Boar sperm plasma membrane Ca(2+)-selective channels in planar lipid bilayers

Human sperm ion channel (dys)function: implications for fertilization

BACKGROUND

Intensive research on sperm ion channels has identified members of several ion channel families in both mouse and human sperm. Gene knock-out studies have unequivocally demonstrated the importance of the calcium and potassium conductances in sperm for fertility. In both species, the calcium current is carried by the highly complex cation channel of sperm (CatSper). In mouse sperm, the potassium current has been conclusively shown to be carried by a channel consisting of the pore forming subunit SLO3 and auxiliary subunit leucine-rich repeat-containing 52 (LRRC52). However, in human sperm it is controversial whether the pore forming subunit of the channel is composed of SLO3 and/or SLO1. Deciphering the role of the proton-specific Hv1 channel is more challenging as it is only expressed in human sperm. However, definitive evidence for a role in, and importance for, human fertility can only be determined through studies using clinical samples.

OBJECTIVE AND RATIONALE

This review aims to provide insight into the role of sperm ion channels in human fertilization as evidenced from recent studies of sperm from infertile men. We also summarize the key discoveries from mouse ion channel knock-out models and contrast the properties of mouse and human CatSper and potassium currents. We detail the evidence for, and consequences of, defective ion channels in human sperm and discuss hypotheses to explain how defects arise and why affected sperm have impaired fertilization potential.

SEARCH METHODS

Relevant studies were identified using PubMed and were limited to ion channels that have been characterized in mouse and human sperm. Additional notable examples from other species are included as appropriate.

OUTCOMES

There are now well-documented fundamental differences between the properties of CatSper and potassium channel currents in mouse and human sperm. However, in both species, sperm lacking either channel cannot fertilize in vivo and CatSper-null sperm also fail to fertilize at IVF. Sperm-lacking potassium currents are capable of fertilizing at IVF, albeit at a much lower rate. However, additional complex and heterogeneous ion channel dysfunction has been reported in sperm from infertile men, the causes of which are unknown. Similarly, the nature of the functional impairment of affected patient sperm remains elusive. There are no reports of studies of Hv1 in human sperm from infertile men.

WIDER IMPLICATIONS

Recent studies using sperm from infertile men have given new insight and critical evidence supporting the supposition that calcium and potassium conductances are essential for human fertility. However, it should be highlighted that many fundamental questions remain regarding the nature of molecular and functional defects in sperm with dysfunctional ion channels. The development and application of advanced technologies remains a necessity to progress basic and clinical research in this area, with the aim of providing effective screening methodologies to identify and develop treatments for affected men in order to help prevent failed ART cycles. Conversely, development of drugs that block calcium and/or potassium conductances in sperm is a plausible strategy for producing sperm-specific contraceptives.

Localization and functional modification of L-type voltage-gated calcium channels in equine spermatozoa from fresh and frozen semen

It is well known that insemination of cryopreserved semen always results in lower fertility when compared with fresh semen, but there is an increased interest and demand for frozen equine semen by the major breeder associations because of the utility arising from semen already "on hand" at breeding time. In this article, we report that equine sperm cells express L-type voltage-gated calcium channels; their localization is restricted to sperm neck and to the principal piece of the tail in both fresh and frozen-thawed spermatozoa. We also studied the causes of cryoinjury at the membrane level focusing on the function of L-type calcium channels. We report that in cryopreserved spermatozoa the mean basal value of [Ca(2+)]i is higher than that of spermatozoa from fresh semen (447.130 vs. 288.3 nM; P < 0.001) and L-type channels function differently in response to their agonist and antagonist in relation to semen condition (fresh or frozen-thawed). We found that on addition of agonist to the culture medium, the increase in intracellular calcium concentrations ([Ca(2+)]i) was greater in frozen semen than in fresh semen (Δ[Ca(2+)]i = 124.59 vs. 16.04 nM; P < 0.001), whereas after the addition of antagonist the decrease in [Ca(2+)]i was lower in frozen semen than in fresh semen (Δ[Ca(2+)]i = 32.5 vs. 82.5 nM; P < 0.001). In this article, we also discuss the impact of cryopreservation on sperm physiology.

