Sodium and Epithelial Sodium Channels Participate in the Regulation of the Capacitation-associated Hyperpolarization in Mouse Sperm

Abstract

In a process called capacitation, mammalian sperm gain the ability to fertilize after residing in the female tract. During capacitation the mouse sperm plasma membrane potential (E m ) hyperpolarizes. However, the mechanisms that regulate sperm E m are not well understood. Here we show that sperm hyperpolarize when external Mammalian sperm are not able to fertilize after ejaculation. They acquire this ability only after residing in the female uterine tract for a finite period of time that varies depending on the species. The molecular, biochemical, and physiological changes that occur in sperm while in the female tract are collectively referred to as capacitation (1). Capacitation is associated with changes in membrane properties, enzyme activities, and motility that prepare the sperm for the acrosome reaction and for penetration of the egg vestments prior to fertilization. The molecular basis of capacitation has been partially defined and includes: the removal of cholesterol from the sperm plasma membrane by cholesterol acceptors such as bovine serum albumin (2, 3), modifications in plasma membrane phospholipids, fluxes of HCO 3 Ϫ (4) and other intracellular ions, and increased tyrosine phosphorylation of proteins (5-7). These events are likely to play a role in the induction of hyperactivated motility and the ability of the sperm to undergo a regulated acrosome reaction (for review see Ref. 8).Bovine and mouse sperm capacitation is also accompanied by a plasma membrane hyperpolarization. E m decreases in mouse sperm from Ϫ38 to Ϫ55 mV (4, 9, 10) and in bovine sperm from Ϫ33 to Ϫ66 mV (9). Because capacitation prepares sperm for the acrosome reaction, the capacitation-associated hyperpolarization may regulate the ability of sperm to generate transient Ca 2ϩ elevations during the acrosome reaction induced by physiological agonists (e.g. zona pellucida) (11). In this respect, low voltage-activated T-type Ca 2ϩ channels have been detected in mouse spermatogenic cells (12, 13), and these channels are also present in mature mouse sperm (14, 15). One unique property of low voltage-activated Ca 2ϩ channels is that they inactivate at the resting E m of sperm prior to capacitation (around Ϫ35 mV) (12,14). Thus, if low voltage-activated Ca 2ϩ channels are involved in the regulation of the acrosome reaction, the capacitation-associated sperm hyperpolarization may be required to remove this inactivation (11,16,17).Although the molecular mechanisms by which the sperm E m hyperpolarizes during capacitation are not clear, there exist several potential candidates. demonstrated with patch clamp techniques that inward rectifying K ϩ channels are expressed in mouse spermatogenic cells and proposed that these channels may contribute to the capacitation-associated sperm membrane hyperpolarization. An increase in sperm K ϩ permeability should lead to an E m hyperpolarization, according to the K ϩ equilibrium potential (18). Alternatively, the sperm plasma membrane may become less permeable to Na ϩ . The relatively depolarized mamma...

Compartmentalization of Distinct cAMP Signaling Pathways in Mammalian Sperm

“…In mammalian sperm, cAMP has been reported to be involved in the regulation of several capacitation-associated processes such as motility hyperactivation (5), hyperpolarization of the sperm plasma membrane (51,52), and the increase in protein tyrosine phosphorylation (45). Because of its relevance for cell processes, cAMP levels are tightly regulated both temporally and spatially by different types of adenylyl cyclases and cAMP phosphodiesterases (53)(54)(55).…”

Involvement of Cystic Fibrosis Transmembrane Conductance Regulator in Mouse Sperm Capacitation

“…In addition, we documented that an electrogenic Na ϩ /HCO 3 Ϫ cotransporter hyperpolarizes mouse sperm when external HCO 3 Ϫ is elevated (7). Furthermore, we recently reported the presence of epithelial Na ϩ channels (ENaCs) in mouse sperm and showed that a reduction in the sperm Na ϩ permeability is essential for the capacitation-associated hyperpolarization (8). This later work strongly suggests that mouse sperm (ENaCs) are constitutively active in noncapacitated sperm and close during capacitation, resulting in E m hyperpolarization.…”

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

“…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).…”