A Review on the Role of Bicarbonate and Proton Transporters during Sperm Capacitation in Mammals

Abstract

Alkalinization of sperm cytosol is essential for plasma membrane hyperpolarization, hyperactivation of motility, and acrosomal exocytosis during sperm capacitation in mammals. The plasma membrane of sperm cells contains different ion channels implicated in the increase of internal pH (pH_i) by favoring either bicarbonate entrance or proton efflux. Bicarbonate transporters belong to the solute carrier families 4 (SLC4) and 26 (SLC26) and are currently grouped into Na⁺/HCO₃^- transporters and Cl^-/HCO₃^- exchangers. Na⁺/HCO₃^- transporters are reported to be essential for the initial and fast entrance of HCO₃^- that triggers sperm capacitation, whereas Cl^-/HCO₃^- exchangers are responsible for the sustained HCO₃^- entrance which orchestrates the sequence of changes associated with sperm capacitation. Proton efflux is required for the fast alkalinization of capacitated sperm cells and the activation of pH-dependent proteins; according to the species, this transport can be mediated by Na⁺/H⁺ exchangers (NHE) belonging to the SLC9 family and/or voltage-gated proton channels (HVCN1). Herein, we discuss the involvement of each of these channels in sperm capacitation and the acrosome reaction.

Keywords: HVCN1 channels; NHE; SLC26 channels; SLC4 channels; mammals; sperm capacitation.

Figure 1

Structural characteristics of SLC4 channels. Bicarbonate transporters from the SLC4 family have slightly different functions, but the structure is generally conserved between them. The structure presented in this figure corresponds to SLC4A1 channel (PDB reference 4YZF). (A) SLC4 channels present 14 transmembrane α-helices (TM1–14) connected through loops that occasionally present amphipathic helices (H1–6). TM3 and TM10 are two half-helices, and their N-termini regions form the active site (red circles). In the TM8 segment, Glu681 (orange, E) is a blocker of the active site. (B) The folded protein conforms two different domains: the core domain, which is formed by helices TM1–4 and TM8–11; and the gate domain, which consists of TM5–7 and TM12–14. N-termini of TM3 and TM10 face each other to form the active site (red circle). (C) SLC4 channels form dimers that interact through a dimerization interface constituted by TM6 and TM7, which are part of the gate domain.

Figure 2

Structural characteristics of SLC26 channels. Bicarbonate exchangers from the SLC26 family present a highly similar structure to SLC4 channels. The structure presented in this figure corresponds to the SLC26A9 channel (PDB reference 7CH1). (A) SLC26 channels present 14 transmembrane α-helices (TM1–14) connected through loops that occasionally present amphipathic helices (H). TM3 and TM10 are two half-helices, and their N-termini regions form the active site (red circles). At the C-terminal region, a sulphate transporter anti-sigma factor antagonist domain (STAS), which is involved in SLC26 trafficking, and protein–protein interaction and regulation, which includes the interaction with the cystic fibrosis transmembrane conductance regulator channel (CFTR). (B) The folded protein conforms two different domains: the core domain, which is formed by helices TM1–4 and TM8–11; and the gate domain, which consists of TM5–7 and TM12–14. The N-termini of TM3 and TM10 face each other to form the active site (red circle). (C) SLC26 channels form dimers, with the STAS domain being excluded from the dimerization domain, since it must be free to interact with other proteins.

Figure 3

Structural characteristics of NHE channels. Sodium–proton exchangers from the SLC9 family of channels present slightly variable structures, with the sNHE channel presenting some characteristic features. (A) SLC9 channels generally present 12 transmembrane α-helices (TM1–12) connected through loops. An ion permeation pathway is formed between TM4 and TM5 (yellow). The C-terminal domain presents multiples sites of interaction with other proteins. (B) Quaternary conformation of SLC9 channels, which tend to form dimers, acquire a more stable conformation. The structure presented in this figure corresponds to the SLC9A1 channel (PDB reference 7DSW). (C) The sNHE channel structure is different from other NHE channels, mainly for TM11–14. This domain resembles the voltage-sensing domain of other channels. TM14 has a high number of basic residues, which corresponds to the voltage-sensing region (blue, +). The C-terminal region contains, among the sites of interaction with other molecules, a cyclic nucleotide-binding domain (CNBD).

Figure 4

Structural characteristics of the voltage-gated proton channel (HVCN1). (A) The proton channel HVCN1 presents four transmembrane α-helices (TM1–4) connected through three loops. In TM1, the residue Asp112 is highly conserved and is critical for proton selectivity (yellow, D). The TM4 segment presents multiple basic residues, which correspond to the voltage-sensing region (blue, +). Both the loop between TM1 and TM2, and the C-terminal coiled coil are essential for dimerization. (B) Each monomer folds to form a single ion permeation pathway that also contains the voltage-sensing domain (VSD) (PDB reference 5OQK, lacking the coiled-coil domain located at the C-terminal region). (C) The quaternary structure of HVCN1 consists of the formation of dimers, and the two coiled coils interact intracellularly (PDB reference 3VMX). (D) Dimers present two different permeation pathways, each corresponding to a different monomer.

Figure 5

Membrane channels in sperm capacitation and the acrosome reaction. When sperm enter the female tract, the plasma membrane is partially destabilized because of cholesterol depletion. Albumin does not only contribute to cholesterol depletion, but also activates HVCN1. This channel is also activated by the higher pH in the female tract and the lower concentration of Zn²⁺ compared to the male tract. One of the first events in the capacitation pathway is the activation of SLC4 (NBC) channels that contribute to the increase in bicarbonate concentration. This increase is also achieved through the cooperation of SLC26 channels with CFTR. The activation of CFTR inhibits ENaC channels, and these two events contribute to plasma membrane hyperpolarization. Intracellular pH alkalinization occurs in response to both HCO₃⁻ influx and H⁺ efflux through the plasma membrane, which activates both CatSper and SLO channels. Both high pH and increased concentration of Ca²⁺ activate the soluble adenylate cyclase (sAC), which increases cAMP levels and, in turn, PKA activity. The increase in tyrosine phosphorylation prepares the sperm for the AR, and hyperactivates sperm motility. In addition, downstream of the signaling pathways of tyrosine phosphorylation, SLC4A1 channels in the acrosome region are phosphorylated, which activates actin depolymerization in this region. The increase in progesterone levels that sperm encounter in the oocyte vicinity activates CatSper channels in the sperm head, which trigger the acrosome reaction.