It is predicted that Japan and European Union will soon experience appreciable decreases in their populations due to persistently low total fertility rates (TFR) below replacement level (2.1 child ...

https://www.physiology.org/doi/pdf/10.1152/physrev.00017.2015

Male Reproductive Disorders and Fertility Trends: Influences of Environment and Genetic Susceptibility

Steroid hormone progesterone released by cumulus cells surrounding the egg is a potent stimulator of human spermatozoa. It attracts spermatozoa towards the egg and helps them penetrate the egg's protective vestments. Progesterone induces Ca(2+) influx into spermatozoa and triggers multiple Ca(2+)-dependent physiological responses essential for successful fertilization, such as sperm hyperactivation, acrosome reaction and chemotaxis towards the egg. As an ovarian hormone, progesterone acts by regulating gene expression through a well-characterized progesterone nuclear receptor. However, the effect of progesterone upon transcriptionally silent spermatozoa remains unexplained and is believed to be mediated by a specialized, non-genomic membrane progesterone receptor. The identity of this non-genomic progesterone receptor and the mechanism by which it causes Ca(2+) entry remain fundamental unresolved questions in human reproduction. Here we elucidate the mechanism of the non-genomic action of progesterone on human spermatozoa by identifying the Ca(2+) channel activated by progesterone. By applying the patch-clamp technique to mature human spermatozoa, we found that nanomolar concentrations of progesterone dramatically potentiate CatSper, a pH-dependent Ca(2+) channel of the sperm flagellum. We demonstrate that human CatSper is synergistically activated by elevation of intracellular pH and extracellular progesterone. Interestingly, human CatSper can be further potentiated by prostaglandins, but apparently through a binding site other than that of progesterone. Because our experimental conditions did not support second messenger signalling, CatSper or a directly associated protein serves as the elusive non-genomic progesterone receptor of sperm. Given that the CatSper-associated progesterone receptor is sperm specific and structurally different from the genomic progesterone receptor, it represents a promising target for the development of a new class of non-hormonal contraceptives.

Progesterone activates the principal Ca2+ channel of human sperm.

A proper dialogue between spermatozoa and the egg is essential for conception of a new individual in sexually reproducing animals. Ca(2+) is crucial in orchestrating this unique event leading to a new life. No wonder that nature has devised different Ca(2+)-permeable channels and located them at distinct sites in spermatozoa so that they can help fertilize the egg. New tools to study sperm ionic currents, and image intracellular Ca(2+) with better spatial and temporal resolution even in swimming spermatozoa, are revealing how sperm ion channels participate in fertilization. This review critically examines the involvement of Ca(2+) channels in multiple signaling processes needed for spermatozoa to mature, travel towards the egg, and fertilize it. Remarkably, these tiny specialized cells can express exclusive channels like CatSper for Ca(2+) and SLO3 for K(+), which are attractive targets for contraception and for the discovery of novel signaling complexes. Learning more about fertilization is a matter of capital importance; societies face growing pressure to counteract rising male infertility rates, provide safe male gamete-based contraceptives, and preserve biodiversity through improved captive breeding and assisted conception initiatives.

https://www.physiology.org/doi/pdf/10.1152/physrev.00028.2010

Calcium Channels in the Development, Maturation, and Function of Spermatozoa

Ion channels control the sperm ability to fertilize the egg by regulating sperm maturation in the female reproductive tract and by triggering key sperm physiological responses required for successful fertilization such as hyperactivated motility, chemotaxis, and the acrosome reaction. CatSper, a pH-regulated, calcium-selective ion channel, and KSper (Slo3) are core regulators of sperm tail calcium entry and sperm hyperactivated motility. Many other channels had been proposed as regulating sperm activity without direct measurements. With the development of the sperm patch-clamp technique, CatSper and KSper have been confirmed as the primary spermatozoan ion channels. In addition, the voltage-gated proton channel Hv1 has been identified in human sperm tail, and the P2X2 ion channel has been identified in the midpiece of mouse sperm. Mutations and deletions in sperm-specific ion channels affect male fertility in both mice and humans without affecting other physiological functions. The uniqueness of sperm ion channels makes them ideal pharmaceutical targets for contraception. In this review we discuss how ion channels regulate sperm physiology.

/pdf/the-control-of-male-fertility-by-spermatozoan-ion-channels-32sov64se8.pdf

The control of male fertility by spermatozoan ion channels.

Class II PI3Ks (EC 2.7.1.154) phosphorylate phosphatidylinositol to generate phosphatidylinositol 3-phosphate (and possibly phosphatidylinositol 4-phosphate to generate phosphatidylinositol 3,4-bisphosphate). Three monomeric members exist, PI3K-C2α, β and γ, and include Ras-binding, Phox homology and two C2domains.

/pdf/the-concise-guide-to-pharmacology-2013-14-ion-channels-519s8y4sbh.pdf

The concise guide to PHARMACOLOGY 2013/14: Ion channels

The female steroid hormone progesterone is produced by the ovaries and the placenta, and supports gestation and embryogenesis through its actions on a well-characterized nuclear progesterone receptor. But progesterone released by cells surrounding the egg also stimulates sperm cells within the Fallopian tubes and increases their fertilizing ability, and the mechanism of this action of progesterone has remained elusive. Two independent research groups now report that progesterone potently activates CatSper, the principal Ca2+ channel of the sperm flagellum. Their data demonstrate that the CatSper channel or a directly associated membrane protein serves as a novel progesterone receptor that can mediate a fast, non-genomic effect of progesterone at the level of the sperm plasma membrane. These results should help to define the physiological role of progesterone and CatSper in sperm, and could lead to the development of new classes of non-hormonal contraceptives. Progesterone stimulates an increase in Ca2+ levels in human sperm, but the underlying signalling mechanism is poorly understood. Two studies now show that progesterone activates the sperm-specific, pH-sensitive CatSper calcium channel, leading to a rapid influx of Ca2+ ions into the spermatozoa. These results should help to define the physiological role of progesterone and CatSper in sperm, and could lead to the development of new classes of non-hormonal contraceptives. In the oviduct, cumulus cells that surround the oocyte release progesterone. In human sperm, progesterone stimulates a Ca2+ increase by a non-genomic mechanism1,2,3. The Ca2+ signal has been proposed to control chemotaxis, hyperactivation and acrosomal exocytosis of sperm4,5,6,7,8. However, the underlying signalling mechanism has remained mysterious. Here we show that progesterone activates the sperm-specific, pH-sensitive CatSper Ca2+ channel9,10,11. We found that both progesterone and alkaline pH stimulate a rapid Ca2+ influx with almost no latency, incompatible with a signalling pathway involving metabotropic receptors and second messengers. The Ca2+ signals evoked by alkaline pH and progesterone are inhibited by the Cav channel blockers NNC 55-0396 and mibefradil. Patch-clamp recordings from sperm reveal an alkaline-activated current carried by mono- and divalent ions that exhibits all the hallmarks of sperm-specific CatSper Ca2+ channels10,11. Progesterone substantially enhances the CatSper current. The alkaline- and progesterone-activated CatSper current is inhibited by both drugs. Our results resolve a long-standing controversy over the non-genomic progesterone signalling. In human sperm, either the CatSper channel itself or an associated protein serves as the non-genomic progesterone receptor. The identification of CatSper channel blockers will greatly facilitate the study of Ca2+ signalling in sperm and help to define further the physiological role of progesterone and CatSper.

Nachiket D. Kashikar

Papers

The CatSper channel mediates progesterone-induced Ca2+ influx in human sperm

Channel Currents in Rat Intracardiac Neurons Muscarinic Receptor Activation Modulates Calcium