Nifedipine reveals the existence of two discrete components of the progesterone-induced [Ca2+]i transient in human spermatozoa

Ceramide induces a multicomponent intracellular calcium increase triggering the acrosome secretion in human sperm

Exocytosis of spermatozoon's secretory vesicle, named acrosome reaction (AR), is a regulated event that plays a central role in fertilization. It is coupled to a complex calcium signaling. Ceramide is a multitasking lipid involved in exocytosis. Nevertheless, its effect on secretion is controversial and the underlying cellular and molecular mechanisms remain unknown. Human spermatozoa are useful to dissect the role of ceramide in secretion given that the gamete is not capable to undergo any trafficking mechanisms other than exocytosis. We report for the first time, the presence of sphingolipid metabolism enzymes such as neutral-sphingomyelinase and ceramide synthase in sperm. Ceramidases are also present and active. Both the addition of cell-permeable ceramide and the rise of the endogenous one, increase intracellular calcium acting as potent inducers of exocytosis. Ceramide triggers AR in capacitated spermatozoa and enhances the gamete response to progesterone. The lipid induces physiological ultrastructural changes in the acrosome and triggers an exocytosis-signaling cascade involving protein tyrosine phosphatase 1B and VAMP2. Real-time imaging showed an increment of calcium in the cytosol upon ceramide treatment either in the absence or in the presence of extracellular calcium. Pharmacological experiments demonstrate that at early stages the process involves ryanodine receptors, CatSper (calcium channel of sperm), and store-operated calcium channels. We set out the signaling sequence of events that connect ceramide to internal calcium mobilization and external calcium signals during secretion. These results allow the coordination of lipids and proteins in a pathway that accomplishes secretion. Our findings contribute to the understanding of ceramide's role in regulated exocytosis and fertilization.

Panax ginseng induces the expression of CatSper genes and sperm hyperactivation

The cation channel of sperm (CatSper) protein family plays important roles in male reproduction and infertility. The four members of this family are expressed exclusively in the testis and are localized differently in sperm. To investigate the effects of Panax ginseng treatment on the expression of CatSper genes and sperm hyperactivation in male mice, sperm motility and CatSper gene expression were assessed using a computer-assisted semen analysis system, a Fluoroskan Ascent microplate fluorometer to assess Ca²⁺ influx, real-time polymerase chain reaction, Western blotting and immunofluorescence. The results suggested that the Ca²⁺ levels of sperm cells treated with P. ginseng were increased significantly compared with the normal group. The P. ginseng-treated groups showed increased sperm motility parameters, such as the curvilinear velocity and amplitude of lateral head displacement. Taken together, the data suggest that CatSper messenger ribonucleic acid levels were increased significantly in mouse testes in the P. ginseng-treated group, as was the protein level, with the exception of CatSper2. In conclusion, P. ginseng plays an important role in improving sperm hyperactivation via CatSper gene expression.

Cell-penetrating peptides, targeting the regulation of store-operated channels, slow decay of the progesterone-induced [Ca2+]i signal in human sperm

Previous work has provided evidence for involvement of store-operated channels (SOCs) in [Ca(2+)]i signalling of human sperm, including a contribution to the transient [Ca(2+)]i elevation that occurs upon activation of CatSper, a sperm-specific cation channel localized to the flagellum, by progesterone. To further investigate the potential involvement of SOCs in the generation of [Ca(2+)]i signals in human sperm, we have used cell-penetrating peptides containing the important basic sequence KIKKK, part of the STIM-Orai activating region/CRAC activating domain (SOAR/CAD) of the regulatory protein stromal interaction molecule 1. SOAR/CAD plays a key role in controlling the opening of SOCs, which occurs upon mobilization of stored Ca(2+). Resting [Ca(2+)]i temporarily decreased upon application of KIKKK peptide (3-4 min), but scrambled KIKKK peptide had a similar effect, indicating that this action was not sequence-specific. However, in cells pretreated with KIKKK, the transient [Ca(2+)]i elevation induced by stimulation with progesterone decayed significantly more slowly than in parallel controls and in cells pretreated with scrambled KIKKK peptide. Examination of single-cell responses showed that this effect was due, at least in part, to an increase in the proportion of cells in which the initial transient was maintained for an extended period, lasting up to 10 min in a subpopulation of cells. We hypothesize that SOCs contribute to the progesterone-induced [Ca(2+)]i transient, and that interference with the regulatory mechanisms of SOC delays their closure, causing a prolongation of the [Ca(2+)]i transient.

