Physiological role of potassium channels in mammalian germ cell differentiation, maturation, and capacitation

Abstract

Background: Ion channels are essential for differentiation and maturation of germ cells, and even for fertilization in mammals. Different types of potassium channels have been identified, which are grouped into voltage-gated channels (Kv), ligand-gated channels (K_ligand), inwardly rectifying channels (K_ir), and tandem pore domain channels (K_2P).

Material-methods: The present review includes recent findings on the role of potassium channels in sperm physiology of mammals.

Results-discussion: While most studies conducted thus far have been focused on the physiological role of voltage- (Kv1, Kv3, and Kv7) and calcium-gated channels (SLO1 and SLO3) during sperm capacitation, especially in humans and rodents, little data about the types of potassium channels present in the plasma membrane of differentiating germ cells exist. In spite of this, recent evidence suggests that the content and regulation mechanisms of these channels vary throughout spermatogenesis. Potassium channels are also essential for the regulation of sperm cell volume during epididymal maturation and for preventing premature membrane hyperpolarization. It is important to highlight that the nature, biochemical properties, localization, and regulation mechanisms of potassium channels are species-specific. In effect, while SLO3 is the main potassium channel involved in the K⁺ current during sperm capacitation in rodents, different potassium channels are implicated in the K⁺ outflow and, thus, plasma membrane hyperpolarization during sperm capacitation in other mammalian species, such as humans and pigs.

Conclusions: Potassium conductance is essential for male fertility, not only during sperm capacitation but throughout the spermiogenesis and epididymal maturation.

Keywords: cell volume; epididymal maturation; plasma membrane potential; sperm capacitation; spermatogenesis.

FIGURE 1

Classification and general structure of potassium channels. (A) K_ir channels present a 2TM/P structure, which consists of two transmembrane α‐helices and a connecting P‐loop; this corresponds to the canonical structure of K⁺ channels. (B) K_2P channels present a 4TM/2P structure and contain two consecutive 2TM/P sequences. (C) Kv and K_ligand channels present a 6TM/P structure, with four transmembrane α‐helices followed by a connecting loop and a 2TM/P sequence. TM: transmembrane segment.

FIGURE 2

Localization of potassium channels in the spermatozoa of different mammalian species (acrosomal region, post‐acrosomal region, connecting piece, midpiece, annulus, principal piece, and terminal piece). When a channel has been described to be located in the sperm head without specifying the region, it has been considered to be present in both the acrosomal and post‐acrosomal regions. When a channel has been found to be located in the sperm tail without specifying the region, it has been considered to be present in the midpiece, annulus, principal piece, and terminal piece. Kv: voltage‐gated K⁺ channels; K_ir: inwardly rectifying K⁺ channels; K_Ca: calcium‐gated K⁺ channels; K_2P: tandem pore domain K⁺ channels.

FIGURE 3

Structural characteristics of voltage‐gated K⁺ channels (Kv). (A) Voltage‐gated K⁺ channels present a large N‐terminal domain, which includes the tetramerization domain (T1) and the inactivation ball (IB). The transmembrane segment TM4 presents a series of positively charged residues that form a voltage‐sensing domain (VSD). The loop between TM4 and TM5 is the inactivation‐ball‐binding loop. (B) Voltage‐gated K⁺ channels arrange in tetramers, and the regions corresponding to T1 and IB form a large intracellular domain. (C) Transmembrane domains from different monomers overlap. Darker residues highlight the TM4–5 connecting loop. Positively charged residues from the VSD in TM4 are highlighted (light blue). (D) The tetrameric structure forms a central pore that involves TM5–6 and the connecting loop between TM4 and TM5 (squares) of each monomer, whereas TM1–4 are not directly involved in the structure of the central pore. (E) P‐loops (darker residues) are oriented toward the central pore, and the VSD in TM4 (light blue) is found outside of the pore. (F) Each P‐loop presents a GYG sequence, which contributes to the selectivity filter (PDB reference 7EJ1). TM: transmembrane segment.

FIGURE 4

Structural characteristics of inwardly rectifying K⁺ channels (K_ir). (A) Inwardly rectifying K⁺ channels present a basic structure that consists of two transmembrane segments linked by a connecting P‐loop. (B) Inwardly rectifying K⁺ channels arrange in tetramers that present a large intracellular domain that controls the access to the transmembrane pore. Four cytoplasmic loops form a girdle around the central pore, the G‐loop, which contributes to the modulation of inward rectification. (C) Transmembrane domains from different monomers overlap, and at the cytosolic end of the transmembrane pore, the TM2 segments cross to form a bundle crossing, which also contributes to the modulation of channel gating. (D) The tetrameric structure forms a central pore that involves TM2 segments from the different monomers. (E) P‐loops (darker residues) are oriented toward the central pore. (F) Each P‐loop presents a GYG sequence, which contributes to the selectivity filter (PDB reference 7ZDZ). TM: transmembrane segment.

FIGURE 5

Structural characteristics of tandem pore domain K⁺ channels (K_2P). (A) Tandem pore domain K⁺ channels present two P‐loops. The linker between the TM1 and the first P‐loop is prominent, and forms the extracellular cap. (B and C) Tandem pore domain K⁺ channels arrange in dimers. Self‐interacting‐domain (SID) allows dimerization through the formation of disulfide bonds between cysteine residues (Cys69) that are present at the extracellular cap. Two different conformations of the TM segments have been identified: the classical configuration (B), and the domain‐swapped configuration (C). (D) The tetrameric structure forms a central pore that involves TM2 and TM4, whereas TM1 and TM3 are not directly involved in the structure of the central pore. In the classical configuration, TM segments from one monomer do not overlap with the other, and the outer helix from each TM1‐P‐loop 1 linker interacts with the inner helix from the same monomer. Meanwhile, in the domain‐swapped configuration, the TM1 segments present swapped positions, and the outer helix from each TM1‐P‐loop 1 linker interacts with the inner helix from the other monomer. P‐loops (darker residues) are oriented toward the central pore. The SID (blue residues) is present at the TM1‐P‐loop 1 linker, in the extracellular cap. (E) P‐loops (darker residues) are oriented toward the central pore. (F) Each P‐loop presents a GYG sequence that contributes to the selectivity filter (PDB references 3UM7 [classical configuration] and 4BW5 [swapped domain configuration]). TM: transmembrane segment.

FIGURE 6

Structural characteristics of ligand‐gated K⁺ channels (K_ligand). (A) Ligand‐gated K⁺ channels present an additional TM segment (TM0). The transmembrane segment TM4 presents a series of positively charged residues that form a voltage‐sensing domain (VSD). The loop between TM4 and TM5 is the inactivation‐ball‐binding loop. They also present a large C‐terminal domain that presents binding sites for their ligands. In this case, a calcium‐gated K⁺ channel (SLO1) is represented, which presents two regulators of K⁺ conductance (RCK) domains, each of them formed by two segments (S). (B) Ligand‐gated K⁺ channels arrange in tetramers. Positively charged residues from the VSD in TM4 are highlighted (light blue). Each RCK domain from calcium‐gated K⁺ channels present a binding site for calcium. (C) Transmembrane domains from different monomers overlap. Darker residues highlight the TM4–5 connecting loop. (D) The tetrameric structure forms a central pore that involves TM5–6 of each monomer, whereas TM1–4 are not directly involved in the structure of the central pore. (E) P‐loops (darker residues) are oriented toward the central pore, and the VSD in TM4 (light blue) is found outside of the pore. (F) Each P‐loop presents a GYG sequence, which contributes to the selectivity filter (PDB reference 6V38). TM: transmembrane segment.