Identification of osteopontin in water buffalo semen

The competitiveness of buffalo breeding will depend on the utilization of reproductive biotechnologies that allows acceleration of genetic progress. A major factor hampering the efficiency of both artificial insemination and in vitro embryo production programs in this species is male hypofertility. Reports for several species suggest that seminal plasma contains factors that influence male fertility. Osteopontin is a glycoprotein found in several biological fluids including seminal plasma, and its presence is associated with spermatozoa concentration. In cattle, expression of osteopontin was highly correlated with bull fertility, and it was proposed to be a marker to predict male fertility (Cancel et al. 1999 Biol. Reprod. 60, 454–460). No data are available about the presence or activity of osteopontin in water buffalo. The aim of this preliminary study was to determine if osteopontin is present in buffalo semen and to evaluate whether freezing procedures cause the loss of osteopontin from spermatozoa. Semen was collected in authorized semen collection centers from 6 buffalo bulls by using an artificial vagina. A collection of bovine semen was used as a positive control. An aliquot from each sample was frozen using standard procedures for semen storage. Each ejaculate was centrifuged at 600g for 10 min at room temperature, and the supernatant was recovered and centrifuged at 10 000g for 1 h at 4°C. The total protein concentration in seminal plasma and spermatozoa was determined by the Bradford method, using ovoalbumin as the standard. Proteins (50 µg) were separated by electrophoresis and analyzed by western blotting (Cancel et al. 1999). Polyclonal antibodies against bovine milk osteopontin were prepared as previously described (Cancel et al. 1997 Biol. Reprod. 57, 1293–1301). The intensities of bands indicated by western blot were quantified by densitometer. Osteopontin was detected in all samples of buffalo semen. Most of the osteopontin detected was in the seminal plasma. Relative amounts of osteopontin detected in spermatozoa were 50% or less of that detected in seminal plasma; furthermore, the protein was not found in sperm from all bulls. These results suggest that most osteopontin is produced by the ampullae and seminal vesicles, similar to what was reported for cattle (Cancel et al. 1999). Semen frozen by standard procedures showed a reduction in amount of osteopontin by up to 50%. These studies suggest that the fertility-associated protein osteopontin may be useful as a sensitive tool to evaluate whether sperm storage procedures are detrimental and result in excess loss of osteopontin from sperm. In conclusion, the results have demonstrated that osteopontin is present in buffalo seminal plasma and sperm. Further studies will examine whether the expression of osteopontin is correlated with the fertility of buffalo bulls, as has been demonstrated in bovine bulls.