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.

Pharmacological investigation of voltage-dependent Ca2+ channels in human ejaculatory sperm in vitro

The types of the voltage-dependent calcium channels (VDCCs) in human ejaculatory sperm and the effects of calcium channel blocker (CCB) on human sperm motility parameters in vitro were investigated. The human sperm motility parameters in vitro in response to the pharmacological agents nifedipine (NIF, inhibitor of L-type VDCC) and co-conotoxin (GVIA, inhibitor of N-type VDCC) were compared and analyzed statistically. The results showed that NIF (1, 5, 10 micromol/L) could not only significantly affect human sperm's shape but also spermatozoa motility after incubated at least 10 min in vitro (P<0.001). GVIA (0.1, 0.5 and 1 micromol/L) could just only significantly affect human sperm's progressive motility (a %+b %) after incubated for 20 min in vitro (P<0.01), but they both could not significantly affect spermic abnormality rate. It is suggested that L-type VDCC, non L-type VDCCs and isoform of L-type VDCC exist in the cell membrane of human sperm solely or together, and they participate in the spermic physiological processes especially the spermic motility.

Sperm parameters: paradigmatic index of good health and longevity

Since the discovery of spermatozoon by Anton van Leeuwenhoek in 1677, there has been an ever increasing understanding of its role in reproduction. Many factors adversely affect sperm quality, including varicocele, accessory gland infection, immunological factors, congenital abnormalities, and iatrogenic systemic and endocrine causes, such as diabetes mellitus, obesity, metabolic syndrome, and smoking. The mechanisms responsible for the association between poor sperm parameters and ill health may include oxidative stress, low-grade inflammation, low testosterone, and low sex-hormone-binding globulin. Oxidative stress in the testicular microenvironment may result in decreased spermatogenesis and sperm DNA damage, loss of sperm motility, and abnormal sperm morphology. Low testosterone caused by advanced age, visceral obesity, and inflammation is associated with the development of cardiovascular disease. Hence, semen analysis has an important role in the routine evaluation of idiopathic male infertility, usually manifested as low sperm counts, impaired sperm motility, or absence of sperm, and remains the most common single diagnostic tool. Several studies have shown an inverse relationship between semen quality and medical disorders. This review elucidates the effect of medical disorders and social habits on sperm quality, the mechanisms that are involved in the impairment of sperm quality, and whether or not sperm quality can be used as an index of good health and longevity in a man.

Lack of Voltage-Dependent Calcium Channel Opening During the Calcium Influx Induced by Progesterone in Human Sperm. Effect of Calcium Channel Deactivation and Inactivation

Progesterone induces calcium influx and acrosomal exocytosis in human sperm. Pharmacologic evidence suggests that voltage-dependent calcium channels (VDCCs) are involved. In this study, membrane potential (Vm) and intracellular calcium concentration ([Ca(2+)](i)) were monitored simultaneously to assess the effect of VDCC gating on the calcium influx triggered by progesterone. Holding the Vm to values that maintained VDCCs in a deactivated (-71 mV) closed state inhibited the calcium influx induced by progesterone by approximately 40%. At this Vm, the acrosomal reaction induced by progesterone, but not by A23187, was inhibited. However, when the Vm was held at -15 mV (which maintains VDCCs in an inactivated closed state), the progesterone-induced calcium influx was stimulated. Furthermore, the progesterone and voltage-dependent calcium influxes were additive. These findings indicate that progesterone does not produce VDCC gating in human sperm.

Optimized protocols to analyze sphingosine-1-phosphate signal transduction pathways during acrosomal exocytosis in human sperm

Regulated secretion is a central issue for the specific function of many cells; for instance, mammalian sperm acrosomal exocytosis is essential for egg fertilization. Sphingosine 1-phosphate is a bioactive sphingolipid that regulates crucial physiological processes. We have recently reported that sphingosine 1-phosphate and sphingosine kinase are involved in a novel signaling pathway leading to acrosomal exocytosis (Suhaiman L et al., J Biol Chem 285:1630-16314, 2010). Acrosomal exocytosis in mammalian sperm is a regulated secretion with unusual characteristics. We therefore employed biochemical functional assays to assess the sphingolipid signaling in both permeabilized and nonpermeabilized sperm. The exocytosis of the acrosomal content is regulated by Ca(2+). During exocytosis, changes in [Ca(2+)]i occur induced by either Ca(2+)-influx or Ca(2+)-mobilization from intracellular stores. By using single cell [Ca(2+)] measurements, we detected intracellular Ca(2+) changes after sphingosine 1-phosphate treatment. Additionally, measuring sphingosine kinase activity, we determined that sphingosine 1-phosphate levels increase after an exocytotic stimulus.This chapter is designed to provide the user with sufficient background to analyze sphingosine 1--phosphate signal transduction pathways during acrosomal exocytosis in human sperm.

