Sperm are promiscuous and CatSper is to blame…

Cdc42 localized in the CatSper signaling complex regulates cAMP-dependent pathways in mouse sperm

Sperm acquire the ability to fertilize in a process called capacitation and undergo hyperactivation, a change in the motility pattern, which depends on Ca²⁺ transport by CatSper channels. CatSper is essential for fertilization and it is subjected to a complex regulation that is not fully understood. Here, we report that similar to CatSper, Cdc42 distribution in the principal piece is confined to four linear domains and this localization is disrupted in CatSper1-null sperm. Cdc42 inhibition impaired CatSper activity and other Ca²⁺ -dependent downstream events resulting in a severe compromise of the sperm fertilizing potential. We also demonstrate that Cdc42 is essential for CatSper function by modulating cAMP production by soluble adenylate cyclase (sAC), providing a new regulatory mechanism for the stimulation of CatSper by the cAMP-dependent pathway. These results reveal a broad mechanistic insight into the regulation of Ca²⁺ in mammalian sperm, a matter of critical importance in male infertility as well as in contraception.

CatSper: The complex main gate of calcium entry in mammalian spermatozoa

Calcium ions (Ca²⁺) are involved in nearly every aspect of cellular life. They are one of the most abundant elements in mammals and play a vital role in physiological and biochemical processes acting mainly as intracellular messengers. In spermatozoa, several key functions are regulated by cytoplasmic Ca²⁺ concentration such as sperm capacitation, chemotaxis, hyperactive motility, and acrosome reaction. The sperm-specific ion channel CatSper is the principal calcium channel in sperm mediating the calcium influx into the sperm flagellum and acting as an essential modulator of downstream mechanisms involved in fertilization. This review aims to provide insights into the structure, localization, and function of the mammalian CatSper channel, primarily human and mice. The activation of CatSper by progesterone and prostaglandins, as well as the ligand-independent regulation of the channel by a change in the membrane voltage and intracellular pH are going to be addressed. Finally, major questions, challenges, and perspectives are discussed.

Novel pharmacological actions of trequinsin hydrochloride improve human sperm cell motility and function

BACKGROUND AND PURPOSE

Asthenozoospermia is a leading cause of male infertility, but development of pharmacological agents to improve sperm motility is hindered by the lack of effective screening platforms and knowledge of suitable molecular targets. We have demonstrated that a high-throughput screening (HTS) strategy and established in vitro tests can identify and characterise compounds that improve sperm motility. Here, we applied HTS to identify new compounds from a novel small molecule library that increase intracellular calcium ([Ca²⁺ ]_i ), promote human sperm cell motility, and systematically determine the mechanism of action.

EXPERIMENTAL APPROACH

A validated HTS fluorometric [Ca²⁺ ]_i assay was used to screen an in-house library of compounds. Trequinsin hydrochloride (a PDE3 inhibitor) was selected for detailed molecular (plate reader assays, electrophysiology, and cyclic nucleotide measurement) and functional (motility and acrosome reaction) testing in sperm from healthy volunteer donors and, where possible, patients.

KEY RESULTS

Fluorometric assays identified trequinsin as an efficacious agonist of [Ca²⁺ ]_i , although less potent than progesterone. Functionally, trequinsin significantly increased cell hyperactivation and penetration into viscous medium in all donor sperm samples and cell hyperactivation in 22/25 (88%) patient sperm samples. Trequinsin-induced [Ca²⁺ ]_i responses were cross-desensitised consistently by PGE₁ but not progesterone. Whole-cell patch clamp electrophysiology confirmed that trequinsin activated CatSper and partly inhibited potassium channel activity. Trequinsin also increased intracellular cGMP.

CONCLUSION AND IMPLICATIONS

Trequinsin exhibits a novel pharmacological profile in human sperm and may be a suitable lead compound for the development of new agents to improve patient sperm function and fertilisation potential.

Tyrosine phosphorylation signaling regulates Ca2+ entry by affecting intracellular pH during human sperm capacitation

