SLO3: A Conserved Regulator of Sperm Membrane Potential

Abstract

Sperm cells must undergo a complex maturation process after ejaculation to be able to fertilize an egg. One component of this maturation is hyperpolarization of the membrane potential to a more negative value. The ion channel responsible for this hyperpolarization, SLO3, was first cloned in 1998, and since then much progress has been made to determine how the channel is regulated and how its function intertwines with various signaling pathways involved in sperm maturation. Although Slo3 was originally thought to be present only in the sperm of mammals, recent evidence suggests that a primordial form of the gene is more widely expressed in some fish species. Slo3, like many reproductive genes, is rapidly evolving with low conservation between closely related species and different regulatory and pharmacological profiles. Despite these differences, SLO3 appears to have a conserved role in regulating sperm membrane potential and driving large changes in response to stimuli. The effect of this hyperpolarization of the membrane potential may vary among mammalian species just as the regulation of the channel does. Recent discoveries have elucidated the role of SLO3 in these processes in human sperm and provided tools to target the channel to affect human fertility.

Keywords: SLO channels; SLO3; acrosomal exocytosis; capacitation; contraception; hyperactivated motility; male fertility; membrane hyperpolarization; potassium channels; sperm.

Figure 2

Human SLO3 gating ring structure determined by X-ray crystallography. (a) Cartoon of domain topology of two opposing SLO3 α-subunits. (b) Crystal structure of the gating ring of a hSLO3 tetramer with RCK1 and RCK2 domains colored in blue and red, respectively. (c) A single subunit of the hSLO3 channel and (d) highlight of RCK1. (e) A closeup of the hSLO3 assembly interface and (f) the corresponding region of SLO1 bound to Ca²⁺. The RCK1 N-terminal residue that connects to the transmembrane pore is shown as a green sphere. Ca²⁺ ion is shown as a yellow sphere. Reprinted/adapted with permission from [69].

Figure 1

Amino acid sequence homology of mouse (mSLO3), human (hSLO3), and bovine (bSLO3) SLO3. Conserved regions are highlighted in blue. Dark highlighting indicates conservation between three species, light highlighting indicates conservation between two species. Sequence alignment performed using Jalview Version 2 [47,48,49,50].

Figure 3

Models of mouse and human SLO3 activity. (a) Mouse: The exposure to a more alkaline pH and high [HCO₃⁻] concentrations in the female tract contribute to an increase in pH_i, potentially through the activation of the sNHE. This rise in pH_i leads to the activation of SLO3 channels, resulting in membrane hyperpolarization. This hyperpolarization enhances calcium influx through CatSper channels, possibly through two distinct mechanisms: Firstly, by increasing the inward driving force of calcium. Secondly, it may further activate sNHE to elevate intracellular pH even more. (b) Human: In human sperm, exposure to an elevated external pH could potentially activate the Hv1 channel, resulting in an increase in pH_i and contributing to the activation of SLO3 and CatSper channels. However, it is important to note that in humans, SLO3 channels are primarily activated by calcium, while CatSper channels are activated by progesterone. On the other hand, activation of SLO3 leads to membrane hyperpolarization, which has been proposed to remove [Ca²⁺]_i oscillations that inhibit CatSper activation. This raises the question of whether SLO3 is activated upstream or downstream of CatSper channels.