Calcium Channels and Ca2+ Fluctuations in Sperm Physiology

Generating new life in animals by sexual reproduction depends on adequate communication between mature and competent male and female gametes. Ion channels are instrumental in the dialogue between sperm, its environment, and the egg. The ability of sperm to swim to the egg and fertilize it is modulated by ion permeability changes induced by environmental cues and components of the egg outer layer. Ca(2+) is probably the key messenger in this information exchange. It is therefore not surprising that different Ca(2+)-permeable channels are distinctly localized in these tiny specialized cells. New approaches to measure sperm currents, intracellular Ca(2+), membrane potential, and intracellular pH with fluorescent probes, patch-clamp recordings, sequence information, and heterologous expression are revealing how sperm channels participate in fertilization. Certain sperm ion channels are turning out to be unique, making them attractive targets for contraception and for the discovery of novel signaling complexes.

Molecular Mechanisms Underlying Pig Oocyte Maturation and Fertilization

Since the pig is not only an important farm animal, but also a model animal for biomedical applications, the development of reproductive technologies in this species has been very important. In vitro oocyte maturation and fertilization (IVM-IVF) are basic techniques for a number of oocyte- or embryo-related technologies. The practical aspects for pig oocyte IVM-IVF have been reviewed, while the molecular mechanisms underlying oocyte meiotic maturation and fertilization have not been well summarized, although accumulating data have been obtained in recent one decade. This review will focus on what is known about the molecular mechanisms of porcine oocyte maturation and fertilization such as first meiosis resumption, meiotic spindle assembly, second meiosis metaphase (MII) arrest during oocyte maturation, sperm-egg recognition and fusion, sperm acrosome reaction, second meiosis resumption, sperm chromatin decondensation, and pronucleus formation during fertilization, as well as the establishment of polyspermy block.

Identification and localization of T-type voltage-operated calcium channel subunits in human male germ cells. Expression of multiple isoforms

Low voltage activated, voltage-operated Ca(2+) channels are expressed in rodent male germ cells and are believed to be pivotal in induction of the acrosome reaction in mouse spermatozoa. However, in humans, very little is known about expression of voltage-operated Ca(2+) channels in male germ cells or their function. We have used reverse transcription-polymerase chain reaction, in situ hybridization, and patch clamp recording to investigate the expression of low voltage activated voltage-operated Ca(2+) channels in human male germ cells. We report that full-length transcripts for both alpha(1G) and alpha(1H) low voltage activated channel subunits are expressed in human testis. Multiple isoforms of alpha(1G) are present in the testis and at least two isoforms of alpha(1H), including a splice variant not previously described in the human. Transcripts for all the isoforms of both alpha(1G) and alpha(1H) were detected by reverse transcription-polymerase chain reaction on mRNA isolated from human spermatogenic cells. In situ hybridization for alpha(1G) and alpha(1H) localized transcripts both in germ cells and in other cell types in the testis. Within the seminiferous tubules, alpha(1H) was detected primarily in germ cells. Using the whole cell patch clamp technique, we detected T-type voltage-operated Ca(2+) channel currents in isolated human male germ cells, although the current amplitude and frequency of occurrence were low in comparison to the occurrence of T-currents in murine male germ cells. We conclude that low voltage activated voltage-operated Ca(2+) channels are expressed in cells of the human male germ line.

