PDC-109 (BSP-A1/A2) promotes bull sperm binding to oviductal epithelium in vitro and may be involved in forming the oviductal sperm reservoir

Abstract

Sperm reservoirs have been found in the oviducts of several species of mammals. In cattle, the reservoir is formed by the binding of sperm to fucose-containing glycoconjugates on the surface of oviductal epithelial cells. A fucose-binding molecule was purified from sperm extracts and identified as PDC-109 (BSP-A1/A2), a protein that is secreted by the seminal vesicles and associates with the plasma membrane of sperm upon ejaculation. The objective of this study was to demonstrate that PDC-109 promotes bull sperm binding to oviductal epithelium. PDC-109 was purified from bovine seminal plasma, and polyclonal antibodies were produced in rabbits. The antibodies detected PDC-109 on ejaculated sperm by indirect immunofluorescence and Western blots of extracts, but PDC-109 was not detected on epididymal sperm. When added to epididymal sperm, purified PDC-109 was absorbed onto the plasma membrane overlying the acrosome, as demonstrated by indirect immunofluorescence and by labeling sperm directly with fluorescein-conjugated PDC-109. When added to explants of oviductal epithelium, significantly fewer epididymal sperm than ejaculated sperm became bound. Addition of PDC-109 to epididymal sperm increased epithelial binding to the level observed for ejaculated sperm. In addition, binding of ejaculated sperm to oviductal epithelium was inhibited by addition of excess soluble PDC-109. Ejaculated sperm lost the ability to bind to oviductal epithelium after heparin-induced capacitation, but treatment with PDC-109 restored binding. These results demonstrate that PDC-109 enables sperm to bind to oviductal epithelium and plays a major role in formation of the bovine oviductal sperm reservoir.