Voltage-dependent calcium channels in mammalian spermatozoa revisited

Semen Proteomics of COVID-19 Convalescent Men Reveals Disruption of Key Biological Pathways Relevant to Male Reproductive Function

A considerable section of males suffered from COVID-19, with many experiencing long-term repercussions. Recovered males have been documented to have compromised fertility, albeit the mechanisms remain unclear. We investigated the impact of COVID-19 on semen proteome following complete clinical recovery using mass spectrometry. A label-free quantitative proteomics study involved 10 healthy fertile subjects and 17 COVID-19-recovered men. With 1% false discovery rate and >1 unique peptide stringency, MaxQuant analysis found 1099 proteins and 8503 peptides. Of the 48 differentially expressed proteins between the healthy and COVID-19-recovered groups, 21 proteins were downregulated and 27 were upregulated in COVID-19-recovered males. The major pathways involved in reproductive functions, such as sperm-oocyte recognition, testosterone response, cell motility regulation, adhesion regulation, extracellular matrix adhesion, and endopeptidase activity, were downregulated in COVID-19-recovered patients according to bioinformatics analysis. Furthermore, the targeted approach revealed significant downregulation of semenogelin 1 and prosaposin, two proteins related to male fertility. Therefore, we demonstrate the alteration of semen proteome in response to COVID-19, thus disrupting the male reproductive function despite the patient's clinical remission. Hence, to understand fertility-related biological processes triggered by this infection, a protracted evaluation of the consequences of COVID-19 in recovered men is warranted.

Influence of antifertility agents Dutasteride and Nifedipine on CatSper gene level in epididymis during sperm maturation in BALB/c mice

Calcium (Ca²⁺) signaling is critical for successful fertilization. In spermatozoa, capacitation, hyperactivation of motility and the acrosome reaction are all mediated by increases in intracellular Ca²⁺ through CatSper (sperm-specific cation channel). The CatSper channel complex contains four pore-forming α subunits (CatSper1-4) and five accessory subunits called β, δ, ε, γ and ζ. Genetic deletion of any of the four CatSper genes in mice results in loss of hyperactivated motility and male infertility. Despite their vital role in male fertility, almost very little is known about influence of antifertility agents on CatSper gene expression in epididymis and epididymal spermatozoa. Therefore, we performed quantitative real-time qPCR analysis for CatSper expression in the epididymis and epididymal sperm of BALB/c mice after treatment with Dutasteride (DS), a dual 5-α reductase inhibitor and Nifedipine (NF) a calcium channel blocker as positive control. We observed that treatment with antifertility agents Dutasteride and Nifedipine induced significant decreases in the caput and cauda epididymal sperm counts, motility and fertility which could partly be attributed to alteration in the normal morphology of the sperm associated with downregulation/upregulation of CatSper mRNAs in epididymis and epididymal spermatozoa of male BALB/c mice. These can be explained on the basis of interference with mechanisms affecting calcium ion signaling resulting in changes in intracellular calcium required for sperm activity, finally affecting sperm maturation and fertility of male BALB/c mice. These studies provide some novel avenues for developing new male contraceptives in future.

New insights into epididymal function in relation to sperm maturation

Testicular spermatozoa acquire fertility only after 1 or 2 weeks of transit through the epididymis. At the end of this several meters long epididymal tubule, the male gamete is able to move, capacitate, migrate through the female tract, bind to the egg membrane and fuse to the oocyte to result in a viable embryo. All these sperm properties are acquired after sequential modifications occurring either at the level of the spermatozoon or in the epididymal surroundings. Over the last few decades, significant increases in the understanding of the composition of the male gamete and its surroundings have resulted from the use of new techniques such as genome sequencing, proteomics combined with high-sensitivity mass spectrometry, and gene-knockout approaches. This review reports and discusses the most relevant new results obtained in different species regarding the various cellular processes occurring at the sperm level, in particular, those related to the development of motility and egg binding during epididymal transit.

