CaMKII prevents spontaneous acrosomal exocytosis in sperm through induction of actin polymerization

• Published in: Developmental biology 2016-07, Vol.415 (1), p.64-74
• Main Authors: Shabtay, Ortal, Breitbart, Haim
• Format: Article
• Language: English
• Subjects: Acrosome - enzymology; Acrosome reaction; Acrosome Reaction - physiology; Actin; Actin Cytoskeleton - ultrastructure; Actins - metabolism; Animals; Calcimycin - pharmacology; Calcium Signaling - drug effects; Calcium-Calmodulin-Dependent Protein Kinase Type 2; CaMKII; Cattle; Enzyme Activation - drug effects; Exocytosis - physiology; Gelsolin - metabolism; Gelsolin - pharmacology; Hydrogen Peroxide - pharmacology; Male; Oxazoles - pharmacology; Peptide Fragments - pharmacology; Phospholipase D - antagonists & inhibitors; Phospholipase D - physiology; PLD; Polymerization; ROS; Sperm Capacitation - physiology; Spermatozoa; Spermatozoa - drug effects; Spermatozoa - ultrastructure
• ISSN: 0012-1606; 1095-564X
• DOI: 10.1016/j.ydbio.2016.05.008

In order to interact with the egg and undergo acrosomal exocytosis or the acrosome reaction (AR), mammalian spermatozoa must undergo a series of biochemical changes in the female reproductive tract, collectively called capacitation. We showed that F-actin is formed during sperm capacitation and fast depolymerization occurs prior to the AR. We hypothesized that F-actin protects the sperm from undergoing spontaneous-AR (sAR) which decreases fertilization rate. We show that activation of the actin-severing protein gelsolin induces a significant increase in sAR. Moreover, inhibition of CaMKII or PLD during sperm capacitation, caused an increase in sAR and inhibition of F-actin formation. Spermine, which leads to PLD activation, was able to reverse the effects of CaMKII inhibition on sAR-increase and F-actin-decrease. Furthermore, the increase in sAR and the decrease in F-actin caused by the inactivation of the PLD-pathway, were reversed by activation of CaMKII using H2O2 or by inhibiting protein phosphatase 1 which enhance the phosphorylation and oxidation states of CaMKII. These results indicate that two distinct pathways lead to F-actin formation in the sperm capacitation process which prevents the occurrence of sAR. A model describing the role of CaMKII and PLD pathways in actin polymerization during sperm capacitation. During the capacitation process, there is an increase in [Ca2+]i leading to CaMKII activation by the Ca2+/CaM complex. CaMKII activation is also induced by H2O2 produced during capacitation, which oxidizes CaMKII and also activates adenylyl-cyclase (Rivlin et al., 2004) to form cAMP, which activates PKA. CaMKII is autophosphorylated on threonine residues, which can inhibit or enhance its activation. The phosphorylation of CaMKII is regulated by Ca2+/CaM and PP1. PP1 activity is regulated by Src, which inhibits its activity, and Src is activated -in turn- by PKA (Rotfeld et al., 2014). PLD is activated by PKC or PKA (Cohen et al., 2004) and PIP2 is a cofactor for its activation. The elevation of PIP2 levels and gelsolin phosphorylation by Src maintain gelsolin in an inactive state (Finkelstein et al., 2010), allowing increased F-actin levels and CaMKII activation during sperm capacitation. CaMKII and PLD pathways maintain actin in a polymerized state and protect the sperm from undergoing sAR. Notes: steps/interactions that were identified for the first time in this study, are marked in pale blue. This scheme includes the effects of all the in