Non-genomic effects of vitamin D in human spermatozoa

The spectrum for vitamin D (VD) mediated effects has expanded in recent years. Activated VD (1,25(OH)(2)D(3)) binds to the VD receptor (VDR) and mediates non-genomic effects through the alternative ligand binding-pocket (VDR-ap) or regulates gene transcription through the genomic binding-pocket. VDR and VD-metabolizing enzymes are expressed in human testis, male reproductive tract and mature spermatozoa, and VD is considered important for male reproduction. Expression of the VD-inactivating enzyme CYP24A1 at the annulus of human spermatozoa distinguish normal and infertile men with high specificity, and CYP24A1 expression is positively correlated with all semen variables and suggested as a marker for both semen quality and VD responsiveness. Moreover, spermatozoa are transcriptionally silent and are therefore a unique model to study non-genomic effects. 1,25(OH)(2)D(3) induced a rapid increase in intracellular calcium concentration [Ca(2+)](i) in human spermatozoa. The [Ca(2+)](i) increase was abrogated by the non-genomic VDR antagonist 1β,25(OH)(2)D(3), while the specific agonist for VDR-ap (JN) increased [Ca(2+)](i) with similar kinetics as 1,25(OH)(2)D(3). The rise in [Ca(2+)](i) originated as a Ca(2+)-release from intracellular stores since inhibition of phospholipase-C diminished the 1,25(OH)(2)D(3) mediated Ca(2+) response, while suspending spermatozoa in a nominally Ca(2+)-free medium did not affect the VD mediated Ca(2+) rise. The spatio-temporal kinetics of the VD-response differed from the progesterone-mediated increase in [Ca(2+)](i) as the VD-mediated Ca(2+) rise was not observed in the tail region and was independent of extracellular Ca(2+). A functional role of the VD-mediated Ca(2+) increase was supported by showing that 1,25(OH)(2)D(3) increased sperm motility and induced the acrosome reaction in vitro.

Diacylglycerol stimulates acrosomal exocytosis by feeding into a PKC- and PLD1-dependent positive loop that continuously supplies phosphatidylinositol 4,5-bisphosphate

Acrosomal exocytosis involves a massive fusion between the outer acrosomal and the plasma membranes of the spermatozoon triggered by stimuli that open calcium channels at the plasma membrane. Diacylglycerol has been implicated in the activation of these calcium channels. Here we report that this lipid promotes the efflux of intraacrosomal calcium and triggers exocytosis in permeabilized human sperm, implying that diacylglycerol activates events downstream of the opening of plasma membrane channels. Furthermore, we show that calcium and diacylglycerol converge in a signaling pathway leading to the production of phosphatidylinositol 4,5-bisphosphate (PIP(2)). Addition of diacylglycerol promotes the PKC-dependent activation of PLD1. Rescue experiments adding phosphatidic acid or PIP(2) and direct measurement of lipid production suggest that both PKC and PLD1 promote PIP(2) synthesis. Inhibition of different steps of the pathway was reverted by adenophostin, an agonist of IP(3)-sensitive calcium channels, indicating that PIP(2) is necessary to keep these channels opened. However, phosphatidic acid, PIP(2), or adenophostin could not trigger exocytosis by themselves, indicating that diacylglycerol must also activate another factor. We found that diacylglycerol and phorbol ester stimulate the accumulation of the GTP-bound form of Rab3A. Together our results indicate that diacylglycerol promotes acrosomal exocytosis by i) maintaining high levels of IP(3) - an effect that depends on a positive feedback loop leading to the production of PIP(2) - and ii) stimulating the activation of Rab3A, which in turn initiates a cascade of protein interactions leading to the assembly of SNARE complexes and membrane fusion.

