Disruption of protein kinase A localization induces acrosomal exocytosis in capacitated mouse sperm.

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 Ca2+ influx through the sperm-specific Ca2+ channel CatSper, starting a tail-to-head Ca2+ 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 Ca2+ channels in the acrosome reaction, sensitive to PKA stimulation upon release from AKAP.

Sp-tetraKCNG: A novel cyclic nucleotide gated K(+) channel.

The sequence of a novel cGMP-regulated, tetrameric, K(+) selective channel (Sp-tetraKCNG) was discovered in the sea urchin Strongylocentrotus purpuratus. The Sp-tetraKCNG is a single polypeptide made of four KCNG domains similar to voltage-dependent Na(+) and Ca(2+) channels. Each KCNG domain has six transmembrane segments (S1-S6), the ion pore having the K(+) selectivity signature GYGD and a cyclic nucleotide-binding domain (CNBD). This novel channel is evolutionary located between K(+)-selective and voltage-dependent EAG channels and voltage-independent cationic CNG channels. Bilayer reconstitutions demonstrate such a cGMP-regulated K(+) selective channel in sea urchin spermatozoa.

KATP channels in mouse spermatogenic cells and sperm, and their role in capacitation.

Mammalian sperm must undergo a series of physiological changes after leaving the testis to become competent for fertilization. These changes, collectively known as capacitation, occur in the female reproductive tract where the sperm plasma membrane is modified in terms of its components and ionic permeability. Among other events, mouse sperm capacitation leads to an increase in the intracellular Ca(2+) and pH as well as to a hyperpolarization of the membrane potential. It is well known that ion channels play a crucial role in these events, though the molecular identity of the particular channels involved in capacitation is poorly defined. In the present work, we report the identification and potential functional role of K(ATP) channels in mouse spermatogenic cells and sperm. By using whole-cell patch clamp recordings in mouse spermatogenic cells, we found K(+) inwardly rectifying (K(ir)) currents that are sensitive to Ba(2+), glucose and the sulfonylureas (tolbutamide and glibenclamide) that block K(ATP) channels. The presence of these channels was confirmed using inhibitors of the ATP synthesis and K(ATP) channel activators. Furthermore, RT-PCR assays allowed us to detect transcripts for the K(ATP) subunits SUR1, SUR2, K(ir)6.1 and K(ir)6.2 in total RNA from elongated spermatids. In addition, immunoconfocal microscopy revealed the presence of these K(ATP) subunits in mouse spermatogenic cells and sperm. Notably, incubation of sperm with tolbutamide during capacitation abolished hyperpolarization and significantly decreased the percentage of AR in a dose-dependent fashion. Together, our results provide evidence for the presence of K(ATP) channels in mouse spermatogenic cells and sperm and disclose the contribution of these channels to the capacitation-associated hyperpolarization.