Niflumic acid blocks native and recombinant T-type channels

Voltage-dependent calcium channels are widely distributed in animal cells, including spermatozoa. Calcium is fundamental in many sperm functions such as: motility, capacitation, and the acrosome reaction (AR), all essential for fertilization. Pharmacological evidence has suggested T-type calcium channels participate in the AR. Niflumic acid (NA), a non-steroidal anti-inflammatory drug commonly used as chloride channel blocker, blocks T-currents in mouse spermatogenic cells and Cl(-) channels in testicular sperm. Here we examine the mechanism of NA blockade and explore if it can be used to separate the contribution of different Ca(V)3 members previously detected in these cells. Electrophysiological patch-clamp recordings were performed in isolated mouse spermatogenic cells and in HEK cells heterologously expressing Ca(V)3 channels. NA blocks mouse spermatogenic cell T-type currents with an IC(50) of 73.5 µM, without major voltage-dependent effects. The NA blockade is more potent in the open and in the inactivated state than in the closed state of the T-type channels. Interestingly, we found that heterologously expressed Ca(V)3.1 and Ca(V)3.3 channels were more sensitive to NA than Ca(V)3.2 channels, and this drug substantially slowed the recovery from inactivation of the three isoforms. Molecular docking modeling of drug-channel binding predicts that NA binds preferentially to the extracellular face of Ca(V)3.1 channels. The biophysical characteristics of mouse spermatogenic cell T-type currents more closely resemble those from heterologously expressed Ca(V)3.1 channels, including their sensitivity to NA. As Ca(V)3.1 null mice maintain their spermatogenic cell T-currents, it is likely that a novel Ca(V)3.2 isoform is responsible for them.

LRRC52 (leucine-rich-repeat-containing protein 52), a testis-specific auxiliary subunit of the alkalization-activated Slo3 channel

KSper, a pH-dependent K(+) current in mouse spermatozoa that is critical for fertility, is activated by alkalization in the range of pH 6.4-7.2 at membrane potentials between -50 and 0 mV. Although the KSper pore-forming subunit is encoded by the Slo3 gene, heterologously expressed Slo3 channels are largely closed at potentials negative to 0 mV at physiological pH. Here we identify a Slo3-associating protein, LRRC52 (leucine-rich repeat-containing 52), that shifts Slo3 gating into a range of voltages and pH values similar to that producing KSper current activation. Message for LRRC52, a homolog of the Slo1-modifying LRRC26 protein, is enriched in testis relative to other homologous LRRC subunits and is developmentally regulated in concert with that for Slo3. LRRC52 protein is detected only in testis. It is markedly diminished from Slo3(-/-) testis and completely absent from Slo3(-/-) sperm, indicating that LRRC52 expression is critically dependent on the presence of Slo3. We also examined the ability of other LRRC subunits homologous to LRRC26 and LRRC52 to modify Slo3 currents. Although both LRRC26 and LRRC52 are able to modify Slo3 function, LRRC52 is the stronger modifier of Slo3 function. Effects of other related subunits were weaker or absent. We propose that LRRC52 is a testis-enriched Slo3 auxiliary subunit that helps define the specific alkalization dependence of KSper activation. Together, LRRC52 and LRRC26 define a new family of auxiliary subunits capable of critically modifying the gating behavior of Slo family channels.

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 signaling through CatSper channels in mammalian fertilization

The molecular mechanisms underlying Ca(2+) entry into sperm are now much more well defined thanks to direct recordings of mature sperm cells. This article reviews the function of a sperm-specific ion channel, CatSper. CatSpers have a clearly defined function in sperm's hyperactivated motility and are essential for male fertility. We propose that physiological stimuli such as zona pellucida and cyclic nucleotides induce Ca(2+) entry through CatSper channels instead of acting on Ca(V) and CNG channels as previously thought.