A caged progesterone analog alters intracellular Ca2+ and flagellar bending in human sperm

Progesterone is a physiological agonist for mammalian sperm, modulating its flagellar movement and facilitating the acrosome reaction. To study the initial action of progesterone, we developed a caged analog with a photosensitive group: nitrophenylethanediol, at position 20. Using this compound combined with stroboscopic illumination, we performed Ca(2)(+) imaging of human spermatozoa and analyzed the effects of progesterone on the intracellular Ca(2)(+) concentration ([Ca(2)(+)](i)) of beating flagella for the first time. We observed a transient [Ca(2)(+)](i) increase in the head and the flagellum upon photolysis of the caged progesterone and an increase in flagellar curvature. Detailed kinetic analysis revealed that progesterone elicits an increase in the [Ca(2)(+)](i) immediately in the flagellum (mid-piece and principal piece), thereafter in the head with a short time lag. This observation is different from the progesterone-induced Ca(2)(+) mobilization in mouse spermatozoa, where the Ca(2)(+) rise initiates at the base of the sperm head. Our finding is mostly consistent with the recent discovery that progesterone activates CatSper channels in human spermatozoa, but not in mouse spermatozoa.

Vitamin D is positively associated with sperm motility and increases intracellular calcium in human spermatozoa

BACKGROUND

The vitamin D receptor (VDR) is expressed in human spermatozoa, and VDR-knockout mice and vitamin D (VD) deficiency in rodents results in impaired fertility, low sperm counts and a low number of motile spermatozoa. We investigated the role of activated VD (1,25(OH)(2)D(3)) in human spermatozoa and whether VD serum levels are associated with semen quality.

METHODS

Cross-sectional association study of semen quality and VD serum level in 300 men from the general population, and in vitro studies on spermatozoa from 40 men to investigate the effects of VD on intracellular calcium, sperm motility and acrosome reaction. All men delivered samples for routine semen analysis and blood for measurements of follicle stimulating hormone, Inhibin B, 25-hydroxy-VD, albumin, alkaline phosphatase, calcium and parathyroid hormone (PTH).

RESULTS

In the association study, 44% were VD insufficient (<50 nM), and VD was inversely correlated with PTH (P < 0.0005). VD serum levels correlated positively with sperm motility and progressive motility (P < 0.05), and men with VD deficiency (<25 nM) had a lower proportion of motile (P = 0.027), progressive motile (P = 0.035) and morphologically normal spermatozoa (P = 0.044) compared with men with high VD levels (>75 nM). 1,25(OH)(2)D(3) increased intracellular calcium concentration in human spermatozoa through VDR-mediated calcium release from an intracellular calcium storage, increased sperm motility and induced the acrosome reaction in vitro.

CONCLUSIONS

1,25(OH)(2)D(3) increased intracellular calcium concentration, sperm motility and induced the acrosome reaction in mature spermatozoa, and VD serum levels were positively associated with sperm motility, suggesting a role for VD in human sperm function.

Effect of FSH and progesterone on human spermatozoa cytosolic calcium

Ejaculated spermatozoa must undergo a number of modifications before fertilizing the oocyte: among these the capacitation and the acrosome reaction. Calcium signals play an essential role in these functional and structural modifications. Mature spermatozoa have few organelles and a very small cytoplasmic volume but maintain the homeostasis of [Ca(2+)](c) with great accuracy. We study Ca(2+) mobilization in human spermatozoa exposed to FSH and progesterone by measuring the [Ca(2+)](c) with the FURA-2AM method and report for the first time that the exposure to FSH (up to 98ng/ml) produced an increase of [Ca(2+)](c) to an extent comparable to that observed with 1muM progesterone. FSH and progesterone increase the spermatozoa [Ca(2+)](c) by acting primarily on calcium entry from the external medium. The effects of the two hormones on [Ca(2+)](c) were similar but not identical; the pre-treatment of progesterone blocks the effects of FSH, but not vice-versa. The increase of [Ca(2+)](c) due to FSH was more sensitive to nifedipine (VOCCs inhibitor) than that of progesterone. The effects of these hormones on calcium homeostasis may be relevant for sperm activation.

