection of how the various defects of sperm-oocyte interaction can affect fertilization failure, different experimental methods have been applied. This review summarizes the current understanding of sperm-egg interaction mechanism during fertilization and also accumulates the different types of sperm-egg interaction abnormalities and their association with infertility. Several detection approaches regarding sperm-egg protein interactions and the associated defects are reviewed in this paper.

Sperm plasma-membrane-associated glutathione S-transferases as gamete recognition molecules

Glutathione S-transferases (GSTs) are enzymes that detoxify electrophilic compounds. Earlier studies from our laboratory showed that anti-GST antibodies interfered with the fertilising ability of spermatozoa from Capra hircus (goat) in vitro, suggesting that GSTs are localised at the cell surface. In this study, we provide evidence for the presence of GSTs of 24 kDa on the sperm plasma membrane attached by non-covalent interactions. The GST activity associated with the spermatozoal plasma membrane was significantly higher than the activity present in the plasma membranes of brain cells,hepatocytes, spleenocytes and ventriculocytes. Analysis of GST isoforms demonstrates the presence of GST Pi and Mu on the sperm plasma membranes. Both isoforms were able to bind to solubilised as well as intact zona pellucida(ZP) through their N-terminal regions but failed to bind to ZP once the oocytes were fertilised. Solubilised goat ZP separates into three components,one of which, the ZP3-like component, bound to sperm GSTs. High concentrations of anti-GST antibodies or solubilised ZP led to aggregation of sperm GSTs,resulting in the release of acrosin. In contrast, inhibition of sperm GST binding to ZP, by saturation of binding sites for sperm GSTs on the solubilised ZP using peptides designed from the N-terminii of GST Pi or Mu or blocking of binding sites for ZP on sperm GSTs with antibodies raised against the N-terminal GST peptides, inhibited essential prefertilisation changes in sperm.

These data therefore demonstrate the strategic location of catalytically active defensive enzymes on the sperm surface that also act as zona-binding proteins. Therefore, sperm-surface GSTs serve as bifunctional molecules in a transcriptionally inactive cell whose requirement for cellular defense and economy of molecules that it can carry is greater than that of any somatic cell type.

Analysis of loss of adhesive function in sperm lacking cyritestin or fertilin beta

We produced mice lacking the sperm surface protein cyritestin (ADAM 3) and found mutant males are infertile. Similar to fertilin beta (ADAM 2) null sperm (C. Cho et al., 1998, Science 281, 1857-1859), cyritestin null sperm are drastically deficient in adhesion to the egg zona pellucida (0.3% of wild type) and to the egg plasma membrane (9% of wild type). Thus sperm from male mice with a gene deletion of either ADAM have a loss of adhesive function in at least two steps of fertilization. We found deletion of either ADAM gene resulted in the loss of multiple gene products. This loss of multiple gene products (sperm membrane proteins) appears to result from a novel, developmental mechanism during sperm differentiation. Because the altered sperm protein expression must be responsible for the fertilization defects, our data suggest new models for the molecular basis of the affected steps in fertilization.

A profile of fertilization in mammals

Fertilization is defined as the process of union of two gametes, eggs and sperm. When mammalian eggs and sperm come into contact in the female oviduct, a series of steps is set in motion that can lead to fertilization and ultimately to development of new individuals. The pathway begins with species-specific binding of sperm to eggs and ends a relatively short time later with fusion of a single sperm with each egg. Although this process has been investigated extensively, only recently have the molecular components of egg and sperm that participate in the mammalian fertilization pathway been identified. Some of these components may participate in gamete adhesion and exocytosis, whereas others may be involved in gamete fusion. Here we describe selected aspects of mammalian fertilization and address some of the latest experimental evidence that bears on this important area of research.

Molecular and biochemical regulation of early mammalian development

Fertilization initiates a rapid series of changes that restructures the egg into the zygote and initiates the program of early development. These changes in the cell occur while the genetic complement of the egg and sperm are in a highly condensed state and unable to participate in transcription. The egg cytoplasm, formed by the maternal genome, contains the necessary components that mediate the early restructuring of egg into zygote. These changes are mediated by a series of cytoplasmic signal transduction events initiated by the rise in [Ca2+]i caused when the sperm penetrates the egg. The structural changes that the egg undergoes are rapid and result in the extensive remodeling of this specialized cell. Protein kinase C (PKC) and calcium/calmodulin-dependent protein kinase II (CaM KII) are two pivotal signaling agents that mediate several of these rapid modifications in cell structure. Studies indicate the meiotic spindle serves as an architectural element in the egg that acts to colocalize elements from several of the key signaling pathways and may provide a means for these pathways to interact. In mammals, transcription begins earlier than in zygotes from other classes of organisms, starting several hours after fertilization in the male and female pronuclei and continuing in the embryonic nuclei. Studies indicate that nuclei undergo an initial state that is permissive for transcription, and then in Gap 2 of the two-cell embryo, enter a transcriptionally repressive state. These changes have been linked to the times during the cell cycle when the DNA is replicated, and also have been proposed as a requirement for proper initiation of the program of early development.