Ion channels in sperm physiology

Fertilization is a matter of life or death. In animals of sexual reproduction, the appropriate communication between mature and competent male and female gametes determines the generation of a new individual. Ion channels are key elements in the dialogue between sperm, its environment, and the egg. Components from the outer layer of the egg induce ion permeability changes in sperm that regulate sperm motility, chemotaxis, and the acrosome reaction. Sperm are tiny differentiated terminal cells unable to synthesize protein and difficult to study electrophysiologically. Thus understanding how sperm ion channels participate in fertilization requires combining planar bilayer techniques, in vivo measurements of membrane potential, intracellular Ca2+ and intracellular pH using fluorescent probes, patch-clamp recordings, and molecular cloning and heterologous expression. Spermatogenic cells are larger than sperm and synthesize the ion channels that will end up in mature sperm. Correlating the presence and cellular distribution of various ion channels with their functional status at different stages of spermatogenesis is contributing to understand their participation in differentiation and in sperm physiology. The multi-faceted approach being used to unravel sperm ion channel function and regulation is yielding valuable information about the finely orchestrated events that lead to sperm activation, induction of the acrosome reaction, and in the end to the miracle of life.

Expression of sperm Ca2+-activated K+ channels in Xenopus oocytes and their modulation by extracellular ATP

Ionic fluxes across the sperm membrane have been shown to be important in the initiating process of sperm activation and gamete interaction; however, electrophysiological investigation of the ion channels involved has been precluded by the small size of the sperm, especially in mammalian species. In the present study sperm ion channels were expressed in Xenopus oocytes by injection of RNAs of spermatogenic cells isolated from the rat testes. The RNA-injected oocytes responded to ATP, a factor known to regulate sperm activation, with the activation of an outwardly rectifying whole-cell current which was dependent on K+ concentrations and inhibitable by K+ channel blockers, charybdotoxin (CTX) and tetraethylammonium (TEA). The ATP-induced current could be mimicked by a Ca2+ ionophore but suppressed by a Ca2+ chelator applied intracellularly, indicating a Ca2+ dependence of the current. Single-channel measurements on RNA-injected oocytes revealed channels of large conductance which could be blocked by CTX and TEA. Co-injection of germ cell RNAs with the antisense RNA for a mouse gene encoding slowpoke 'Maxi' Ca2+-activated K+ channels resulted in significant reduction of the ATP- and ionomycin-induced current. The expression of the 'Maxi' Ca2+-activated K+ channels in sperm collected from the rat epididymis was also confirmed by Western blot analysis. These results suggest that sperm possess Ca2+-activated K+ channels which may be involved in the process of sperm activation.

Functional expression of a Ca2+-activated K+ channel in Xenopus oocytes injected with RNAs from the rat testis

The present study investigated the feasibility of using Xenopus oocytes to express sperm ion channel by injection of RNAs extracted from the rat testis. The RNA-injected oocytes expressed an outwardly rectifying current which was dependent on K+ concentration and inhibitable by K+ channel blockers, charybdotoxin (CTX) and tetraethylammonium (TEA). The Ca2+ ionophore, ionomycin, could also stimulate current activation with similar current characteristics in the RNA-injected oocytes, suggesting the expression of a Ca2+-activated K+ channel. Immunolocalization indicated predominant Ca2+-activated K+ channel immunoreactivity associated with spermatogenic cells. Reverse transcriptase-polymerase chain reaction studies confirmed the expression of the Ca2+-activated K+ channel mRNA in isolated spermatogenic cells. Our results suggest that ion channels and/or receptors of spermatogenic cells could be investigated using the Xenopus oocyte as an expression system. The present study also suggests that sperm may possess a Ca2+-activated K+ channel which has been implicated in the process of sperm activation and gamete interaction.

