Physiological changes that endow mammalian sperm with fertilizing capacity are known as sperm capacitation. As part of capacitation, sperm develop an asymmetrical flagellar beating known as hyperactivation and acquire the ability to undergo the acrosome reaction. Together, these processes promote fertilizing competence in sperm. At the molecular level, capacitation involves a series of signal transduction events which include activation of cAMP-dependent phosphorylation pathways, removal of cholesterol, hyperpolarization of the sperm plasma membrane, and changes in ion permeability. In recent years, new technologies have aided in the study of sperm signaling molecules with better resolution, at both spatial and temporal levels, unraveling how different cascades integrate and cooperate to render a fertilizing sperm. Despite this new information, the molecular mechanisms connecting capacitation with acrosomal exocytosis and hyperactivation are not well understood. This review brings together results obtained in mammalian species in the field of sperm capacitation with special focus on those pathways involved in the preparation to undergo the acrosomal reaction.

Background: Membrane potential (E m ) hyperpolarization during sperm capacitation is necessary for acrosome reaction. Results: cSrc is activated downstream of PKA, regulating the SLO3 K ϩ channel and promoting membrane hyperpolarization.

Protein kinase A (PKA) is a broad-spectrum Ser/Thr kinase involved in the regulation of several cellular activities. Thus, its precise activation relies on being localized at specific subcellular places known as discrete PKA signalosomes. A-Kinase anchoring proteins (AKAPs) form scaffolding assemblies that play a pivotal role in PKA regulation by restricting its activity to specific microdomains. Because one of the first signaling events observed during mammalian sperm capacitation is PKA activation, understanding how PKA activity is restricted in space and time is crucial to decipher the critical steps of sperm capacitation. Here, we demonstrate that the anchoring of PKA to AKAP is not only necessary but also actively regulated during sperm capacitation. However, we find that once capacitated, the release of PKA from AKAP promotes a sudden Ca influx through the sperm-specific Ca channel CatSper, starting a tail-to-head Ca propagation that triggers the acrosome reaction. Three-dimensional super-resolution imaging confirmed a redistribution of PKA within the flagellar structure throughout the capacitation process, which depends on anchoring to AKAP. These results represent a new signaling event that involves CatSper Ca channels in the acrosome reaction, sensitive to PKA stimulation upon release from AKAP.