Membrane hyperpolarization abolishes calcium oscillations that prevent induced acrosomal exocytosis in human sperm

Sperm capacitation is essential to gain fertilizing capacity. During this process, a series of biochemical and physiological modifications occur that allow sperm to undergo acrosomal exocytosis (AE). At the molecular level, hyperpolarization of the sperm membrane potential (Em) takes place during capacitation. This study shows that human sperm incubated under conditions that do not support capacitation (NC) can become ready for an agonist stimulated AE by pharmacologically inducing Em hyperpolarization with Valinomycin or Amiloride. To investigate how Em hyperpolarization promotes human sperm's ability to undergo AE, live single-cell imaging experiments were performed to simultaneously monitor changes in [Ca²⁺ ]_i and the occurrence of AE. Em hyperpolarization turned [Ca²⁺ ]_i dynamics in NC sperm from spontaneously oscillating into a sustained slow [Ca²⁺ ]_i increase. The addition of progesterone (P4) or K⁺ to Valinomycin-treated sperm promoted that a significant number of cells displayed a transitory rise in [Ca²⁺ ]_i which then underwent AE. Altogether, our results demonstrate that Em hyperpolarization is necessary and sufficient to prepare human sperm for the AE. Furthermore, this Em change decreased Ca²⁺ oscillations that block the occurrence of AE, providing strong experimental evidence of the molecular mechanism that drives the acquisition of acrosomal responsiveness.