Capacitation is a mandatory process for the acquisition of mammalian sperm fertilization competence and involves the activation of a complex and still not fully understood system of signaling pathways. Under in vitro conditions, there is an increase in both protein tyrosine phosphorylation (pTyr) and intracellular Ca²⁺ levels in several species. In human sperm, results from our group revealed that pTyr signaling can be blocked by inhibiting proline-rich tyrosine kinase 2 (PYK2). Based on the role of PYK2 in other cell types, we investigated whether the PYK2-dependent pTyr cascade serves as a sensor for Ca ²⁺ signaling during human sperm capacitation. Flow cytometry studies showed that exposure of sperm to the PYK2 inhibitor N-[2-[[[2-[(2,3-dihydro-2-oxo-1 H-indol-5-yl)amino]-5-(trifluoromethyl)-4-pyrimidinyl]amino]methyl]phenyl]- N-methyl-methanesulfonamide hydrate (PF431396) produced a significant and concentration-dependent reduction in intracellular Ca ²⁺ levels during capacitation. Further studies revealed that PF431396-treated sperm exhibited a decrease in the activity of CatSper, a key sperm Ca ²⁺ channel. In addition, time course studies during capacitation in the presence of PF431396 showed a significant and sustained decrease in both intracellular Ca ²⁺ and pH levels after 2 hr of incubation, temporarily coincident with the activation of PYK2 during capacitation. Interestingly, decreases in Ca ²⁺ levels and progressive motility caused by PF431396 were reverted by inducing intracellular alkalinization with NH ₄ Cl, without affecting the pTyr blockage. Altogether, these observations support pTyr as an intracellular sensor for Ca ²⁺ entry in human sperm through regulation of cytoplasmic pH. These results contribute to a better understanding of the modulation of the polymodal CatSper and signaling pathways involved in human sperm capacitation.

Intracellular pH in sperm physiology

Intracellular pH (pHi) regulation is essential for cell function. Notably, several unique sperm ion transporters and enzymes whose elimination causes infertility are either pHi dependent or somehow related to pHi regulation. Amongst them are: CatSper, a Ca(2+) channel; Slo3, a K(+) channel; the sperm-specific Na(+)/H(+) exchanger and the soluble adenylyl cyclase. It is thus clear that pHi regulation is of the utmost importance for sperm physiology. This review briefly summarizes the key components involved in pHi regulation, their characteristics and participation in fundamental sperm functions such as motility, maturation and the acrosome reaction.

Calcium influx and male fertility in the context of the sperm proteome: an update

Freshly ejaculated spermatozoa are incapable or poorly capable of fertilizing an oocyte. The fertilization aptness of spermatozoa depends on the appropriate and time-dependent acquisition of hyperactivation, chemotaxis, capacitation, and the acrosome reaction, where calcium (Ca²⁺) is extensively involved in almost every step. A literature review showed that several ion channel proteins are likely responsible for regulation of the Ca²⁺ uptake in spermatozoa. Therefore, manipulation of the functions of channel proteins is closely related to Ca²⁺ influx, ultimately affecting male fertility. Recently, it has been shown that, together with different physiological stimuli, protein-protein interaction also modifies the Ca²⁺ influx mechanism in spermatozoa. Modern proteomic analyses have identified several sperm proteins, and, therefore, these findings might provide further insight into understanding the Ca²⁺ influx, protein functions, and regulation of fertility. The objective of this review was to synthesize the published findings on the Ca²⁺ influx mechanism in mammalian spermatozoa and its implications for the regulation of male fertility in the context of sperm proteins. Finally, Pathway Studio (9.0) was used to catalog the sperm proteins that regulate the Ca²⁺ influx signaling by using the information available from the PubMed database following a MedScan Reader (5.0) search.

IFN-gamma alters the human sperm membrane permeability to Ca(2+)

Inflammation in the male genitourinary tract has been associated with the release of pro-inflammatory cytokines such as interferon-gamma (IFN-γ) and elevated reactive oxygen species, which affects spermatozoa capacitation, motility, and the acrosome reaction, along with functions regulated by the concentration of cytoplasmic Ca(2+) ([Ca(2+)]cyto). Though Ca(2+) signaling is of particular significance in sperm, the effect of IFN-γ intracellular calcium on these cells is still unknown. The present study evaluated the effect of IFN-γ on the [Ca(2+)]cyto and Ca(2+) permeability on human sperm. A cell suspension loaded with fura-2 was incubated with or without IFN-γ (from 0 to 2000 pg/ml) for 0, 30, 60, and 120 minutes, and the [Ca(2+)]cyto was measured. The permeability to Ca(2+) was evaluated by the change of the intracellular concentration following an extracellular Ca(2+) pulse. IFN-γ at low concentrations (≤ 500  pg/ml) did not affect the [Ca(2+)]cyto and Ca(2+) permeability of sperm. At a high concentration (2000  pg/ml), IFN-γ did not alter the [Ca(2+)](cyto), but significantly decreased the magnitude and velocity of Ca(2+) entry into the cell. This effect was dependent on incubation time and IFN-γ concentration. This alteration induced by IFN-γ was prevented by the simultaneous incubation of sperm with the antioxidant butylhydroxytoluene (BHT). In conclusion, in vitro, IFN-γ modifies Ca(2+) sperm membrane permeability, probably via lipid peroxidation. IFN-γ in high concentration, as observed in inflammation/infection, can affect [Ca(2+)](cyto) regulation and alter sperm fertilizing capacity.

The clinical significance of calcium-signalling pathways mediating human sperm hyperactivation

STUDY QUESTION

What is the prevalence of defects in the Ca²⁺-signalling pathways mediating hyperactivation (calcium influx and store mobilization) among donors and sub-fertile patients and are they functionally significant, i.e. related to fertilization success at IVF?

