PKA-dependent phosphorylation of LIMK1 and Cofilin is essential for mouse sperm acrosomal exocytosis

Mammalian sperm must acquire their fertilizing ability after a series of biochemical modifications in the female reproductive tract collectively called capacitation to undergo acrosomal exocytosis, a process that is essential for fertilization. Actin dynamics play a central role in controlling the process of exocytosis in somatic cells as well as in sperm from several mammalian species. In somatic cells, small GTPases of the Rho family are widely known as master regulators of actin dynamics. However, the role of these proteins in sperm has not been studied in detail. In the present work we characterized the participation of small GTPases of the Rho family in the signaling pathway that leads to actin polymerization during mouse sperm capacitation. We observed that most of the proteins of this signaling cascade and their effector proteins are expressed in mouse sperm. The activation of the signaling pathways of cAMP/PKA, RhoA/C and Rac1 is essential for LIMK1 activation by phosphorylation on Threonine 508. Serine 3 of Cofilin is phosphorylated by LIMK1 during capacitation in a transiently manner. Inhibition of LIMK1 by specific inhibitors (BMS-3) resulted in lower levels of actin polymerization during capacitation and a dramatic decrease in the percentage of sperm that undergo acrosomal exocytosis. Thus, we demonstrated for the first time that the master regulators of actin dynamics in somatic cells are present and active in mouse sperm. Combining the results of our present study with other results from the literature, we have proposed a working model regarding how LIMK1 and Cofilin control acrosomal exocytosis in mouse sperm.

Src Kinase Is the Connecting Player between Protein Kinase A (PKA) Activation and Hyperpolarization through SLO3 Potassium Channel Regulation in Mouse Sperm

Plasma membrane hyperpolarization is crucial for mammalian sperm to acquire acrosomal responsiveness during capacitation. Among the signaling events leading to mammalian sperm capacitation, the immediate activation of protein kinase A plays a pivotal role, promoting the subsequent stimulation of protein tyrosine phosphorylation that associates with fertilizing capacity. We have shown previously that mice deficient in the tyrosine kinase cSrc are infertile and exhibit improper cauda epididymis development. It is therefore not clear whether lack of sperm functionality is due to problems in epididymal maturation or to the absence of cSrc in sperm. To further address this problem, we investigated the kinetics of cSrc activation using anti-Tyr(P)-416-cSrc antibodies that only recognize active cSrc. Our results provide evidence that cSrc is activated downstream of PKA and that inhibition of its activity blocks the capacitation-induced hyperpolarization of the sperm plasma membrane without blocking the increase in tyrosine phosphorylation that accompanies capacitation. In addition, we show that cSrc inhibition also blocks the agonist-induced acrosome reaction and that this inhibition is overcome by pharmacological hyperpolarization. Considering that capacitation-induced hyperpolarization is mediated by SLO3, we evaluated the action of cSrc inhibitors on the heterologously expressed SLO3 channel. Our results indicate that, similar to SLO1 K(+) channels, cSrc blockers significantly decreased SLO3-mediated currents. Together, these results are consistent with findings showing that hyperpolarization of the sperm plasma membrane is necessary and sufficient to prepare the sperm for the acrosome reaction and suggest that changes in sperm membrane potential are mediated by cSrc activation.

Redistribution of the intra-acrosomal EGFP before acrosomal exocytosis in mouse spermatozoa

Mammalian spermatozoa must undergo complex physiological and morphological alterations within the female reproductive tract before they become fertilization competent. Two important alterations are capacitation and the acrosome reaction (AR), by which spermatozoa become capable of penetrating the zona pellucida (ZP) of the oocyte. Although various biochemical stimulants have been reported to induce the AR, the true physiological inducer in vivo remains to be identified. Previously, it has been reported that most fertilizing spermatozoa undergo the AR before contacting the ZP and that only a small fraction of in vitro-capacitated spermatozoa can penetrate the ZP. Therefore, it is important to identify which capacitating spermatozoa undergo the AR in response to potential AR inducers such as progesterone. Here we show that spermatozoa undergo a dynamic rearrangement of the acrosome during in vitro capacitation. This involves the rapid movement of an artificially introduced soluble component of the acrosome, enhanced green fluorescent protein (EGFP), from the acrosomal cap region to the equatorial segment (EQ) of the sperm head. Spermatozoa exhibiting the EQ pattern were more sensitive to progesterone than were those without it. We suggest that spermatozoa that are ready to undergo acrosomal exocytosis can be detected by real-time EGFP imaging. This offers a promising new method for identifying where spermatozoa undergo the AR in the female reproductive tract in vivo.

Heterogeneous distribution of histone methylation in mature human sperm

PURPOSE

To analyze the presence of various histone modifications in ejaculated human spermatozoa

METHODS

In this prospective study, seminal ejaculates from 39 normozoospermic individuals were evaluated for semen analysis and the presence of histone modifications in isolated nuclei.

RESULTS

We observed heterogeneous presence of histone methylation in normal mature human sperm. We observed that 12 to 30 % of the nuclei of normal sperm contain a heterogeneous distribution of the marks H3K4Me1, H3K9Me2, H3K4Me3, H3K79Me2, and H3K36Me3. Moreover, the presence of these marks is higher in the poor motile fraction of the ejaculate, which is associated with poor morphology and functional quality. In contrast, we did not observe histone acetylation (H3K4Ac and H4K5Ac) in normal or abnormal mature human sperm

CONCLUSIONS

Defects in the process of spermatogenesis may alter the correct epigenetic programming in mature sperm. Further studies are required to evaluate the impact of these findings in human infertility.