Abstract
Upon ejaculation, mammalian spermatozoa have to undergo a sequence of physiological transformations within the female reproductive tract that will allow them to reach and fertilize the egg. These include initiation of motility, hyperactivation of motility and perhaps chemotaxis toward the egg, and culminate in the acrosome reaction that permits sperm to penetrate the protective vestments of the egg. These physiological responses are triggered through the activation of sperm ion channels that cause elevations of sperm intracellular pH and Ca(2+) in response to certain cues within the female reproductive tract. Despite their key role in sperm physiology and their absolute requirement for the process of fertilization, sperm ion channels remain poorly understood due to the extreme difficulty in application of the patch-clamp technique to spermatozoa. This review covers the topic of sperm ion channels in the following order: first, we discuss how the intracellular Ca(2+) and pH signaling mediated by sperm ion channels controls sperm behavior during the process of fertilization. Then, we briefly cover the history of the methodology to study sperm ion channels, which culminated in the recent development of a reproducible whole-cell patch-clamp technique for mouse and human cells. We further discuss the main approaches used to patch-clamp mature mouse and human spermatozoa. Finally, we focus on the newly discovered sperm ion channels CatSper, KSper (Slo3) and HSper (H(v)1), identified by the sperm patch-clamp technique. We conclude that the patch-clamp technique has markedly improved and shifted our understanding of the sperm ion channels, in addition to revealing significant species-specific differences in these channels. This method is critical for identification of the molecular mechanisms that control sperm behavior within the female reproductive tract and make fertilization possible.
Highlights
Because the application of the patch-clamp technique has demonstrated that properties of sperm ion channels differ significantly between species, in this review, we focused on the physiology of human spermatozoa
Under the recording conditions used to identify the mouse CatSper current and where it can be recorded in isolation from other sperm ion channels, we identified a highly Ca2+-selective flagellar channel in human sperm (Lishko et al, 2010, 2011)
Characterization of sperm ion channels is not complete, it is very unlikely that the 40 or so different ion channel proteins supposedly expressed in spermatozoa are important for sperm physiology in any one particular species (Darszon et al, 2006a)
Summary
Intracellular Ca2+ and intracellular H+ are two key messengers with opposite effects on sperm activity and fertilizing ability: increasing intracellular [Ca2+] stimulates sperm motility and fertility (Darszon et al, 2005; Publicover et al, 2007; Fraser, 2010), whereas increasing intracellular [H+] inhibits them (Babcock et al, 1983; Carr and Acott, 1989; Florman et al, 1989, 1992; Hamamah and Gatti, 1998; Suarez, 2008). Because the application of the patch-clamp technique has demonstrated that properties of sperm ion channels differ significantly between species, in this review, we focused on the physiology of human spermatozoa.
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