SUMMARY ANSWER

This study identifies, for the first time, the prevalence of Ca²⁺ store defects in sperm from research donors, IVF and ICSI patients. It highlights the biological role and importance of Ca²⁺ signalling (Ca²⁺ store mobilization) for fertilization at IVF.

WHAT IS KNOWN ALREADY

Sperm motility and hyperactivation (HA) are important for fertility, mice with sperm incapable of HA are sterile. Recently, there has been significant progress in our knowledge of the factors controlling these events, in particular the generation and regulation of calcium signals. Both pH-regulated membrane Ca²⁺ channels (CatSper) and Ca²⁺ stores (potentially activating store-operated Ca²⁺ channels) have been implicated in controlling HA.

STUDY DESIGN, SIZE, AND DURATION

This was a prospective study examining a panel of 68 donors and 181 sub-fertile patients attending the Assisted Conception Unit, Ninewells Hospital Dundee for IVF and ICSI. Twenty-five of the donors gave a second sample (∼4 weeks later) to confirm consistency/reliability of the recorded responses. Ca²⁺ signalling was manipulated using three agonists, NH₄Cl (activates CatSper via pH), progesterone (direct activation of CatSper channels, potentially enhancing mobilization of stored Ca²⁺ by CICR) and 4-aminopyridine (4-AP) (effect on pH equivalent to NH₄Cl and mobilizes stored Ca²⁺). The broad-spectrum phosphodiesterase inhibitor 3-isobutyl-1-methyxanthine (IBMX), a potent activator of HA was also used for comparison. For patient samples, an aliquot surplus to requirements for IVF/ICSI treatment was examined, allowing direct comparison of Ca²⁺ signalling and motility data with functional competence of the sperm.

MATERIALS, SETTING, METHODS

The donors and sub-fertile patients were screened for HA (using CASA) and changes in intracellular Ca²⁺ were assessed by loading with Fura-2 and measuring fluorescence using a plate reader (FluoStar).

MAIN RESULTS AND THE ROLE OF CHANCE

The relative efficacy of the stimuli in inducing HA was 4-AP >> IBMX > progesterone. NH₄Cl increased [Ca²⁺]_i similarly to 4-AP and progesterone but did not induce a significant increase in HA. Failure of samples to generate HA (no significant increase in response to stimulation with 4-AP) was seen in just 2% of research donors but occurred in 10% of IVF patients (P = 0.025). All donor samples generated a significant [Ca²⁺]_i increase when stimulated with 4-AP but 3.3% of IVF and 28.6% of ICSI patients failed to respond. Amplitudes of HA and [Ca²⁺]_i responses to 4-AP were correlated with fertilization rate at IVF (P= 0.029; P = 0.031, respectively). Progesterone reliably induced [Ca²⁺]_i responses (97% of donors, 100% of IVF patients) but was significantly less effective than 4-AP in inducing HA. Twenty seven per cent of ICSI patients failed to generate a [Ca²⁺]_i response to progesterone (P= 0.035). Progesterone-induced [Ca²⁺]_i responses were correlated with fertilization rate at IVF (P= 0.037) but induction of HA was not. In donor samples examined on more than one occasion consistent responses for 4-AP-induced [Ca²⁺]_i (R² = 0.97) and HA (R² = 0.579) were obtained. In summary, the data indicate that defects in Ca²⁺ signalling leading to poor HA do occur and that ability to undergo Ca²⁺ -induced HA affects IVF fertilizing capacity. The data also confirm that release of stored Ca²⁺ is the crucial component of Ca²⁺ signals leading to HA and that Ca²⁺ store defects may therefore underlie HA failure.

LIMITATIONS, REASONS FOR CAUTION

This is an in vitro study of sperm function. While the repeatability of the [Ca²⁺]_i and HA responses in samples from the same donor were confirmed, data for patients were from 1 assessment and thus the robustness of the failed responses in patients’ needs to be established. The focus of this study was on using 4AP, which mobilizes stored Ca²⁺ and is a potent inducer of HA. The n values for other agonists, especially calcium assessments, are smaller.

WIDER IMPLICATIONS OF THE FINDINGS

Previous studies have shown a significant relationship between basal levels of HA, calcium responses to progesterone and IVF fertilization rates. Here, we have systematically investigated the ability/failure of human sperm to generate Ca²⁺ signals and HA in response to targeted pharmacological challenge and, related defects in these responses to IVF success. [Ca²⁺]_i signalling is fundamental for sperm motility and data from this study will lead to assessment of the nature of these defects using techniques such as single-cell imaging and patch clamping.

STUDY FUNDING/COMPETING INTEREST(S)

Resources from a Wellcome Trust Project Grant (#086470, Publicover and Barratt PI) primarily funded the study. The authors have no competing interests.