Surfing the wave, cycle, life history, and genes/proteins expressed by testicular germ cells. Part 4: intercellular bridges, mitochondria, nuclear envelope, apoptosis, ubiquitination, membrane/voltage-gated channels, methylation/acetylation, and transcription factors

As germ cells divide and differentiate from spermatogonia to spermatozoa, they share a number of structural and functional features that are common to all generations of germ cells and these features are discussed herein. Germ cells are linked to one another by large intercellular bridges which serve to move molecules and even large organelles from the cytoplasm of one cell to another. Mitochondria take on different shapes and features and topographical arrangements to accommodate their specific needs during spermatogenesis. The nuclear envelope and pore complex also undergo extensive modifications concomitant with the development of germ cell generations. Apoptosis is an event that is normally triggered by germ cells and involves many proteins. It occurs to limit the germ cell pool and acts as a quality control mechanism. The ubiquitin pathway comprises enzymes that ubiquitinate as well as deubiquitinate target proteins and this pathway is present and functional in germ cells. Germ cells express many proteins involved in water balance and pH control as well as voltage-gated ion channel movement. In the nucleus, proteins undergo epigenetic modifications which include methylation, acetylation, and phosphorylation, with each of these modifications signaling changes in chromatin structure. Germ cells contain specialized transcription complexes that coordinate the differentiation program of spermatogenesis, and there are many male germ cell-specific differences in the components of this machinery. All of the above features of germ cells will be discussed along with the specific proteins/genes and abnormalities to fertility related to each topic.

Recombinant human ZP3-induced sperm acrosome reaction: evidence for the involvement of T- and L-type voltage-gated calcium channels

For successful fertilization mammalian spermatozoa must undergo the acrosome reaction (AR), an exocytotic event that allows this cell to penetrate the outer layer of the oocyte, the zona pellucida (ZP). Four glycoproteins (ZP1-ZP4) compose the human ZP, being ZP3 the physiological inductor of the AR. This process requires changes in intracellular Ca(2+) concentration ([Ca(2+)](i)) involving not fully understood mechanisms. Even in mouse sperm, the pharmacologically documented participation of voltage-gated Ca(2+) (Ca(V)) channels and store-operated channels (SOCs) in the ZP-induced AR is being debated. The situation in human sperm is even less clear due to the limited availability of human ZP. Here, we used recombinant human ZP3 (rhZP3) produced in baculovirus-infected Sf9 cells to investigate the involvement of Ca(V) channels in the human sperm AR. Our findings showed that Ni(2+) and mibefradil at concentrations that block T-type or Ca(V)3 channels, and nimodipine and diltiazem that block L-type or Ca(V)1 channels, significantly inhibited the rhZP3-initiated AR. On the other hand, the AR was insensitive to concentrations of omega-Agatoxin IVA, omega-Conotoxin GVIA and SNX-482 that block P/Q, N and R-type channels, respectively (Ca(V)2 channels). Our overall findings suggest that Ca(V)1 and Ca(V)3 channels participate in human sperm AR. Consistent with this, we detected in human sperm transcripts for the Ca(V)1 auxiliary subunits, alpha(2)delta, beta(1), beta(2) and beta(4), but not the neuronal specific isoforms beta(3) and gamma(2).

Cadmium concentrations in blood and seminal plasma: correlations with sperm number and motility in three male populations (infertility patients, artificial insemination donors, and unselected volunteers)

