Carbohydrate-based interactions on the route of a spermatozoon to fertilization

E European Society of Human Reproduction and Embryology Human Reproduction Update 1999, Vol. 5, No.4 pp. 314–329 Carbohydrate-based interactions on the route of a spermatozoon to fertilization Edda Töpfer-Petersen1 Institute of Reproductive Medicine, Veterinary School of Hanover, Germany Male and female intercommunication along the route which the spermatozoon takes to fertilization utilizes the information potential of carbohydrates. A hierarchy of carbohydrate-based binding events exists ranging from spermatozoa–oviduct interaction to primary and secondary binding between spermatozoon and oocyte. Before in-vivo fertilization can occur, spermatozoa are stored in the caudal part of the isthmus, in tight contact with the epithelium cells lining the oviduct. The sperm reservoir seems to be created by surface-associated sperm lectins recognizing epithelial glycoconjugates. With the changing conditions in the oviduct at the time of ovulation, spermatozoa may shed those sperm lectins, creating new surfaces which allow spermatozoa to be released from the epithelium, complete capacitation and interact with the oocyte in the appropriate manner. The first contact between both gametes occurs at the spermatozoa–zona pellucida interface. The ‘primary’ binding initiates the acrosomal exocytosis of the spermatozoa, followed by the ‘secondary’ binding of the acrosome-reacted spermatozoon that in consequence leads to sperm penetration through the zona pellucida. Primary and secondary binding events are directed by the cooperative interactions of multiple carbohydrate-recognition systems that may act in a hierarchical and redundant manner. The current perspective will focus on the role of carbohydrate-binding sperm proteins in the sequence of binding events during fertilization in the pig. Keywords: gamete recognition/oviduct/spermatozoa/zona pellucida/zona pellucida-binding proteins TABLE OF CONTENTS Introduction Formation of the oviductal sperm reservoir is a carbohydrate-mediated event Carbohydrates are the signals for gamete recognition Zona pellucida-binding proteins Uncapacitated and capacitated porcine spermatozoa bind to the zona pellucida in vitro Is carbohydrate-mediated gamete recognition really species-specific? Conclusions Acknowledgements References 314 315 317 321 324 325 326 326 327 Introduction Fertilization is a fundamental event which involves a highly coordinated sequence of cellular interactions between the male and female gamete, that is, between the sperm cell and the egg, in order to form a diploid zygote and, ultimately, the new individual. In mammals, fertilization occurs in the female reproductive tract. At ejaculation, millions of spermatozoa are deposited in the female reproductive tract, though only a few thousand enter the oviduct, a few reach the ampulla at the time of fertilization, and only one spermatozoon fertilizes the egg. To guarantee the meeting of the two highly specialized gametes at the right time, and in the right place, the oviduct and the egg itself coordinate sperm functions. On reaching the oviduct, spermatozoa are held back in the reservoir of the lower isthmus due to binding of the spermatozoa to the epithelium (reviewed by Hunter, 1988, 1996; Smith, 1998; Suarez, 1998) (Figure 1). Sperm interactions with the oviductal epithelium appear to increase the viability of the spermatozoa during storage, and suppress sperm motility (Smith, 1998; Suarez, 1998). Before fertilization can occur, however, spermatozoa must enter a functionally activated or capacitated state and develop a hyperactivated motility which enables them to respond to the egg in the appropriate manner (Bedford, 1983). The capacitation process appears to be coordinated temporally by the oviductal epithelium in a still unknown fashion. Close to the time when the egg is ovulated into the ampulla, spermatozoa start or continue the capacitation process and are released from 1Address for correspondence: Institute of Reproductive Medicine, Veterinary School, Hanover, Bünteweg 15, D-30559 Hanover, Germany. Tel: 0511 9538520; Fax: 0511 9538504; e-mail: Spermatozoan–egg interactions in fertilization the oviductal epithelium, whereby the newly developed hyperactivated motility may help the detachment of spermatozoa and facilitate their swimming to the site of fertilization (Hunter, 1996; Suarez, 1996, 1998; Smith, 1998 and references therein). On approaching the oocyte, the spermatozoon must first be recognized by the oocyte. This interaction occurs when a spermatozoon first makes contact with the zona pellucida (ZP), the extracellular coat enveloping the oocyte. The ZP not only mediates the recognition between both gametes, but also regulates sperm functions, enabling the spermatozoon to complete fertilization. Capacitation is a prerequisite for the subsequent activation of the sperm transmembrane signalling system(s) by structures of the ZP, leading to the exocytosis of the sperm acrosome, referred to as the acrosome reaction. Thereby, the enzymatic equipment of the acrosome is activated, and is made available to aid sperm passage through the ZP, finally allowing fusion with the egg vitelline membrane. After fusion and oocyte activation have been completed by the spermatozoon, the sperm nucleus decondenses and delivers the male genome into the egg cytoplasm, thus marking the start of the programme for embryonic development. As one consequence of oocyte activation, the ZP is altered by components released from the oocyte cortical granules, contributing to the establishment of the egg-induced block to polyspermy (reviewed by Yanagimachi, 1994; Storey, 1995). Whereas the capacitation process is modulated by the oviduct, the ensuing physiologically significant acrosome reaction is coordinated by the ZP. It has long been accepted that recognition and initial binding between spermatozoa and egg involves the binding of multiple carbohydrate receptors of the sperm cell to the complementary oligosaccharide chains attached to the ZP proteins. Recently, sperm binding to the oviduct has also been shown to involve carbohydrate–protein interactions (Suarez, 1998). Thus, two important regulatory steps on the journey of the spermatozoon to union with the egg may be initiated by carbohydrates (Figure 1). In recent years, current knowledge of the mechanisms of spermatozoa–oocyte interaction has been excellently reviewed, covering different aspects of the role of carbohydrates in fertilization (for example Wassarman and Litscher, 1995; Chapman and Barratt, 1996; Clark et al., 1996; Snell and White, 1996; Benoff, 1997; Sinowatz et al., 1997; Tulsiani et al., 1997; McLesky et al., 1998). The current perspective will therefore focus on the role of ZP-binding proteins in fertilization under in-vitro conditions in the pig, and the assumed fate during in-vivo transit to the site of fertilization. Formation of the oviductal sperm reservoir is a carbohydrate-mediated event In mammals, millions of (...truncated)