Modulation Of Mammalian Sperm Motility Is Crucial To Fertilization

Hyperactivation, a motility pattern of mammalian sperm in the oviduct, is essential to fertilization. Hyperactivation helps sperm to swim effectively through oviductal mucus, to escape from the sperm reservoir, and to penetrate the cumulus matrix and zona pellucida of the oocyte. Hyperactivation is characterized by asymmetrical flagellar beating and an increase of cytoplasmic Ca2+. We observed that when mouse sperm hyperactivate during capacitation in vitro, the amplitude of the flagellar bend that forms in the same orientation as the hook of the head is greatly increased. We defined this as pro-hook beating. We established that pro-hook beating is stimulated by an influx of Ca2+ through CATSPER channels and is associated with an increase of intracellular pH. However, an increase of flagellar bend amplitude in the opposite direction can be induced by treating sperm with thimerosal to stimulate release of Ca2+ from internal stores. We defined this as anti-hook beating. We also demonstrated that anti-hook beating is dominant over pro-hook beating; that is, when sperm are simultaneously or sequentially subjected to treatments that stimulate prohook and anti-hook beating, then the sperm will swim using the anti-hook beating pattern. Different protein phosphorylation patterns were detected in extracts of sperm that produced these two different flagellar beat patterns, indicating the involvement of different signaling pathways. In order to better understand how sperm movement is regulated in the oviduct, we mated wild-type female mice with Acr-EGFP males, whose sperm with fluorescent acrosomes enabled us to locate sperm moving within the oviduct easily. Oviducts were removed shortly before or after ovulation to record sperm movement within. Hyperactivated sperm in the isthmic reservoir detached frequently from the epithelium and then reattached. However, most sperm found in the ampulla remained bound to epithelium throughout the observation period of several minutes. In both regions, most sperm produced anti-hook bends. Sperm were observed to detach from epithelium in both the ampulla and isthmus during strong contractions of the wall of the oviduct. These observations indicate that sperm continue to bind to oviductal epithelium after they leave the isthmic reservoir and may be assisted in detaching by muscular contractions; furthermore, the intracellar Ca2+ store in sperm is likely to be involved in modulating sperm movement. Together with the in vitro experiments, we conclude that sperm motility is modulated to guide the ascent of sperm up the oviduct and toward the oocytes, with different Ca2+ signaling pathways underlying this process