To investigate a possible common environmental exposure that may partially explain the observed decrease in human semen quality, we correlated seminal plasma and blood cadmium levels with sperm concentration and sperm motility. We studied three separate human populations: group 1, infertility patients (Long Island, NY, USA); group 2, artificial insemination donors (AID) (Rochester, NY, USA); and group 3, general population volunteers (Rochester, NY, USA). Information about confounding factors was collected by questionnaire. Seminal plasma cadmium did not correlate with blood cadmium (Spearman correlation, n = 91, r = -0.092, P = 0.386, NS). Both blood and seminal plasma cadmium were significantly higher among infertility patients than the other subjects studied (for example, median seminal plasma cadmium was 0.282 microg/L in infertility patients versus 0.091 microg/L in AID and 0.092 microg/L in general population volunteers; Kruskal-Wallis test, P < 0.001). The percentage of motile sperm and sperm concentration correlated inversely with seminal plasma cadmium among the infertility patients (r = -0.201, P < 0.036 and r = -0.189, P < 0.05, respectively), but not in the other two groups. Age (among infertility patients) was the only positive confounder correlating with seminal plasma cadmium. To validate our human findings in an animal model, we chronically exposed adolescent male Wistar rats to low-moderate cadmium in drinking water. Though otherwise healthy, the rats exhibited decreases in epididymal sperm count and sperm motility associated with cadmium dose and time of exposure. Our human and rat study results are consistent with the hypothesis that environmental cadmium exposures may contribute significantly to reduced human male sperm concentration and sperm motility.

Control of hyperactivation in sperm

BACKGROUND

Sperm hyperactivation is critical to fertilization, because it is required for penetration of the zona pellucida. Hyperactivation may also facilitate release of sperm from the oviductal storage reservoir and may propel sperm through mucus in the oviductal lumen and the matrix of the cumulus oophorus. Hyperactivation is characterized by high amplitude, asymmetrical flagellar bending.

METHODS

This is a review of the original literature on the mechanisms that regulate hyperactivation, including physiological factors and signaling pathways.

RESULTS

Computer-assisted semen analysis systems can be used to identify hyperactivated sperm by setting minimum thresholds for curvilinear velocity (VSL) and lateral head movement and a maximum threshold for path linearity. Hyperactivation is triggered by a rise in flagellar Ca(2+) resulting from influx primarily through plasma membrane CatSper channels and possibly also by release of Ca(2+) from a store in the redundant nuclear envelope. It requires increased pH and ATP production. The physiological signals that trigger the rise in Ca(2+) remain elusive, but there is evidence that the increased Ca(2+) acts through a calmodulin/calmodulin kinase pathway. Hyperactivation is considered part of the capacitation process; however, the regulatory pathway that triggers hyperactivation can operate independently from that which prepares sperm to undergo the acrosome reaction. Hyperactivation may be modulated by chemotactic signals to turn sperm toward the oocyte.

CONCLUSIONS

Little is known about exactly what triggers hyperactivation in human sperm. This information could enable clinicians to develop reliable fertility assays to assess normal hyperactivation in human sperm samples.

Link between low-dose environmentally relevant cadmium exposures and asthenozoospermia in a rat model

OBJECTIVE

To define the mechanism(s) underlying an association between asthenozoospermia and elevated blood, seminal plasma, and testicular cadmium levels in infertile human males using a rat model of environmentally relevant cadmium exposures.

SETTING

University medical center andrology research laboratory.

ANIMAL(S)

Male Wistar rats (n = 60), documented to be sensitive to the testicular effects of cadmium.

INTERVENTION(S)

Rats were given ad libitum access to water supplemented with 14% sucrose and 0 mg/L, 5 mg/L, 50 mg/L, or 100 mg/L cadmium for 1, 4, or 8 weeks beginning at puberty.

MAIN OUTCOME MEASURE(S)

Testicular cadmium levels were determined by atomic absorption, cauda epididymal sperm motility by visual inspection, and testicular gene expression by DNA microarray hybridization.

RESULT(S)

Chronic, low-dose cadmium exposures produced a time- and dose-dependent reduction in sperm motility. Transcription of genes regulated by calcium and expression of L-type voltage-dependent calcium channel mRNA splicing variants were altered by cadmium exposure. Expression of calcium binding proteins involved in modulation of sperm motility was unaffected.

CONCLUSION(S)

A causal relationship between elevated testicular cadmium and asthenozoospermia was identified. Aberrrant sperm motility was correlated with altered expression of L-type voltage-dependent calcium channel isoforms found on the sperm tail, which regulate calcium and cadmium influx.