Glycobiology of fertilization in the pig.

By adopting internal fertilization, the meeting of both gametes--the sperm and the egg--and thus the highly coordinated sequence of interactions leading to fertilization, occur in the female reproductive tract. In mammals, the oviduct has been shown to translate the requirements of the female, coordinating sperm activation (capacitation) and sperm transport with the arrival of the ovulated egg. A hierarchy of carbohydrate-based interactions accompanies these events ranging from the binding of uncapacitated sperm to the oviductal epithelium (establishment of the female sperm reservoir), to the primary and secondary binding processes contributing to gamete recognition and sperm penetration of the oocyte zona pellucida. The current perspective will focus on the carbohydrate-recognition systems in the binding events during fertilization in the pig. The roles of the major carbohydrate-binding proteins, the spermadhesins and the acrosomal serine proteinase, pro/acrosin are discussed under consideration of recent structural data. The glycans and the glycoproteins of the porcine oviduct with a focus on the candidate sperm receptors as well as the zona pellucida N-glycans of prepuberal pigs have been characterized by a mass spectrometric approach. Furthermore, some preliminary data supporting the hypothesis that the zona pellucida has to undergo a maturation process during oocyte development are presented.

Spermadhesin AQN1 is a candidate receptor molecule involved in the formation of the oviductal sperm reservoir in the pig.

Sperm are stored in the isthmic region of the oviduct under conditions that maintain viability and suppress early capacitation steps until ovulation occurs. The initial contact between sperm and oviductal epithelium is mediated by carbohydrate-protein interactions. In the pig, the carbohydrate recognition system has been shown to involve oligomannosyl structures. The spermadhesins AWN and AQN1 are the dominant porcine carbohydrate-binding sperm proteins. The objective of this study was to demonstrate that AQN1 contributes to sperm binding to the oviductal epithelium. AQN1 showed a broad carbohydrate-binding pattern as it recognizes both alpha- and beta-linked galactose as well as Manalpha1-3(Manalpha1-6)Man structures, whereas AWN bound only the galactose species. Binding of ejaculated sperm to oviductal epithelium was inhibited by addition of AQN1 but not by AWN. Mannose-binding sites were localized over the rostral region of the sperm head. Flow cytometry showed that, under capacitating conditions, the population of live sperm was shifted within 30 min toward an increase in the proportion of cells with low mannose- and high galactose-binding. The loss of mannose-binding sites was accompanied by the loss of AQN1 in sperm extracts and the significant reduction in the sperm-oviduct binding. The oviductal epithelium was shown by GNA-lectin histochemistry and by SDS-PAGE and lectin blotting of the apical membrane fraction to express mannose components that could be recognized by AQN1. These results demonstrate that the sperm lectin AQN1 fulfils the criteria for an oviduct receptor in the pig and may play a role in the formation of the oviductal sperm reservoir.