Sphingosine 1-phosphate and sphingosine kinase are involved in a novel signaling pathway leading to acrosomal exocytosis

Regulated secretion is a central issue for the specific function of many cells; for instance, mammalian sperm acrosomal exocytosis is essential for egg fertilization. Sphingosine 1-phosphate is a bioactive sphingolipid that regulates crucial physiological processes. Here we report that this lipid triggers acrosomal exocytosis in human sperm by a mechanism involving a G(i)-coupled receptor. Real-time imaging showed a remarkable increase of cytosolic calcium upon activation with sphingosine 1-phosphate and pharmacological experiments indicate that the process requires extracellular calcium influx through voltage and store-operated calcium channels and efflux from intracellular stores through inositol 1,4,5-trisphosphate-sensitive calcium channels. Sphingosine 1-phosphate-induced exocytosis requires phospholipase C and protein kinase C activation. We investigated possible sources of the lipid. Western blot indicates that sphingosine kinase 1 is present in spermatozoa. Indirect immunofluorescence showed that phorbol ester, a potent protein kinase C activator that can also trigger acrosomal exocytosis, redistributes sphingosine kinase 1 to the acrosomal region. Functional assays showed that phorbol ester-induced exocytosis depends on the activation of sphingosine kinase 1. Furthermore, incorporation of (32)P to sphingosine demonstrates that cells treated with the phorbol ester increase their sphingosine kinase activity that yields sphingosine 1-phosphate. We present here the first evidence indicating that human spermatozoa produce sphingosine 1-phosphate when challenged with an exocytic stimulus. These observations point to a new role of sphingosine 1-phosphate in a signaling cascade that facilitates acrosome reaction providing some clues about novel lipid molecules involved in exocytosis.

Delineation of downstream signalling components during acrosome reaction mediated by heat solubilized human zona pellucida

BACKGROUND

Human egg is enveloped by a glycoproteinaceous matrix, zona pellucida (ZP), responsible for binding of the human spermatozoa to the egg and induction of acrosomal exocytosis in the spermatozoon bound to ZP. In the present manuscript, attempts have been made to delineate the downstream signalling components employed by human ZP to induce acrosome reaction.

METHODS

Heat-solubilized human ZP (SIZP) was used to study the induction of acrosome reaction in capacitated human spermatozoa using tetramethylrhodamine isothiocyanate conjugated Pisum sativum agglutinin (TRITC-PSA) in absence or presence of various pharmacological inhibitors. In addition, intracellular calcium ([Ca2+]i) levels in sperm using Fluo-3 acetoxymethyl ester as fluorescent probe were also estimated in response to SIZP.

RESULTS

SIZP induces acrosomal exocytosis in capacitated human sperm in a dose dependent manner accompanied by an increase in [Ca2+]i. Human SIZP mediated induction of acrosome reaction depends on extracellular Ca2+ and involves activation of Gi protein-coupled receptor, tyrosine kinase, protein kinases A & C and phosphoinositide 3 (PI3)- kinase. In addition, T-type voltage operated calcium channels and GABA-A receptor associated chloride (Cl-) channels play an important role in SIZP mediated induction of acrosome reaction.

CONCLUSIONS

Results described in the present study provide a comprehensive account of the various downstream signalling components associated with human ZP mediated acrosome reaction.

Molecular mechanism for human sperm chemotaxis mediated by progesterone

Sperm chemotaxis is a chemical guiding mechanism that may orient spermatozoa to the egg surface. A picomolar concentration gradient of Progesterone (P), the main steroidal component secreted by the cumulus cells that surround the egg, attracts human spermatozoa. In order to elucidate the molecular mechanism of sperm chemotaxis mediated by P, we combine the application of different strategies: pharmacological inhibition of signaling molecules, measurements of the concentrations of second messengers and activation of the chemotactic signaling. Our data implicate a number of classic signal transduction pathways in the response and provide a model for the sequence of events, where the tmAC-cAMP-PKA pathway is activated first, followed by protein tyrosine phosphorylation (equatorial band and flagellum) and calcium mobilization (through IP(3)R and SOC channels), whereas the sGC-cGMP-PKG cascade, is activated later. These events lead to sperm orientation towards the source of the chemoattractant. The finding proposes a molecular mechanism which contributes to the understanding of the signal transduction pathway that takes place in a physiological process as chemotaxis.

