Ion channels from the mouse sperm plasma membrane in planar lipid bilayers

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

A proper dialogue between spermatozoa and the egg is essential for conception of a new individual in sexually reproducing animals. Ca2+ is crucial in orchestrating this unique event leading to a new life. No wonder that nature has devised different Ca2+-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 Ca2+ 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 Ca2+ 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 Ca2+ 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.

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.

Intracellular sodium changes during the speract response and the acrosome reaction in sea urchin sperm

The sperm-activating peptide speract and fucose-sulphate glycoconjugate (FSG) are sea urchin egg-envelope components that modulate sperm ion permeability. They influence motility and induce acrosomal reaction (AR), respectively. A fluorescent Na(+)-sensitive dye (Na(+)-binding benzofuran isophthalate, SBFI) was used to determine how these egg envelope components influence sperm Na(+) permeability. [Ca(2+)](i) and pH(i) were also measured to correlate their changes in response to speract and FSG with those observed in [Na(+)](i). SBFI determinations indicate that the resting [Na(+)](i) is 20 +/- 8 mM in sea urchin sperm. Saturating levels of speract increased [Na(+)](i) by approximately 15 mM, while similar levels of FSG caused a further elevation of approximately 30 mM. The kinetics of the [Na(+)](i), [Ca(2+)](i) and pH(i) changes induced by saturating levels of speract were faster than those induced by FSG. Both egg ligands appeared to activate more than one Na(+) transport system. Nifedipine, Ni(2+) and TEA(+) inhibited the ionic changes and the AR induced by FSG but, importantly, did not alter those caused by speract. Thus, there are differences in some of the ionic transport mechanisms that operate in the speract and FSG responses. ZD2788, a blocker of hyperpolarization and cyclic-nucleotide-gated (HCN) channels such as SpHCN present in sea urchin sperm, did not decrease the speract-induced [Na(+)](i) increase, but slowed its kinetics. Therefore, SpHCN does not play a major role in the uptake of Na(+) triggered by this decapeptide. KB-R7943, an inhibitor of Na(+)/Ca(2+) exchangers, decreased the resting [Na(+)](i) and did not change significantly the speract-induced [Ca(2+)](i) increase, but slowed its recovery.

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.

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.

A dihydropyridine-sensitive T-type Ca2+ current is the main Ca2+ current carrier in mouse primary spermatocytes

Ca2+ entry through Ca2+ channels is likely to play an important role in the differentiation of male germ cells as well as in fertilization by mature sperm. Here we present a detailed analysis of Ca2+ currents expressed in acutely dissociated mouse primary spermatocytes. Patch-clamp recordings demonstrated that the only voltage-gated Ca2+ channels present belong to the family of T-type Ca2+ currents. Accordingly, Ni2+ (200 microM) and amiloride (500 microM) reduced current amplitude by 75 and 62%, respectively. To our knowledge, this is the first report of a system where T-type Ca2+ channels are expressed in isolation. Unexpectedly, 5 and 10 microM nifedipine also reduced peak currents by 38 and 53%, respectively significant inhibition of the Ca2+ current occurred at concentrations as low as 2 microM. Because mature sperm cells are unable to synthesize new proteins, these Ca2+ channels are also likely to be present in these cells, where they may contribute to the Ca2+ influx required to trigger the acrosome reaction. This notion is supported by the fact that concentrations of Ni2+ and nifedipine, which block these Ca2+ currents, also inhibit the acrosome reaction. Because these channels represent the primary pathway for voltage-gated Ca2+ entry in mouse spermatocytes, they may also participate in regulating meiotic cell division and sperm differentiation.

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.