Ion-channels reconstituted into lipid bilayer from human sperm plasma membrane

Ion environment and ionic fluxes through membrane are thought to be important in the spermatozoa's maturation, capacitation, and the initiating process of gamete interaction. In this work, the membrane proteins isolated from human sperm plasma membrane were reconstituted into planar lipid bilayers via fusion, and the ion channels activities were observed under voltage clamp mode. In cis 200//trans 100 mM KCl solution, a TEA-sensitive cation-selective channel with a unit conductance of 40 pS was recorded. In a gradient of 200//100 mM NaCl solutions, a Na(+)-selective channel with a unit conductance of 26 pS was recorded. In both cases, reversal potential was about-18 mV, which is close to the predicated value of a perfect Nernst K+ or Na+ electrode. In 50//10 mM CaCl2 solution, a cation channel activity with a unit conductance of 40 pS and reversal potential of about -20 mV was usually observed. In 200//100 mM NMDG(N-methyl-D-glucamine)-Cl solution, where the cation ions were substituted with NMDG, a 30-pS anion-selective channel activity was also detected. The variety in the types of ion channels observed in human spermatozoa plasma membrane suggests that ion channels may play a range of different roles in sperm physiology and gamete interaction.

Properties of a novel pH-dependent Ca2+ permeation pathway present in male germ cells with possible roles in spermatogenesis and mature sperm function

Rises of intracellular Ca2+ ([Ca2+]i) are key signals for cell division, differentiation, and maturation. Similarly, they are likely to be important for the unique processes of meiosis and spermatogenesis, carried out exclusively by male germ cells. In addition, elevations of [Ca2+]i and intracellular pH (pHi) in mature sperm trigger at least two events obligatory for fertilization: capacitation and acrosome reaction. Evidence implicates the activity of Ca2+ channels modulated by pHi in the origin of these Ca2+ elevations, but their nature remains unexplored, in part because work in individual spermatozoa are hampered by formidable experimental difficulties. Recently, late spermatogenic cells have emerged as a model system for studying aspects relevant for sperm physiology, such as plasmalemmal ion fluxes. Here we describe the first study on the influence of controlled intracellular alkalinization on [Ca2+]i on identified spermatogenic cells from mouse adult testes. In BCECF [(2',7')-bis(carboxymethyl)- (5, 6)-carboxyfluorescein]-AM-loaded spermatogenic cells, a brief (30-60 s) application of 25 mM NH4Cl increased pHi by approximately 1.3 U from a resting pHi approximately 6.65. A steady pHi plateau was maintained during NH4Cl application, with little or no rebound acidification. In fura-2-AM-loaded cells, alkalinization induced a biphasic response composed of an initial [Ca2+]i drop followed by a two- to threefold rise. Maneuvers that inhibit either Ca2+ influx or intracellular Ca2+ release demonstrated that the majority of the Ca2+ rise results from plasma membrane Ca2+ influx, although a small component likely to result from intracellular Ca2+ release was occasionally observed. Ca2+ transients potentiated with repeated NH4Cl applications, gradually obliterating the initial [Ca2+]i drop. The pH-sensitive Ca2+ permeation pathway allows the passage of other divalents (Sr2+, Ba2+, and Mn2+) and is blocked by inorganic Ca2+ channel blockers (Ni2+ and Cd2+), but not by the organic blocker nifedipine. The magnitude of these Ca2+ transients increased as maturation advanced, with the largest responses being recorded in testicular sperm. By extrapolation, these findings suggest that the pH-dependent Ca2+ influx pathway could play significant roles in mature sperm physiology. Its pharmacology and ion selectivity suggests that it corresponds to an ion channel different from the voltage-gated T-type Ca2+ channel also present in spermatogenic cells. We postulate that the Ca2+ permeation pathway regulated by pHi, if present in mature sperm, may be responsible for the dihydropyridine-insensitive Ca2+ influx required for initiating the acrosome reaction and perhaps other important sperm functions.