Membrane-permeant Rab3A triggers acrosomal exocytosis in living human sperm

The acrosome reaction is a regulated Ca2+-dependent secretion event required for sperm-egg interaction. Previous studies indicate that the process requires Rab3-dependent tethering of membranes, SNARE complex assembly, and Ca2+-mediated activation of synaptotagmin. Sperm are transcriptionally and translationally inactive; hence, most studies of the exocytosis mechanism are limited to membrane-permeant reagents. The effect of proteins involved in exocytosis has been assessed only in permeabilized cells. Polyarginine peptides are a powerful tool for delivering macromolecules to cells. Most reports indicate that membrane translocation of arginine-containing proteins requires endocytosis; therefore, this strategy might not be useful in sperm. However, our results indicate that GST and Rab3A, when fused with an arginine-rich peptide, were able to translocate into sperm. Moreover, membrane-permeant Rab3A initiated exocytosis when prenylated and activated with GTP. We show here that a key event after the cytoplasmic Ca2+ increase caused by progesterone is the activation of Rab3A. When active Rab3A is introduced into sperm, Ca2+ in the extracellular medium and in the cytoplasm is dispensable. However, a Ca2+ efflux from inside the acrosome is still required to achieve exocytosis. In conclusion, arginine-containing proteins can penetrate the sperm plasma membrane and thus are valuable tools to study sperm physiology in intact cells.

Patterns of [Ca2+]i mobilization and cell response in human spermatozoa exposed to progesterone

Human spermatozoa stimulated with progesterone (a product of the cumulus and thus encountered by sperm prior to fertilization in vivo) apparently mobilize Ca(2+) and respond very differently according to the way in which the steroid is presented. A progesterone concentration ramp (0-3 microM) induces [Ca(2+)](i) oscillations (repetitive store mobilization) which modify flagellar beating, whereas bolus application of micromolar progesterone causes a single large transient (causing acrosome reaction) which is apparently dependent upon Ca(2+) influx. We have investigated Ca(2+)-mobilization and functional responses in human sperm exposed to 3 muM progesterone. The [Ca(2+)](i) response to progesterone was abolished by 4 min incubation in 0 Ca(2+) medium (2 mM EGTA) but in nominally Ca(2+)-free medium (no added Ca(2+); 0 EGTA) a smaller, slow response occurred. Single cell imaging showed a similar effect of nominally Ca(2+)-free medium and approximately 5% of cells generated a small transient even in the presence of EGTA. When cells were exposed to EGTA-containing saline (5 min) and then returned to nominally Ca(2+)-free medium before stimulation, the [Ca(2+)](i) transient was greatly delayed (approximately 50 s) and rise time was doubled in comparison to cells not subjected to EGTA pre-treatment. We conclude that mobilization of stored Ca(2+) contributes a 'slow' component to the progesterone-induced [Ca(2+)](i) transient and that incubation in EGTA-buffered saline is able rapidly to deplete this store. Analysis of flagellar activity induced by 3 muM progesterone showed an effect (modified beating) associated with the [Ca(2+)](i) transient, in >80% of cells bathed in nominally Ca(2+)-free medium. This was reduced greatly in cells subjected to 5 min EGTA pre-treatment. The store-mediated transient showed a pharmacological sensitivity similar to that of progesterone-induced [Ca(2+)](i) oscillations (consistent with filling of the store by an SPCA) suggesting that the transient induced by micromolar progesterone is a 'single shot' activation of the same store that generates Ca(2+) oscillations.

Calcium signalling in human spermatozoa: a specialized 'toolkit' of channels, transporters and stores

Ca(2+) is a ubiquitous intracellular messenger which encodes information by temporal and spatial patterns of concentration. In spermatozoa, several key functions, including acrosome reaction and motility, are regulated by cytoplasmic Ca(2+) concentration. Despite the very small size and apparent structural simplicity of spermatozoa, evidence is accumulating that they possess sophisticated mechanisms for regulation of cytoplasmic Ca(2+) concentration and generation of complex Ca(2+) signals. In this review, we consider the various components of the Ca(2+)-signalling 'toolkit' that have been characterized in somatic cells and summarize the evidence for their presence and activity in spermatozoa. In particular, data accumulated over the last few years show that spermatozoa possess one (and probably two) Ca(2+) stores as well as a range of plasma membrane pumps and channels. Selective regulation of the various components of the 'toolkit' by agonists probably allows spermatozoa to generate localized Ca(2+) signals despite their very small cytoplasmic volume, permitting the discrete and selective activation of cell functions.

Reassessing the role of progesterone in fertilization—compartmentalized calcium signalling in human spermatozoa?

