KSper, a pH-sensitive K ⁺ current that controls sperm membrane potential

Abstract

Mature mammalian spermatozoa are quiescent in the male reproductive tract. Upon ejaculation and during their transit through the female reproductive tract, they undergo changes that enable them to fertilize the egg. During this process of capacitation, they acquire progressive motility, develop hyperactivated motility, and are readied for the acrosome reaction. All of these processes are regulated by intracellular pH. In the female reproductive tract, the spermatozoan cytoplasm alkalinizes, which in turn activates a Ca 2؉ -selective current (ICatSper) required for hyperactivated motility. Here, we show that alkalinization also has a dramatic effect on membrane potential, producing a rapid hyperpolarization. This hyperpolarization is primarily mediated by a weakly outwardly rectifying K ؉ current (IKSper) originating from the principal piece of the sperm flagellum. Alkalinization activates the pH i-sensitive I KSper, setting the membrane potential to negative potentials where Ca 2؉ entry via ICatSper is maximized. IKSper is one of two dominant ion currents of capacitated sperm cells.capacitation ͉ fertilization ͉ ion channels ͉ mSlo3 ͉ reproduction B efore fertilization, mammalian sperm must undergo a poorly understood process called capacitation (1). Capacitation refers to a series of biochemical and physiological changes in the sperm cell that enable it to reach and fertilize the egg. Among these changes is an increase in the intracellular pH (pH i ) (2) and hyperpolarization of the sperm plasma membrane potential (V m ) (3, 4). Resting pH i of mammalian sperm is Ϸ6.5 as determined by pH-sensitive fluorescent probes (2, 5-7). After in vitro capacitation in bicarbonate-containing media at pH 7.4, pH i increases Ͼ0.3 units (2,8,9). When intracellular alkalinization is prevented by glucose incubation, bovine sperm fail to capacitate (8, 10).Measurements with voltage-sensitive fluorescent dyes indicate that noncapacitated murine sperm are relatively depolarized (approximately Ϫ30 mV) and hyperpolarize to approximately Ϫ60 mV during capacitation (4,11,12). Indirect measurements attribute the capacitation-associated hyperpolarization to an increase in K ϩ permeability (4) and a block of epithelial sodium channels (ENaCs) (13). Interestingly, addition of BaCl 2 (a nonspecific K ϩ channel blocker) to the capacitating medium prevented the hyperpolarization and the acrosome reaction (12). Several K ϩ channels have been reported in mammalian sperm (ref. 14; see also reviews in refs. 15 and 16), but only recently have patch-clamp methods been developed that allow whole-spermcell currents to be recorded under voltage clamp (17). To date, the inward Ca 2ϩ -selective current, I CatSper , but not outward K ϩ -selective current, has been described in detail (17, 18). By using whole-spermatozoan current clamp, we find that pH i largely determines mouse sperm membrane potential. Under whole-sperm and whole-flagellum voltage clamp, a pH i -sensitive K ϩ current (I KSper ) was identified that sets the epididymal spermatozoan membra...

Flagellar ion channels of sperm: similarities and differences between species

“…Noncapacitated epididymal murine spermatozoa are slightly depolarized at about −40 mV, however they hyperpolarize up to −60 mV upon capacitation [137]. This effect is attributed to potassium permeability and two members of the Slo family of potassium channels have been recently proposed to play a role in this process [21,25,26,29,31,33,[138][139][140][141]. Slo3 (Kcnu1), a pH-sensitive, calcium-independent and weakly voltage-sensitive channel, has been identified as the principal potassium channel in murine sperm [25,26,29,31,33,[141][142][143].…”

Deletion of theSlo3gene abolishes alkalization-activated K+current in mouse spermatozoa

“…An alkalization-activated K + conductance, termed KSper, appears to be the primary K + conductance in mouse spermatozoa (1). It has been proposed that hyperpolarization resulting from alkalizationinduced KSper activation will work in concert with the simultaneous activation of the pH-sensitive Ca 2+ -selective CatSper channel to promote Ca 2+ influx essential for the initiation of hyperactivation (1,2).…”

“…An alkalization-activated K + conductance, termed KSper, appears to be the primary K + conductance in mouse spermatozoa (1). It has been proposed that hyperpolarization resulting from alkalizationinduced KSper activation will work in concert with the simultaneous activation of the pH-sensitive Ca 2+ -selective CatSper channel to promote Ca 2+ influx essential for the initiation of hyperactivation (1,2). Thus, KSper activation is proposed to play a critical role in the complex set of physiological and biochemical changes, collectively termed "capacitation" (3)(4)(5), that are essential for the ability of sperm to produce fertilization.…”

Cl− Channels and Transporters in Sperm Physiology

“…CatSper null male mice are infertile mainly because of a failure to hyperactivate (Carlson et al 2005 ;Carlson et al 2003 ;Quill et al 2001 ;Ren et al 2001 ). It has been proposed that the hyperpolarization of the sperm plasma associated with capacitation increases the driving force for Ca 2+ , facilitating Ca 2+ infl ux through CatSper channels during cytosolic alkalinization (Navarro et al 2007 ).…”

“…Achieving whole-cell patch-clamp recordings became more feasible when Kirichok et al ( 2006 ) were able to seal the cytoplasmic droplet of mouse epididymal spermatozoa and then mature human spermatozoa (Kirichok and Lishko 2011 ). This novel strategy is allowing the characterization of spermspecifi c channels such as CatSper (Kirichok et al 2006 ) and SLO3 (Navarro et al 2007 ;Santi et al 2010 ;Schreiber et al 1998 ;Zeng et al 2011 ), and of sperm anion channels that are present in other cell types Ferrera et al 2010 ). Additionally, a voltage-sensitive H + channel involved in the intracellular pH (pH i ) regulation in human sperm and less importantly in mouse sperm (Kirichok and Lishko 2011 ), and ATP-gated channels of the purinergic family, P2X2, in mouse epididymal sperm have been recorded (Navarro et al 2011 ).…”