Human Reproduction Update Advance Access published online on July 24, 2008
Human Reproduction Update, doi:10.1093/humupd/dmn029
Control of hyperactivation in sperm
Department of Biomedical Sciences, T5-002B Veterinary Research Tower, Cornell University Ithaca, NY 14853, USA
To whom correspondence should be addressed at: 1 Correspondence address. Tel: +1-607-253-3589; Fax: +1-607-253-3541; E-mail: sss7{at}cornell.edu
BACKGROUND: Sperm hyperactivation is critical to fertilization, because it is required for penetration of the zona pellucida. Hyperactivation may also facilitate release of sperm from the oviductal storage reservoir and may propel sperm through mucus in the oviductal lumen and the matrix of the cumulus oophorus. Hyperactivation is characterized by high amplitude, asymmetrical flagellar bending.
METHODS: This is a review of the original literature on the mechanisms that regulate hyperactivation, including physiological factors and signaling pathways.
RESULTS: Computer-assisted semen analysis systems can be used to identify hyperactivated sperm by setting minimum thresholds for curvilinear velocity (VSL) and lateral head movement and a maximum threshold for path linearity. Hyperactivation is triggered by a rise in flagellar Ca2+ resulting from influx primarily through plasma membrane CatSper channels and possibly also by release of Ca2+ from a store in the redundant nuclear envelope. It requires increased pH and ATP production. The physiological signals that trigger the rise in Ca2+ remain elusive, but there is evidence that the increased Ca2+ acts through a calmodulin/calmodulin kinase pathway. Hyperactivation is considered part of the capacitation process; however, the regulatory pathway that triggers hyperactivation can operate independently from that which prepares sperm to undergo the acrosome reaction. Hyperactivation may be modulated by chemotactic signals to turn sperm toward the oocyte.
CONCLUSIONS: Little is known about exactly what triggers hyperactivation in human sperm. This information could enable clinicians to develop reliable fertility assays to assess normal hyperactivation in human sperm samples.
Key words: sperm hyperactivation / intracellular calcium / CatSper / signaling pathways / human sperm
Received on March 31, 2008; revised May 27, 2008; accepted on June 23, 2008