Progesterone is present at micromolar concentrations in the vicinity of the oocyte. Human spermatozoa generate a biphasic rise in intracellular calcium concentration ([Ca(2+)](i)) and undergo the acrosome reaction upon progesterone stimulation, suggesting that the hormone acts as a secondary inducer or 'primer' of the acrosome reaction in association with the zona pellucida. However, the sensitivity of human spermatozoa to progesterone is such that many cells may undergo the acrosome reaction prematurely, compromising their ability to fertilize. We have shown that exposing human spermatozoa to a progesterone gradient, simulating the stimulus encountered as sperm approach the oocyte, results in a novel response. A slow rise in [Ca(2+)](i) occurs, upon which, in many cells, [Ca(2+)](i) oscillations are superimposed. Cells showing this pattern of response do not undergo the acrosome reaction, but instead show an alternating pattern of flagellar activity associated with peaks and troughs of [Ca(2+)](i). A Ca(2+) store in the rear of the sperm head apparently generates this complex signal, functioning as an '[Ca(2+)](i) oscillator'. We propose that: (i) the acrosome reaction and flagellar beat are regulated by separate Ca(2+) stores; (ii) these stores are mobilized through different mechanisms by different agonists; and (iii) progesterone in vivo acts as a switch for the oscillator which regulates the flagellar beat mode.

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.

Slow calcium oscillations in human spermatozoa

We have used single-cell imaging to investigate intracellular Ca2+ signalling in human spermatozoa stimulated with progesterone (3 microM). In approx. 9% of cells progesterone caused the activation of slow repetitive [Ca2+]i (intracellular Ca2+ concentration) oscillations, with a period of 1-4 min, which persisted for the duration of recording (20-30 min). Pretreatment with nifedipine, which blocks T- and L-type voltage-operated Ca2+ channels in spermatogenic cells, did not modify the characteristics of the oscillations, but reduced the proportion of cells in which they were observed. Stimulation with Bay K 8644 or FPL64176 induced [Ca2+]i oscillations in 5-10% of cells, but their frequency was low (period, 4-5 min). Application of valinomycin (1 microM) to clamp membrane potential at E(K) (equilibrium potential for potassium) did not modify activity in oscillating cells, showing that plasma membrane potential and activation of voltage-operated conductances are not involved in the mechanism by which sperm [Ca2+]i oscillations are generated.

Role of human prostasomes in the activation of spermatozoa

Prostasomes are small vesicles of prostatic origin contained in human semen. Their composition is peculiar under many aspects. Cholesterol is abundant and many proteins are endowed with enzymatic or other activities. The function of prostasomes has been amply debated and several hypotheses have been put forward. The liquefaction of semen, spermatozoa motility, antibacterial activity and immunological functions have been related to prostasomes. Under certain aspects, prostasomes resemble synaptosomes. The fusion of prostasomes to spermatozoa enriches spermatozoa with cholesterol and causes bursts of cytoplasmic sperm calcium. The interaction of spermatozoa and prostasomes should be limited to vagina since prostasomes are immobile and do not follow spermatozoa in the superior female genital tract. Calcium bursts would increase spermatozoa motility, where cholesterol would decapacitate spermatozoa, so preventing untimely activation. Since spermatozoa receive many different molecules from prostasomes, additional effects are also possible. Prostasomes makes spermatozoa more apt to be activated by progesterone in the proximity of the ovum. Therefore, the fusion between spermatozoa and prostasomes would influence spermatozoa behaviour under many aspects and might be relevant for fecundation. The richness of molecular species in prostasomes is amazing and these small vesicles are expected to lead to many more discoveries in the field of human reproduction.

Physiological and proteomic approaches to studying prefertilization events in the human

This research aims firstly to understand, in cellular and molecular terms, how a mature human spermatozoon is prepared for fertilization, and secondly, to identify what factors are involved in the initial signalling interactions between the egg and spermatozoon. In order to achieve these objectives, a combination of approaches is being used, including single-cell imaging, patch clamping and proteomics. Single-cell imaging reveals hidden complexity and heterogeneity in signalling responses in spermatozoa. Characterization of cell physiology at the single-cell level must be a future aim, including the study of ion channel expression and function by patch clamping. Proteomic experiments are aimed at identifying defects in protein expression in specific subgroups of men, e.g. those with globozoospermia. A better understanding of prefertilization events will allow the development of non-assisted reproductive therapy, drug-based treatments for male infertility.