The biological and functional significance of the sperm acrosome and acrosomal enzymes in mammalian fertilization

The mammalian spermatozoon undergoes continuous modifications during spermatogenesis, maturation in the epididymis, and capacitation in the female reproductive tract. Only the capacitated spermatozoa are capable of binding the zona-intact egg and undergoing the acrosome reaction. The fertilization process is a net result of multiple molecular events which enable ejaculated spermatozoa to recognize and bind to the egg's extracellular coat, the zona pellucida (ZP). Sperm-egg interaction is a species-specific event which is initiated by the recognition and binding of complementary molecule(s) present on sperm plasma membrane (receptor) and the surface of the ZP (ligand). This is a carbohydrate-mediated event which initiates a signal transduction cascade resulting in the exocytosis of acrosomal contents. This step is believed to be a prerequisite which enables the acrosome reacted spermatozoa to penetrate the ZP and fertilize the egg. This review focuses on the formation and contents of the sperm acrosome as well as the mechanisms underlying the induction of the acrosome reaction. Special emphasis has been laid on the synthesis, processing, substrate specificity, and mechanism of action of the acid glycohydrolases present within the acrosome. The hydrolytic action of glycohydrolases and proteases released at the site of sperm-zona binding, along with the enhanced thrust generated by the hyperactivated beat pattern of the bound spermatozoon, are important factors regulating the penetration of ZP. We have discussed the most recent studies which have attempted to explain signal transduction pathways leading to the acrosomal exocytosis.

Mouse sperm patch-clamp recordings reveal single Cl- channels sensitive to niflumic acid, a blocker of the sperm acrosome reaction

Ion channels lie at the heart of gamete signaling. Understanding their regulation will improve our knowledge of sperm physiology, and may lead to novel contraceptive strategies. Sperm are tiny (approximately 3 microm diameter) and, until now, direct evidence of ion channel activity in these cells was lacking. Using patch-clamp recording we document here, for the first time, the presence of cationic and anionic channels in mouse sperm. Anion selective channels were blocked by niflumic acid (NA) (IC50 = 11 microM). The blocker was effective also in inhibiting the acrosome reaction induced by the zona pellucida, GABA or progesterone. These observations suggest that Cl- channels participate in the sperm acrosome reaction in mammals.

ZP3-dependent activation of sperm cation channels regulates acrosomal secretion during mammalian fertilization

The sperm acrosome reaction is a Ca(2+)-dependent secretory event required for fertilization. Adhesion to the egg's zona pellucida promotes Ca2+ influx through voltage-sensitive channels, thereby initiating secretion. We used potentiometric fluorescent probes to determine the role of sperm membrane potential in regulating Ca2+ entry. ZP3, the glycoprotein agonist of the zona pellucida, depolarizes sperm membranes by activating a pertussis toxin-insensitive mechanism with the characteristics of a poorly selective cation channel. ZP3 also activates a pertussis toxin-sensitive pathway that produces a transient rise in internal pH. The concerted effects of depolarization and alkalinization open voltage-sensitive Ca2+ channels. These observations suggest that mammalian sperm utilize membrane potential-dependent signal transduction mechanisms and that a depolarization pathway is an upstream transducing element coupling adhesion to secretion during fertilization.

T-type Ca2+ channels and alpha1E expression in spermatogenic cells, and their possible relevance to the sperm acrosome reaction

There is pharmacological evidence that Ca2+ channels play an essential role in triggering the mammalian sperm acrosome reaction, an exocytotic process required for sperm to fertilize the egg. Spermatozoa are small terminally differentiated cells that are difficult to study by conventional electrophysiological techniques. To identify the members of the voltage-dependent Ca2+ channel family possibly present in sperm, we have looked for the expression of the alpha 1A, alpha 1B, alpha 1C, alpha 1D and alpha 1E genes in mouse testis and in purified spermatogenic cell populations with RT-PCR. Our results indicate that all 5 genes are expressed in mouse testis, and in contrast only alpha 1E, and to a minor extent alpha 1A, are expressed in spermatogenic cells. In agreement with these findings, only T-type Ca2+ channels sensitive to the dihydropyridine nifedipine were observed in patch-clamp recordings of pachytene spermatocytes. These results suggest that low-threshold Ca2+ channels are the dihydropyridine-sensitive channels involved in the sperm acrosome reaction.