Regulation mechanisms and implications of sperm membrane hyperpolarization

Abstract

Mammalian sperm are unable to fertilize the egg immediately after ejaculation. In order to gain fertilization competence, they need to undergo a series of biochemical and physiological modifications inside the female reproductive tract, known as capacitation. Capacitation correlates with two essential events for fertilization: hyperactivation, an asymmetric and vigorous flagellar motility, and the ability to undergo the acrosome reaction. At a molecular level, capacitation is associated to: phosphorylation cascades, modification of membrane lipids, alkalinization of the intracellular pH, increase in the intracellular Ca concentration and hyperpolarization of the sperm plasma membrane potential. Hyperpolarization is a crucial event in capacitation since it primes the sperm to undergo the exocytosis of the acrosome content, essential to achieve fertilization of the oocyte.

Membrane Potential Assessment by Fluorimetry as a Predictor Tool of Human Sperm Fertilizing Capacity

“…Alasmari et al (2013) showed that defects in Ca 2+ signaling lead to poor HA and that the ability to undergo Ca 2+ -induced HA affects sperm fertilizing capacity. This is in agreement with our results and with the hypothesis that the Em might play a role in regulating Ca 2+ channels and, consequently, in intracellular calcium and indirectly in sperm motility (reviewed in Ritagliati et al, 2018).…”

“…Sperm were then transferred to a gently stirred cuvette at 37 • C, and the fluorescence was monitored with a Varian Cary Eclipse fluorescence spectrophotometer at 620/670 nm excitation/emission wavelengths. Recordings were initiated when steady-state fluorescence was reached and calibration was performed at the end of each measure by adding 1 µM valinomycin and sequential additions of KCl for internal calibration curves, as previously described (Ritagliati et al, 2018;Baro Graf et al, 2019). Sperm Em was obtained from the initial fluorescence (measured as Arbitrary Fluorescence Units) by linearly interpolating it in the theoretical Em values from the calibration curve against arbitrary fluorescence units of each trace.…”

“…Two capacitation events are essential for successful sperm penetration into oocytes: a vigorous motility called hyperactivation (HA) and the ability to undergo the acrosome reaction (AR) in response to a physiological agonist (Yanagimachi, 1994a,b). In addition, a series of sequential and concomitant biochemical processes must occur during capacitation, including the hyperpolarization of the plasma membrane (Zeng et al, 1995;Arnoult et al, 1999;Muñoz-Garay et al, 2001;Demarco et al, 2003;Ritagliati et al, 2018). In sperm, as in most cells, the internal ion concentrations are markedly different from those in the extracellular medium.…”

“…Hyperpolarization of the plasma membrane during capacitation has been thoroughly studied in mouse sperm. Initially, mouse sperm are relatively depolarized (Em between −35 and −45 mV) and become hyperpolarized (Em around −70 mV) upon capacitation (Zeng et al, 1995;Arnoult et al, 1999;Stival et al, 2015;Ritagliati et al, 2018). This hyperpolarization has been shown to be both necessary and sufficient for sperm to undergo [Ca 2+ ] i increases and the AR in response to an agonist (De La Vega-Beltran et al, 2012).…”

Editorial: Sperm Differentiation and Spermatozoa Function: Mechanisms, Diagnostics, and Treatment

“…Membrane potential is an essential feature during capacitation for sperm fertilization capability (López-González et al, 2014;Ritagliati et al, 2018). Two research papers in this collection develop methods to determine membrane potential.…”